JP2005024612A

JP2005024612A - Image forming method and image forming apparatus

Info

Publication number: JP2005024612A
Application number: JP2003186858A
Authority: JP
Inventors: Naoto Watanabe; 直人渡辺; Kiyonori Tsuda; 清典津田; Atsushi Sanpei; 敦史三瓶; Takeo Suda; 武男須田; Satoshi Hatori; 聡羽鳥; Yuji Arai; 裕司荒井; Tomoji Ishikawa; 知司石川
Original assignee: Ricoh Co Ltd
Current assignee: Ricoh Co Ltd
Priority date: 2003-06-30
Filing date: 2003-06-30
Publication date: 2005-01-27

Abstract

<P>PROBLEM TO BE SOLVED: To obtain stable image density not only in a solid part but also in a high light part over a long period. <P>SOLUTION: In the image forming method, electrostatic latent images of a plurality of reference patterns are formed on a photoreceptor 1, the reference patterns are developed by a developing device 5, toner attached quantity in the developed reference pattern is detected and the image forming condition is compensated based on the detected result of the toner attached quantity. The image forming condition is compensated by deriving at a straight line approximate expression between developing potential and toner attached quantity from the detected result of latent image potential and toner attached quantity in the plurality of reference patterns. Then, developing gamma, which is the inclination of the straight line approximate expression, and developing start voltage are calculated. Next, the target electrifying potential and the target exposure part potential are determined from the developing gamma. The target developing bias potential is determined from the target electrifying potential, the target exposure part potential, developing gamma and developing start voltage. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、複写機、ファクシミリ、プリンタ等の画像形成装置および画像形成方法に関するものである。
【０００２】
【従来の技術】
従来、電子写真方式の画像形成装置では、環境変動や経時における画像品質の安定化を図るため、像担持体上に基準パターンのトナー像を作成してそのトナー付着量を検出し、検出結果に基づき作像条件の制御を行う技術が知られている。
【０００３】
その一例として、特許文献１では以下の制御をおこなう画像形成装置が開示されている。なお、以下の説明においては、像担持体として負極性帯電の感光体を用い、露光部にトナーを付着させて画像部とするネガポジ現像のシステムを用いる。まず、感光体の帯電電位及び現像バイアス電位の値をそれぞれ徐々に変化させながら感光体上に基準パターンであるパッチパターンを形成する。具体的には、帯電電位は通常の画像形成時における一様な帯電電位とは異なり、値を負極性側で徐々に絶対値を大きくしていく。そして、レーザ光等による露光より、帯電電位を減衰させて基準パターン用の静電潜像を形成する。次に、この静電潜像を現像して基準パターンのトナー像を形成し、センサにてトナー付着量を検出する。この現像における現像バイアス電位の値も負極性側で徐々に絶対値を大きくしていく。そして、露光部電位と現像バイアス電位との差から求めた現像ポテンシャルとセンサで検出したトナー付着量とから、現像ポテンシャルとトナー付着量との関係に係わる直線近似式を算出する。この直線近似式（ｙ＝ａｘ＋ｂ）を用いれば、所望の画像濃度（トナー付着量）が得られる現像ポテンシャルを演算することができる。その後、演算された現像ポテンシャルに基づいて、現像バイアス電位と、これと一定電位差を有するように帯電電位を定め、これらを目標電位として、各電位を目標電位に合致するように制御し、制御された帯電電位、現像バイアス電位によって、通常の画像形成を行う。
【０００４】
また、上記レーザ光等による露光は、露光部電位が帯電電位にかかわらず例えば−２０［Ｖ］まで減衰させるような強度としてもよい。この場合、露光部電位が一定であるので、露光部電位と現像バイアス電位との差から現像ポテンシャルを求めなくとも、直接、現像バイアス電位とトナー付着量との関係に係わる直線近似式を算出することができる。この直線近似式を用いれば、所望の画像濃度（トナー付着量）が得られる現像バイアス電位を直接演算することができる。
【０００５】
【特許文献１】
特開２００２−１１６６１６号公報
【０００６】
【発明が解決しようとする課題】
しかしながら、上記画像形成装置では、ベタ部のトナー付着量（画像濃度）は安定するが、ハイライト部（中間調）のトナー付着量に誤差が生じやすく、安定しないという欠点があった。これは、以下の理由によると考えられる。
図１１は、感光体に１ドットの光書き込みをおこなった場合の潜像電位分布をしめす図である。一般的に感光体の潜像特性は、帯電電位と露光部電位とで規定されるが、実際の１ドットでみると、図１１に示すように、帯電電位が露光部電位に減衰するまでの間に中間の電位が存在する。この中間部分の形状は、帯電電位、露光部電位が同一でも異なることがある。ここで、上記画像形成装置では、露光部電位と現像バイアス電位との差から求めた現像ポテンシャルとトナー付着量との関係に基づいて、補正をおこなっているため、露光部電位に対応するベタ部のトナー付着量は安定する。しかしながら、中間部分の電位に対応するハイライト部については特に考慮されていないため、中間部分で形状に差がある場合には、ハイライト部のトナー付着量に差を生じてしまう。
また、ハイライト部のトナー付着量のばらつきが大きくなると、画像地肌部の汚れなどが発生するおそれがあり、画像品質が大きく低下する懸念がある。
【０００７】
本発明は、上記背景に鑑みなされたものであり、その目的とするところは、ベタ部のみならずハイライト部においても安定した画像濃度を長期に渡って得ることができる画像形成装置及び画像形成方法を提供することである。
【０００８】
【課題を解決するための手段】
上記目的を達成するために、請求項１の発明は、像担持体を一様に帯電したのち露光して静電潜像を形成し、該像担持体上の静電潜像に現像装置よりトナーを付着させて現像することで作像をおこなうもので、該像担持体上に複数の基準パターンの静電潜像を形成し、該現像装置で該基準パターンを現像し、現像された該基準パターンのトナー付着量を検出し、該トナー付着量の検出結果に基づいて作像条件を補正する画像形成方法において、上記複数の基準パターンの潜像電位と上記トナー付着量の検出結果とから現像ポテンシャルとトナー付着量との直線近似式を求めて、該直線近似式の傾きである現像ガンマと現像開始電圧とを算出し、該現像ガンマから目標帯電電位及び目標露光部電位を定め、該目標帯電電位及び該目標露光部電位と該現像ガンマと該現像開始電圧とから目標現像バイアス電位を定めて作像条件を補正することを特徴とするものである。
また、請求項２の発明は、請求項１の画像形成方法において、上記目標現像バイアス電位は上記現像ガンマと上記現像開始電圧とから最大現像量を得るための現像ポテンシャルを求め、該現像ポテンシャルを得るように上記目標帯電電位及び上記目標露光電位に基づいて定めることを特徴とするものである。
また、請求項３の発明は、請求項１または２の画像形成方法において、上記目標帯電電位及び上記目標露光電位は上記現像ガンマと帯電電位及び露光部電位との関係を予め定めたテーブルに基づいてを定めることを特徴とするものである。
また、請求項４の発明は、請求項１、２または３の画像形成方法において、上記トナー付着量を検出する検出手段として乱反射型フォトセンサを用いることを特徴とするものである。
また、請求項５の発明は、静電潜像を担持する像担持体と、該像担持体に静電潜像を形成する潜像形成手段と、該像担持体上の静電潜像をトナー像化する現像装置と、該像担持体上に複数の基準のパッチパターンの静電潜像を形成するパッチパターン書き込み手段と、該現像装置で現像された該パッチパターンの画像濃度を検出する画像濃度検知手段と、該画像濃度検知手段による検知結果に基づいて作像形成条件を補正する作像条件補正手段とを備えた画像形成装置において、上記像担持体上に形成した複数のパッチパターンの潜像電位と、上記画像濃度検知手段の検知結果とから現像ポテンシャルとトナー付着量に係わる直線近似式を求めて、該直線近似式の傾きである現像ガンマと現像開始電圧とを算出する現像特性算出手段と、該現像特性算出手段で算出した現像ガンマに基づき目標帯電電位及び目標露光電位を定める潜像特性算出手段と、該潜像特性算出手段で定めた目標帯電電位及び目標露光電位と該現像特性算出手段で算出した現像ガンマと現像開始電圧とから目標現像バイアス電位を定める現像バイアス電位算出手段とを備えたことを特徴とするものである。
また、請求項６の発明は、請求項５の画像形成装置において、上記現像バイアス電位算出手段は上記現像特性算出手段で算出した現像ガンマと現像開始電圧とから最大現像量を得るための現像ポテンシャルを求め、該現像ポテンシャルを得るように上記潜像特性算出手段で定めた目標帯電電位及び目標露光電位に基づいて目標現像バイアス電位を定めることを特徴とするものである。
また、請求項７の発明は、請求項５または６の画像形成装置において、上記潜像特性算出手段で算出する目標帯電電位及び目標露光電位は上記現像特性算出手段で算出した現像ガンマと帯電電位及び露光部電位との関係を予め定めたテーブルに基づいてを定めることを特徴とするものである。
また、請求項８の発明は、請求項５、６または７の画像形成装置において、上記画像濃度検出手段として乱反射型フォトセンサを用いることを特徴とするものである。
また、請求項９の発明は、請求項５、６、７または８の画像形成装置において、上記像担持体と上記現像装置とを一体的に構成した画像形成ユニットを用いることを特徴とするものである。
【０００９】
請求項１乃至９の発明においては、複数の基準パターンの潜像電位とトナー付着量の検出結果とから現像ポテンシャルとトナー付着量に係わる直線近似式を求め、直線近似式からその傾きである現像ガンマと現像開始電圧とを算出する。そして、算出された現像ガンマに基づいて目標帯電電位及び目標露光電位を定める。さらに、この目標帯電電位及び目標露光電位と現像ガンマと現像開始電圧とに基づいて現像バイアス電位を定める。
以下、本発明における制御の考え方および具体的制御フローについて、図８、図９、図１０に基づき詳しく説明する。図８は、露光部電位と現像バイアスから求められる現像ポテンシャルとトナー付着量の関係をしめすグラフである。また、図９は、像担持体の露光量と潜像電位の関係をしめすグラフである。また、図１０は、像担持体の露光量とトナー付着量の関係をしめすグラフである。
例えば、現像ポテンシャルとトナー付着量の関係が、図８にしめす標準的現像能力▲１▼であり、潜像特性が図９にしめす標準的潜像特性１であったとすると、像担持体の露光量とトナー付着量の関係は図１０にしめす現像能力▲１▼＋潜像特性１のライン（以下、標準ラインという）になる。ところが、図８に示すように、標準的現像能力▲１▼に対して、現像能力▲２▼、▲３▼のようにバラツキがあるとすると、像担持体の露光量とトナー付着量の関係は図１０にしめす現像能力▲２▼＋潜像特性１、現像能力▲３▼＋潜像特性１のラインになってしまう。このように現像能力が変化した場合においても、これらの像担持体の露光量とトナー付着量の関係のラインを標準的ラインになるように作像条件を補正することで、ハイライト部、ベタ部共に常に標準的トナー付着量が得られるようになる。
まず、現像ポテンシャルとトナー付着量の関係が現像能力▲２▼となった場合について説明する。現像能力▲２▼の直線近似式は、現像能力▲１▼の直線近似式と較べ現像開始電圧Ｖｋ（Ｘ軸との切片）が等しく、現像ガンマ（傾き）が異なる。このように現像ガンマが小さい場合、潜像特性として標準的な潜像特性１をそのまま使用すると、図１０にしめすように標準ラインよりも傾きが小さい現像能力▲２▼＋潜像特性１のラインとなる。そこで、現像ガンマの傾きが小さくなったことに基づき、図９にしめす露光量に対する電位の傾きが潜像特性１よりも急峻な潜像特性２になるように、目標帯電電位、目標露光部電位を定める。これは、現像ガンマの傾きが小さくなったことにより像担持体の露光量とトナー付着量との関係のラインの傾きが標準ラインより小さくなったことを、潜像特性の傾きを急峻にすることで補正し標準ラインと同じ傾きになるようにするものである。さらに、定められた目標帯電電位、目標露光部電位および、現像ポテンシャルとトナー付着量の関係とから、所望のトナー付着量がえられるよう目標現像バイアス電位を定める。
次に、現像ポテンシャルとトナー付着量の関係が現像能力▲３▼となった場合について説明する。現像能力▲３▼の直線近似式は、現像能力▲１▼の直線近似式と較べ現像開始電圧Ｖｋ（Ｘ軸との切片）が異なり、現像ガンマ（傾き）が等しい。このように現像開始電圧Ｖｋが異なる場合、潜像特性として標準的な潜像特性１をそのまま使用すると、現像ガンマが等しいため標準ラインと傾きが同じで、現像開始電圧Ｖｋが異なる分、同一現像ポテンシャルでのトナー付着量が異なり平行にシフトした現像能力▲３▼＋潜像特性１のラインとなる。そこで、現像ガンマが同じことに基づき潜像特性は標準的な潜像特性１をそのまま用いる。具体的には、それまでと同じ目標帯電電位、目標露光部電位とする。次に、現像開始電圧Ｖｋ小さくなったことに基づき現像ポテンシャルを小さくするよう、目標現像バイアス電位を変更する。これは、現像開始電圧Ｖｋが小さくなって現像能力が高くなったことにより、像担持体の露光量とトナー付着量の関係のラインが平行シフトしたことを目標現像バイアス電位を変更することで補正するものである。
また、現像ガンマ、現像開始電圧Ｖｋの両方が現像能力▲１▼と異なる場合は、上記現像能力▲２▼の補正及び現像能力▲３▼の補正の両方を行えばよい。
具体的な制御フローとしては、まず、像担持体の帯電電位及び現像バイアス電位をそれぞれ徐々に変化させながら像担持体上に基準パターンを形成し、センサにてトナー付着量を検出する。像担持体の露光部電位と現像バイアス電位との差から求めた現像ポテンシャルとセンサで検出したトナー付着量とから、現像ポテンシャルとトナー付着量との関係に係わる直線近似式を求める。この直線近似式を示す関数（ｙ＝ａｘ＋ｂ）の傾きである現像ガンマａ及び現像開始電圧ｂを演算する。この現像ガンマａには、予めきめられたテーブルでこれに適切なドラム帯電電位と露光部電位とが関連付けられており、これより現像ガンマａに最も近いテーブルを選びこれに関連付けられた帯電電位を目標帯電電位に、露光部電位を目標露光部電位と定める。また、目標帯電電位および目標露光部電位と、所望の画像濃度を得るための現像ポテンシャルとから、目標現像バイアス電位を算出する。このような目標電位にて通常の画像形成をおこなうことで、ベタ部からハイライト部にかけて全域でそれぞれ所望の画像濃度のトナー像を形成し得ることができる。
【００１０】
【発明の実施の形態】
以下、本発明を適用した画像形成装置の実施形態の一例として、電子写真方式のプリンタ（以下、単にプリンタという）について説明する。まず、本プリンタの基本的な構成について説明する。
図１は、本プリンタの概略構成図である。このプリンタ１００は、イエロー（Ｙ）、マゼンダ（Ｍ）、シアン（Ｃ）、黒（Ｋ）の各色の画像を形成するための４組のプロセスカートリッジ状のトナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋを備えたタンデム方式のカラープリンタである。以下、各符号の添字Ｙ、Ｍ、Ｃ、Ｋは、それぞれイエロー、マゼンダ、シアン、黒をしめす。４組のトナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋの上方には、後述する中間転写体としての中間転写ベルト８を張架しながら無端移動せしめる中間転写ユニット１５が配設されている。この中間転写ベルト８の移動方向における上流側から、４組のトナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋが順に配置されている。また、これらのトナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋは、各々潜像担持体としてのドラム上の感光体１Ｙ、１Ｍ、１Ｃ、１Ｋを備えている。
【００１１】
また、トナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋの下方には、露光装置７が配設されている。露光装置７は、光源、ポリゴンミラー、ｆ−θレンズ、反射ミラー等を備え、画像データに基づいて各感光体１Ｙ、１Ｍ、１Ｃ、１Ｋの表面にレーザ光を走査しながら照射する。この光照射により、感光体１Ｙ、１Ｍ、１Ｃ、１Ｋ上にイエロー、マゼンダ、シアン、黒の静電潜像が形成される。
【００１２】
トナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋは、現像剤として、互いに異なる色のイエロー、マゼンダ、シアン、黒トナーを用いるが、それ以外は同様の構成になっているので、以下イエローのトナー像形成部１Ｙを代表例として用いて説明する。
【００１３】
図２はイエローのトナー像形成部１Ｙの概略構成図である。このトナー像形成部１Ｙは、ドラム状の感光体１Ｙ、クリーニング装置２Ｙ、付図事の除電装置、帯電装置４Ｙ、現像装置５Ｙ等を備えている。このトナー像形成部６Ｙは、プリンタ１００本体に脱着可能であり、その寿命到達時に一度に消耗部品を交換できるようになっている。帯電装置４Ｙは、図示しない駆動手段によって図中時計回りに回転駆動される感光体１Ｙの表面を一様帯電（例えば−７００Ｖ）させる。一様帯電した感光体１Ｙの表面は、露光装置７によりレーザ光Ｌを照射されイエロー画像用の静電潜像を形成する。この静電潜像は、イエロートナーを用いる現像装置５Ｙによって現像される。そして、中間転写ベルト８上に１次転写される。クリーニング装置２Ｙは、１次転写工程を経た後の感光体１Ｙ表面に残留したトナーを除去する。また、除電装置は、クリーニング後の感光体１Ｙの残留電荷を除電する。この除電により、感光体１Ｙの表面が初期化されて次の画像形成に備えられる。他のトナー像形成部６Ｍ、６Ｃ、６Ｋにおいても、同様にして感光体１Ｍ、１Ｃ、１Ｋ上にマゼンダ、シアン、黒トナー像が形成される。
【００１４】
中間転写ユニット１５は、中間転写ベルト８の他、４つの１次転写バイアスローラ９Ｙ、９Ｍ、９Ｃ、９Ｋ、中間転写ベルトクリーニング装置１０などを備えている。また、２次転写バックアップローラ１２、クリーニングバックアップローラ１３、テンションローラ１４なども備えている。中間転写ベルト８は、これら３つのローラに張架されながら、少なくとも何れか１つのローラの回転駆動によって図中反時計回りに無端移動せしめられる。１次転写バイアスローラ９Ｙ、９Ｍ、９Ｃ、９Ｋは、このように無端移動せしめられる中間転写ベルト８を感光体１Ｙ、１Ｍ、１Ｃ、１Ｋとの間に挟み込んでそれぞれ１次転写ニップを形成している。これらは中間転写ベルト８の裏面（ループ内周面）にトナーとは逆極性（例えばプラス）の転写バイアスを印加する方式のものである。１次転写バイアスローラ９Ｙ、９Ｍ、９Ｃ、９Ｋを除くローラは、全て電気的に接地されている。中間転写ベルト８は、その無端移動に伴ってイエロー、マゼンダ、シアン、黒用の１次転写ニップを順次通過していく過程で、感光体１Ｙ、１Ｍ、１Ｃ、１Ｋ上のイエロー、マゼンダ、シアン、黒トナー像を重ね合わせるように１次転写される。これにより、中間転写ベルト８上に４色重ね合わせトナー像（以下、４色トナー像という）が形成される。
【００１５】
また、露光装置７の下方には、紙収容カセット２６、給紙ローラ２７、レジストローラ対２８など有する給紙手段が配設されている。また、中間転写ベルト８のトナー像形成部６Ｋよりも下流側で、２次転写バックアップローラ１２に対向するように、２次転写ローラ１９を備えている。ここで、２次転写バックアップローラ１２は、２次転写ローラ１９との間に中間転写ベルト８を挟み込んで２次転写ニップを形成している。さらに、２次転写ニップの上方には、定着ユニット２０、排紙ローラ対２９などを備えている。
【００１６】
紙収容カセット２６は、記録媒体としての転写紙Ｐを複数枚重ねて収納しており、一番上の転写紙Ｐには給紙ローラ２７が当接している。給紙ローラ２７は図示しない駆動手段によって図中反時計回りに回転せしめられ、一番上の転写紙Ｐをレジストローラ対２８のローラ間に向けて給紙する。レジストローラ対２８は、転写紙Ｐを挟み込むべく両ローラを回転駆動するが、挟み込んですぐに回転を一旦停止させる。そして、転写紙Ｐを適切なタイミングで、２次転写ニップに向けて送り出す。このような構成の給紙手段においては、給紙ローラ２７と、タイミングローラ対たるレジストローラ対２８との組合せによって搬送手段が構成されている。この搬送手段は、転写紙Ｐを収容手段たる紙収容カセット２６から２次転写ニップまで搬送するものである。中間転写ベルト８上に形成された４色トナー像は、この２次転写ニップで転写紙Ｐに転写される。２次転写ニップを通過した後の中間転写ベルト８には、転写紙Ｐに転写されなかった転写残トナーが付着している。これは、クリーニング装置１０によってクリーニングされる。
【００１７】
２次転写ニップにおいては、転写紙Ｐは互いに順方向に表面移動する中間転写ベルト８と２次転写ローラ１９との間に挟まれて、レジストローラ対２８側とは反対方向に搬送される。２次転写ニップから送り出された転写紙Ｐは、定着装置２０のローラ間を通過する際に熱と圧力とにより、表面に転写された４色トナー像が定着される。その後、転写紙Ｐは、排紙ローラ対２９のローラ間を経て機外へと排出される。プリンタ本体の上面には、スタック部３０が形成されており、上記排紙ローラ対２９によって機外に排出された転写紙Ｐは、このスタック部３０に順次スタックされる。
【００１８】
図３は本レーザプリンタの電気回路の一部を示すブロック図である。図において制御部１５０はそれぞれ電気的に接続されたトナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋ、光書込みユニット７、給紙カセット２６、レジストローラ対２８、中間転写ユニット１５、反射型フォトセンサ４０などを制御する。また、この制御部１５０は、演算部などを制御するＣＰＵ１５０ａと、データを記憶するＲＡＭ１５０ｂとを備えている。
【００１９】
制御部１５０は、図示しない主電源の投入時や、所定時間経過した後の待機時、所定枚数以上のプリントを出力したあとの待機時など、所定のタイミングで、各トナー像形成部６の像形成性能などの作像性能を試験するように構成されている。具体的には、この所定のタイミングが到来すると、まず、感光体１Ｙ、１Ｍ、１Ｃ、１Ｋを回転しながら一様に帯電せしめる。この帯電については、通常のプリント時における一様な帯電（例えば−７００Ｖ）とは異なり、その電位を負極性側で徐々に絶対値を大きくしていく。そして、上記レーザ光の走査によって基準パターン像用の静電潜像を形成しながら、現像装置５Ｙ、５Ｍ、５Ｃ、５Ｋで現像する。この現像により、各色のバイアス現像パターン像が感光体１Ｙ、１Ｍ、１Ｃ、１Ｋ上に形成される。なお、現像の際、制御部１５０は、それぞれの現像ユニット５の現像ローラ５１に印加される現像バイアス電位の値も負極性側で徐々に絶対値を大きくしていくように制御する。
【００２０】
これら各色のバイアス現像パターン像は、中間転写ベルト８上に重なり合わずに並ぶように転写される。この転写により、中間転写ベルト８上には各色の基準パターン像によって構成されるパターンブロックが形成される。このパターンブロックの中の各基準パターン像の基準像は、中間転写ベルト８の無端移動に伴って反射型フォトセンサ４０との対向位置を通過する際、その光反射量が検知され、電気信号として上記制御部１５０に出力される。制御部１５０は、反射型フォトセンサ４０から順次送られてくるこの出力信号に基づいて、各基準像の光反射率を演算し、濃度パターンデータとしてＲＡＭ１５０ａに格納していく。また、反射型フォトセンサ４０との対向位置を通過した上記パターンブロックは、上記クリーニング装置１０によってクリーニングされる。
【００２１】
図４は上記基準パターン像Ｐ（Ｐｙ、Ｐｍ、Ｐｃ、Ｐｋ）を示す模式図である。基準パターン像Ｐは、互いに間隔１５ｍｍを置いて並ぶ３つの基準像で構成されている。本レーザプリンタにおいて、各基準像１０１は縦１５ｍｍ×横１５ｍｍの大きさで、１５ｍｍの間隙を介して形成される。よって、中間転写ベルト８上の基準パターン像Ｐｙ、Ｐｍ、Ｐｃ、Ｐｋの長さはそれぞれＬ２＝７５ｍｍとなる。基準パターン像Ｐｙ、Ｐｍ、Ｐｃ、Ｐｋは、プリントプロセス時に形成される各色のトナー像とは異なり、上記中間転写ベルト８上に重なり合わずに並ぶように転写される。このような転写により、中間転写ベルト８上には各色の基準パターン像Ｐｙ、Ｐｍ、Ｐｃ、Ｐｋによって構成される１つのパターンブロックＰＢが形成される。
【００２２】
図５は、感光体１の設置ピッチを示す模式図である。図示のように、感光体１Ｙ、１Ｍ、１Ｃ、１ＫはそれぞれＬ１＝９０ｍｍに設定されている。上述のように、基準パターン像Ｐｙ、Ｐｍ、Ｐｃ、Ｐｋの長さＬ２はそれぞれ７５ｍｍであり、感光体１Ｙ、１Ｍ、１Ｃ、１Ｋの設置ピッチＬ１よりも短い。このため、基準パターン像Ｐｙ、Ｐｍ、Ｐｃ、Ｐｋは、それぞれの端部を重ね合わせないように独立して転写されることが可能になる。
【００２３】
図６は中間転写ベルト８上に形成される上記パターンブロックを示す模式図である。中間転写ベルト８上には、４つの基準パターンＰｋ、Ｐｃ、Ｐｍ、ＰｙからなるパターンブロックＰＢが２つ形成される。具体的には、基準パターン像Ｐｋ１、Ｐｃ１、Ｐｍ１、Ｐｙ１から構成されるパターンブロックＰＢ１と、基準パターン像Ｐｋ２、Ｐｃ２、Ｐｍ２、Ｐｙ２から構成されるパターンブロックＰＢ２とが形成される。パターンブロックＰＢ１、ＰＢ２は次のようにして形成される。即ち、上記制御部１５０は、１つ目のパターンブロックＰＢ１内の基準パターン像Ｐｋ１、Ｐｃ１、Ｐｍ１、Ｐｙ１が中間転写ベルト８に転写され終わった時点から、最も上流側の基準パターンＰｙ１が最も下流側の感光体１Ｋの転写ニップを通過し終わるまでの間において、中間転写ベルト８上を基準パターン像が移動する。また、上記制御部１５０は、所定のタイミングを見計らって２つ目のパターンブロックＰＢ２の各基準パターン像Ｐｋ２、Ｐｃ２、Ｐｍ２、Ｐｙ２を感光体１Ｙ、１Ｍ、１Ｃ、１Ｋに形成せしめる。この所定のタイミングとは、具体的には１つ目のパターンブロックＰＢ１の後端（基準パターン像Ｐｙ１）が最も下流側の感光体１Ｋの転写ニップを通過してから更に所定量だけ移動した時点から、パターンブロックＰＢ２の基準パターン像Ｐｋ２、Ｐｃ２、Ｐｍ２、Ｐｙ２が中間転写ベルト８上に転写され始めるタイミングである。
【００２４】
図６において、中間転写ベルト８を備える転写ユニットの図中右上側には、画像検知手段としての反射型フォトセンサ４０が配設されており、中間転写ベルト８上の光反射率に応じた信号を出力するように構成されている。この反射型フォトセンサには、拡散光検出型か正反射光検出型のうち、中間転写ベルト８の表面の反射光量と、上記基準パターン像の反射光量との差を十分な値にし得る方が用いられる。なお、本プリンタでは、反射型フォトセンサ４０として、カラートナーの高濃度部を検知できる点で望ましい拡散光検出型を用いた。中間転写ベルト８上の各基準パターン像は無端移動に伴って移動して反射型フォトセンサ４０に検知された後、転写ユニットの中間転写ベルトクリーニング装置１０にて除去される。
【００２５】
反射型フォトセンサ４０は、１つ目のパターンブロックＰＢ１の先端から後端にかけて、基準パターン像Ｐｋ１，Ｐｃ１，Ｐｍ１，Ｐｙ１を構成する各基準像１０１からの反射光量を次のような順序で検知する。即ち、基準パターン像Ｐｋ１の３個の基準像１０１、基準像Ｐｃ１の３個の基準像１０１、基準像Ｐｍ１の３個の基準像１０１、基準像Ｐｙ１の３個の基準像１０１、という順序で検知する。この際、各基準像１０１の光反射量に応じた電圧信号を上記制御部１５０に順次出力する。上記制御部１５０は、反射型フォトセンサ４０から順次送られてくるこの電圧信号に基づいて、各基準像１０１の画像濃度を順次演算してＲＡＭ１５０ｂに格納していく。また、反射型フォトセンサ４０には、２つ目のパターンブロックＰＢ２の先端から後端にかけて、基準パターン像Ｐｋ２、Ｐｃ２、Ｐｍ２、Ｐｙ２を構成する各基準像１０１からの反射光量を、パターンブロックＰＢ１と同様の順序で検知する。制御部１５０は、１つ目のパターンブロックＰＢ１のときと同様に、反射型フォトセンサ４０から順次送られてくる電圧信号に基づいて、各基準像１０１の画像濃度を順次演算してＲＡＭ１５０ｂに格納していく。パターンブロックＰＢ１、ＰＢ２にはそれぞれ４つの基準パターン像Ｐｙ、Ｐｍ、Ｐｃ、Ｐｋが含まれ、更に、これら基準パターン像にはそれぞれ３個の基準像１０１が含まれるため、各色（Ｙ，Ｍ，Ｃ，Ｋ）についてそれぞれ３×２＝６個の基準像１０１の画像濃度が検知されることになる。
【００２６】
各色において、それぞれ６個の基準像１０１は、次の表１に示される作像条件で感光体１Ｙ、１Ｍ、１Ｃ、１Ｋ上に形成される。なお、上記レーザ光の強度については、感光体のドラム帯電電位にかかわらず、基準像１０１用の静電潜像を−２０Ｖまで減衰せしめ得るような強度とする。
【表１】

【００２７】
表１において、（１）、（２）、（３）、（４）、（５）、（６）は、パターンブロックＰＢ１の先端からパターンブロックＰＢ２の後端にかけて１番目、２番目、３番目、４番目、５番目、６番目に形成される基準像１０１を示している。すなわち、（１）〜（３）の基準像１０１はパターンブロックＰＢ１内に存在し、（４）〜（６）の基準像１０１はパターンブロックＰＢ２内に存在している。表１に示すように、本レーザプリンタは、各トナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋにおいて、それぞれドラム帯電電位と現像バイアスとをそれぞれ徐々に高い値に切り替えながら（１）から（６）の基準像１０１を形成する。これら６個の基準像１０１は、後に形成されるものほど、高い現像ポテンシャル（静電潜像の電位と現像バイアスとの差）で現像されるため、画像濃度が高くなる。
【００２８】
表１に示した各現像ポテンシャルと、（１）〜（６）の基準像１０１の画像濃度との関係は、例えば図７に示すグラフのようになる。即ち、現像ポテンシャルと画像濃度（トナー付着量）とには正の直線グラフを示す関数（ｙ＝ａｘ＋ｂ）を用いれば、表２に示すように、上記ドラム帯電電位テーブルでは１５通りの現像ガンマａと、これに適切なドラム帯電電位とが関連付けられている。
【表２】

【００２９】
上記制御部１５０は、トナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋについて、それぞれ上記作像条件テーブルの中から、現像ガンマａに最も近いテーブルを選びこれに関連付けられたドラム帯電電位を特定する。特定したドラム帯電電位については、イエロー、マゼンダ、シアン、黒用の補正ドラム帯電電位としてＲＡＭ１５０ｂに格納する。その後、トナー像形成部６Ｙ、６Ｍ、６Ｃ、６Ｋについて、それぞれ推定露光電位（本実施例の場合−２０Ｖ）からＲＡＭ１５０ｂ格納されている所望の画像濃度を得るための現像ポテンシャルを引いた値をイエロー、マゼンダ、シアン、黒用の補正現像バイアスとしてＲＡＭ１５０ｂに格納する。このような補正により、プリントアウトプロセス時におけるトナー像形成手段６Ｙ、６Ｍ、６Ｃ、６Ｋの作像条件が、それぞれ所望の画像濃度のトナー像を形成し得る条件であり、かつハイライト部も安定するように補正される。上記補正を定期的に実行することで、経時的にもハイライト部が安定するため現像剤が劣化しても地肌部の汚れが起こらず画像品質が安定する。
【００３０】
以上、実施形態で述べた様に、上述の制御を行うことでベタ部のみならずハイライト部においても安定した画像濃度を長期に渡って得ることができるという優れた効果がある。
また、目標現像バイアス電位は現像ガンマと現像開始電圧とから最大現像量を得るための現像ポテンシャルを求め、現像ポテンシャルを得るように目標帯電電位及び目標露光電位に基づいて定める。これにより、現像特性に適した目標帯電電位及び目標露光電位定めることができる。
また、目標帯電電位及び目標露光電位は現像ガンマと帯電電位及び露光部電位との関係を予め定めたテーブルに基づいてを定める。これにより、現像ガンマに適した目標帯電電位及び目標露光電位を選択することができる。
また、トナー付着量を検出する検出手段として乱反射型フォトセンサを用いる。
また、感光体と現像装置とを一体的に構成した画像形成ユニットを用いる画像形成装置において上記制御をおこなう。ここで、画像地肌部の汚れは現像剤の経時変化で発生しやすくなる傾向にあるため、現像装置と像担持体を一体化したプロセスカートリッジを用いる場合は、現像装置のみを交換できず、像担持体も交換せざるを得ないためコストアップにつながっていた。そこで、このような制御をおこなうことで、長期に渡って安定した画像が得られるようになり、画像形成ユニットの長寿命化を図ることができる。
【００３１】
【発明の効果】
請求項１乃至９の発明によれば、ベタ部のみならずハイライト部においても安定した画像濃度を長期に渡って得ることができるという優れた効果がある。
【図面の簡単な説明】
【図１】本実施形態に係る画像形成装置の概略構成図。
【図２】画像形成部の概略構成図。
【図３】本実施形態に係る画像形成装置の制御部のブロック図。
【図４】基準パターン像を示す模式図。
【図５】感光体の設置ピッチを示す模式図。
【図６】中間転写ベルト上に形成されるパターンブロックを示す模式図。
【図７】現像ポテンシャルと画像濃度（トナー付着量）に関する直線近似式。
【図８】現像能力のバラツキをしめすグラフ。
【図９】露光量と潜像電位の関係をしめす明するグラフ。
【図１０】露光量とトナー付着量の関係をしめすグラフ。
【図１１】感光体に１ドットの光書き込みをおこなった場合の潜像電位分布をしめす図。
【図１２】制御のフローチャート。
【符号の説明】
１感光体
２クリーニング装置
４帯電装置
５現像装置
６トナー像形成部
７露光装置
８中間転写ベルト
９１次転写バイアスローラ
１０中間転写ベルトクリーニング装置
１２２次転写バックアップローラ
１３クリーニングバックアップローラ
１４テンションローラ
１５中間転写ユニット
１９２次転写ローラ
２０定着ユニット
２６紙収容カセット
２７給紙ローラ
２８レジストローラ対
２９排紙ローラ対
４０反射型フォトセンサ
１００プリンタ
１５０制御部
１５０ａＣＰＵ
１５０ｂＲＡＭ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, and a printer, and an image forming method.
[0002]
[Prior art]
Conventionally, in an electrophotographic image forming apparatus, in order to stabilize environmental quality and image quality over time, a toner image of a reference pattern is created on an image carrier, and the toner adhesion amount is detected. A technique for controlling image forming conditions based on this is known.
[0003]
As an example, Patent Document 1 discloses an image forming apparatus that performs the following control. In the following description, a negative-positive development system is used in which a negatively charged photoreceptor is used as an image carrier and toner is attached to an exposed portion to form an image portion. First, a patch pattern as a reference pattern is formed on the photosensitive member while gradually changing the charging potential and the developing bias potential of the photosensitive member. Specifically, the charging potential is different from the uniform charging potential during normal image formation, and the value is gradually increased on the negative polarity side. Then, the electrostatic potential image for the reference pattern is formed by attenuating the charging potential by exposure with laser light or the like. Next, the electrostatic latent image is developed to form a toner image having a reference pattern, and the toner adhesion amount is detected by a sensor. The value of the development bias potential in this development is also gradually increased on the negative polarity side. Then, a linear approximation expression relating to the relationship between the development potential and the toner adhesion amount is calculated from the development potential obtained from the difference between the exposure portion potential and the development bias potential and the toner adhesion amount detected by the sensor. By using this linear approximation formula (y = ax + b), it is possible to calculate the development potential for obtaining a desired image density (toner adhesion amount). After that, based on the calculated development potential, the development bias potential and the charging potential are determined so as to have a constant potential difference from this, and these are set as target potentials, and each potential is controlled to match the target potential. Normal image formation is performed by the charged potential and the developing bias potential.
[0004]
Further, the exposure with the laser beam or the like may have such an intensity that the exposure portion potential is attenuated to, for example, −20 [V] regardless of the charging potential. In this case, since the exposed portion potential is constant, the linear approximation formula relating to the relationship between the developing bias potential and the toner adhesion amount is directly calculated without obtaining the developing potential from the difference between the exposed portion potential and the developing bias potential. be able to. By using this linear approximation formula, it is possible to directly calculate the developing bias potential at which a desired image density (toner adhesion amount) is obtained.
[0005]
[Patent Document 1]
JP 2002-116616 A
[0006]
[Problems to be solved by the invention]
However, the above-described image forming apparatus has a disadvantage that the toner adhesion amount (image density) in the solid portion is stable, but an error is likely to occur in the toner adhesion amount in the highlight portion (halftone), which is not stable. This is considered to be due to the following reason.
FIG. 11 is a diagram showing a latent image potential distribution when 1 dot optical writing is performed on the photosensitive member. In general, the latent image characteristic of a photoconductor is defined by a charging potential and an exposure portion potential. However, when viewed with an actual dot, as shown in FIG. 11, the charging potential is reduced to the exposure portion potential. There is an intermediate potential between them. The shape of the intermediate portion may be different even when the charging potential and the exposure portion potential are the same. Here, in the image forming apparatus, since the correction is performed based on the relationship between the developing potential obtained from the difference between the exposed portion potential and the developing bias potential and the toner adhesion amount, the solid portion corresponding to the exposed portion potential. The toner adhesion amount is stable. However, since the highlight portion corresponding to the potential of the intermediate portion is not particularly taken into consideration, if there is a difference in shape in the intermediate portion, a difference occurs in the toner adhesion amount of the highlight portion.
Further, if the variation in the toner adhesion amount in the highlight portion becomes large, there is a risk that the background of the image will be stained, and the image quality may be greatly reduced.
[0007]
The present invention has been made in view of the above background, and an object of the present invention is to provide an image forming apparatus and an image forming apparatus capable of obtaining a stable image density over a long period not only in a solid portion but also in a highlight portion. Is to provide a method.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, according to the first aspect of the present invention, the image carrier is uniformly charged and then exposed to form an electrostatic latent image, and the electrostatic latent image on the image carrier is transferred from the developing device. An image is formed by attaching toner and developing. An electrostatic latent image of a plurality of reference patterns is formed on the image carrier, the reference pattern is developed by the developing device, and the developed image is developed. In an image forming method for detecting a toner adhesion amount of a reference pattern and correcting an image forming condition based on the detection result of the toner adhesion amount, from the latent image potentials of the plurality of reference patterns and the detection result of the toner adhesion amount A linear approximation formula of the development potential and the toner adhesion amount is obtained, a development gamma and a development start voltage that are the slopes of the linear approximation formula are calculated, a target charging potential and a target exposure portion potential are determined from the development gamma, Target charging potential and target exposure part potential It is characterized in that to correct the image forming condition from the developing gamma and developing start voltage to a targeted development bias potential.
According to a second aspect of the present invention, in the image forming method of the first aspect, the target development bias potential is obtained as a development potential for obtaining a maximum development amount from the development gamma and the development start voltage. It is determined based on the target charging potential and the target exposure potential.
According to a third aspect of the present invention, in the image forming method according to the first or second aspect, the target charging potential and the target exposure potential are based on a table in which a relationship between the development gamma, the charging potential, and the exposure portion potential is determined in advance. It is characterized by determining.
According to a fourth aspect of the present invention, in the image forming method of the first, second, or third aspect, an irregular reflection type photosensor is used as the detecting means for detecting the toner adhesion amount.
According to a fifth aspect of the present invention, there is provided an image carrier that carries an electrostatic latent image, a latent image forming unit that forms an electrostatic latent image on the image carrier, and an electrostatic latent image on the image carrier. A developing device for forming a toner image; patch pattern writing means for forming an electrostatic latent image of a plurality of reference patch patterns on the image carrier; and detecting an image density of the patch pattern developed by the developing device In an image forming apparatus comprising: an image density detecting unit; and an image forming condition correcting unit that corrects an image forming condition based on a detection result by the image density detecting unit, a plurality of patch patterns formed on the image carrier Development that calculates a developmental approximation gamma and development start voltage, which are the slopes of the linear approximation formula, by obtaining a linear approximation formula related to the development potential and the toner adhesion amount from the latent image potential and the detection result of the image density detection means. Characteristic calculation means, A latent image characteristic calculating means for determining a target charging potential and a target exposure potential based on the development gamma calculated by the characteristic calculating means, a target charging potential and a target exposure potential determined by the latent image characteristic calculating means, and a calculation by the developing characteristic calculating means. Development bias potential calculating means for determining a target development bias potential from the developed gamma and the development start voltage.
According to a sixth aspect of the present invention, in the image forming apparatus of the fifth aspect, the development bias potential calculating means obtains a development potential for obtaining a maximum development amount from the development gamma and the development start voltage calculated by the development characteristic calculating means. The target developing bias potential is determined based on the target charging potential and the target exposure potential determined by the latent image characteristic calculating means so as to obtain the developing potential.
According to a seventh aspect of the present invention, in the image forming apparatus of the fifth or sixth aspect, the target charging potential and the target exposure potential calculated by the latent image characteristic calculating unit are the development gamma and the charging potential calculated by the developing characteristic calculating unit. And the relationship with the exposure portion potential is determined based on a predetermined table.
According to an eighth aspect of the present invention, in the image forming apparatus according to the fifth, sixth or seventh aspect, an irregular reflection type photosensor is used as the image density detecting means.
According to a ninth aspect of the present invention, in the image forming apparatus according to the fifth, sixth, seventh or eighth aspect, an image forming unit in which the image carrier and the developing device are integrally formed is used. It is.
[0009]
According to the first to ninth aspects of the present invention, a linear approximation expression relating to the developing potential and the toner adhesion amount is obtained from the latent image potentials of a plurality of reference patterns and the detection result of the toner adhesion amount, and development having the inclination is obtained from the linear approximation expression. Gamma and development start voltage are calculated. Then, a target charging potential and a target exposure potential are determined based on the calculated development gamma. Further, the development bias potential is determined based on the target charging potential, the target exposure potential, the development gamma, and the development start voltage.
Hereinafter, the concept of control and the specific control flow in the present invention will be described in detail based on FIG. 8, FIG. 9, and FIG. FIG. 8 is a graph showing the relationship between the developing potential obtained from the exposed portion potential and the developing bias and the toner adhesion amount. FIG. 9 is a graph showing the relationship between the exposure amount of the image carrier and the latent image potential. FIG. 10 is a graph showing the relationship between the exposure amount of the image carrier and the toner adhesion amount.
For example, assuming that the relationship between the development potential and the toner adhesion amount is the standard developing ability (1) shown in FIG. 8, and the latent image characteristic is the standard latent image characteristic 1 shown in FIG. The relationship between the amount and the toner adhesion amount is a line of developing ability (1) + latent image characteristic 1 shown in FIG. 10 (hereinafter referred to as a standard line). However, as shown in FIG. 8, if there is a variation such as the developing ability (2) and (3) with respect to the standard developing ability (1), the relationship between the exposure amount of the image carrier and the toner adhesion amount. Is a line of developing ability (2) + latent image characteristic 1 and developing ability (3) + latent image characteristic 1 shown in FIG. Even when the developing ability is changed in this way, the image forming conditions are corrected so that the line of the relationship between the exposure amount of the image carrier and the toner adhesion amount becomes a standard line, so that A standard toner adhesion amount can always be obtained for both parts.
First, the case where the relationship between the development potential and the toner adhesion amount becomes the development capability (2) will be described. The development approximation voltage Vk (intercept with the X axis) and the development gamma (slope) are different in the linear approximation formula of the development capability (2) compared to the linear approximation formula of the development capability (1). When the development gamma is small as described above, if the standard latent image characteristic 1 is used as it is as the latent image characteristic, the development capability (2) + latent image characteristic 1 line whose inclination is smaller than the standard line as shown in FIG. It becomes. Therefore, based on the fact that the slope of the development gamma is reduced, the target charging potential and the target exposure portion potential are set so that the potential slope with respect to the exposure amount shown in FIG. Determine. This is because the slope of the latent image characteristics is steep, because the slope of the line between the exposure amount of the image carrier and the toner adhesion amount is smaller than the standard line because the slope of the development gamma is small. Is corrected so that the inclination is the same as the standard line. Further, the target development bias potential is determined so as to obtain a desired toner adhesion amount from the determined target charging potential, target exposure portion potential, and the relationship between the development potential and the toner adhesion amount.
Next, a case where the relationship between the development potential and the toner adhesion amount becomes the development capability (3) will be described. The development approximation voltage Vk (intercept with the X axis) is different from the linear approximation formula of the development capability {circle around (3)} and the development gamma (slope) is the same as that of the development capability {circle around (1)}. In this way, when the development start voltage Vk is different, if the standard latent image characteristic 1 is used as it is as the latent image characteristic, the development gamma is equal and the inclination is the same as that of the standard line and the development start voltage Vk is different. The development amount {circle around (3)} + latent image characteristic 1 is a line in which the toner adhesion amount at the potential is different and is shifted in parallel. Therefore, the standard latent image characteristic 1 is used as it is based on the same development gamma. Specifically, the same target charging potential and target exposure portion potential as before are used. Next, the target development bias potential is changed so as to reduce the development potential based on the decrease in the development start voltage Vk. This is achieved by changing the target development bias potential to change the parallel shift of the relationship between the exposure amount of the image carrier and the toner adhesion amount due to the development start voltage Vk becoming smaller and the development capability becoming higher. To do.
If both the development gamma and the development start voltage Vk are different from the development capability (1), both the correction of the development capability (2) and the correction of the development capability (3) may be performed.
As a specific control flow, first, a reference pattern is formed on the image carrier while gradually changing the charging potential and the developing bias potential of the image carrier, and the toner adhesion amount is detected by a sensor. From the development potential obtained from the difference between the exposed portion potential of the image carrier and the development bias potential and the toner adhesion amount detected by the sensor, a linear approximation formula relating to the relationship between the development potential and the toner adhesion amount is obtained. The development gamma a and the development start voltage b, which are the slopes of a function (y = ax + b) representing this linear approximation formula, are calculated. The development gamma a is associated with an appropriate drum charging potential and exposure portion potential in a predetermined table. The table closest to the development gamma a is selected and the charging potential associated therewith is selected. The exposure portion potential is set as the target exposure portion potential as the target charging potential. Further, the target developing bias potential is calculated from the target charging potential and the target exposure portion potential and the developing potential for obtaining a desired image density. By performing normal image formation at such a target potential, a toner image having a desired image density can be formed over the entire area from the solid portion to the highlight portion.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an electrophotographic printer (hereinafter simply referred to as a printer) will be described as an example of an embodiment of an image forming apparatus to which the present invention is applied. First, the basic configuration of the printer will be described.
FIG. 1 is a schematic configuration diagram of the printer. The printer 100 includes four sets of process cartridge-like toner

image forming units

6Y, 6M, 6C for forming images of each color of yellow (Y), magenta (M), cyan (C), and black (K). This is a tandem color printer equipped with 6K. Hereinafter, the subscripts Y, M, C, and K of the respective codes indicate yellow, magenta, cyan, and black, respectively. Above the four sets of toner

image forming portions

6Y, 6M, 6C, and 6K, an intermediate transfer unit 15 that moves endlessly while an intermediate transfer belt 8 as an intermediate transfer member described later is stretched is disposed. Four sets of toner

image forming portions

6Y, 6M, 6C, and 6K are sequentially arranged from the upstream side in the moving direction of the intermediate transfer belt 8. Further, these toner

image forming units

6Y, 6M, 6C, and 6K are respectively provided with photoreceptors 1Y, 1M, 1C, and 1K on the drum as latent image carriers.
[0011]
An exposure device 7 is disposed below the toner

image forming units

6Y, 6M, 6C, and 6K. The exposure device 7 includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and irradiates the surface of each of the photoreceptors 1Y, 1M, 1C, and 1K while scanning with laser light based on image data. By this light irradiation, electrostatic latent images of yellow, magenta, cyan, and black are formed on the photoreceptors 1Y, 1M, 1C, and 1K.
[0012]
The toner

image forming units

6Y, 6M, 6C, and 6K use yellow, magenta, cyan, and black toners of different colors as the developer, but the other configurations are the same. A description will be given using the formation unit 1Y as a representative example.
[0013]
FIG. 2 is a schematic configuration diagram of the yellow toner image forming unit 1Y. The toner image forming unit 1Y includes a drum-shaped photoconductor 1Y, a cleaning device 2Y, a charge eliminating device, a charging device 4Y, a developing device 5Y, and the like. The toner image forming unit 6Y can be attached to and detached from the printer 100 main body, and the consumable parts can be exchanged at the same time when the life is reached. The charging device 4Y uniformly charges (for example, −700 V) the surface of the photoreceptor 1Y that is driven to rotate clockwise in the drawing by a driving unit (not shown). The surface of the uniformly charged photoreceptor 1Y is irradiated with laser light L from the exposure device 7 to form an electrostatic latent image for a yellow image. This electrostatic latent image is developed by the developing device 5Y using yellow toner. Then, primary transfer is performed on the intermediate transfer belt 8. The cleaning device 2Y removes the toner remaining on the surface of the photoreceptor 1Y after the primary transfer process. Further, the static eliminator neutralizes residual charges on the photoreceptor 1Y after cleaning. By this charge removal, the surface of the photoreceptor 1Y is initialized and prepared for the next image formation. In the other toner

image forming units

6M, 6C, and 6K, magenta, cyan, and black toner images are similarly formed on the photoreceptors 1M, 1C, and 1K.
[0014]
In addition to the intermediate transfer belt 8, the intermediate transfer unit 15 includes four primary transfer bias rollers 9Y, 9M, 9C, and 9K, an intermediate transfer belt cleaning device 10, and the like. A secondary transfer backup roller 12, a cleaning backup roller 13, a tension roller 14 and the like are also provided. The intermediate transfer belt 8 is endlessly moved in the counterclockwise direction in the figure by the rotational drive of at least one of the rollers while being stretched around these three rollers. The primary transfer bias rollers 9Y, 9M, 9C, and 9K sandwich the intermediate transfer belt 8 that is moved endlessly in this manner from the photoreceptors 1Y, 1M, 1C, and 1K to form primary transfer nips, respectively. Yes. In these methods, a transfer bias having a polarity opposite to that of toner (for example, plus) is applied to the back surface (loop inner peripheral surface) of the intermediate transfer belt 8. All the rollers except the primary transfer bias rollers 9Y, 9M, 9C, and 9K are electrically grounded. The intermediate transfer belt 8 sequentially passes through the primary transfer nips for yellow, magenta, cyan, and black along with its endless movement, and yellow, magenta, and cyan on the photoreceptors 1Y, 1M, 1C, and 1K. The black toner image is primarily transferred so as to overlap. As a result, a four-color superimposed toner image (hereinafter referred to as a four-color toner image) is formed on the intermediate transfer belt 8.
[0015]
Further, below the exposure device 7, paper supply means having a paper storage cassette 26, a paper supply roller 27, a registration roller pair 28 and the like are disposed. Further, a secondary transfer roller 19 is provided so as to face the secondary transfer backup roller 12 on the downstream side of the toner image forming portion 6K of the intermediate transfer belt 8. Here, the secondary transfer backup roller 12 sandwiches the intermediate transfer belt 8 with the secondary transfer roller 19 to form a secondary transfer nip. Further, a fixing unit 20 and a paper discharge roller pair 29 are provided above the secondary transfer nip.
[0016]
The paper storage cassette 26 stores a plurality of transfer papers P as recording media, and a paper feed roller 27 is in contact with the uppermost transfer paper P. The sheet feeding roller 27 is rotated counterclockwise in the drawing by a driving unit (not shown), and feeds the uppermost transfer sheet P toward the gap between the registration roller pair 28. The registration roller pair 28 rotationally drives both rollers to sandwich the transfer paper P, but temporarily stops rotating immediately after sandwiching. Then, the transfer paper P is sent out toward the secondary transfer nip at an appropriate timing. In the sheet feeding means having such a configuration, the conveying means is configured by a combination of the sheet feeding roller 27 and the registration roller pair 28 as the timing roller. This transport means transports the transfer paper P from the paper storage cassette 26 serving as the storage means to the secondary transfer nip. The four-color toner image formed on the intermediate transfer belt 8 is transferred to the transfer paper P at the secondary transfer nip. Untransferred toner that has not been transferred onto the transfer paper P adheres to the intermediate transfer belt 8 after passing through the secondary transfer nip. This is cleaned by the cleaning device 10.
[0017]
In the secondary transfer nip, the transfer paper P is sandwiched between the intermediate transfer belt 8 and the secondary transfer roller 19 whose surfaces move in the forward direction, and is conveyed in the direction opposite to the registration roller pair 28 side. When the transfer paper P sent out from the secondary transfer nip passes between the rollers of the fixing device 20, the four-color toner image transferred to the surface is fixed by heat and pressure. Thereafter, the transfer paper P is discharged out of the apparatus through a pair of paper discharge rollers 29. A stack unit 30 is formed on the upper surface of the printer body, and the transfer paper P discharged outside the apparatus by the discharge roller pair 29 is sequentially stacked on the stack unit 30.
[0018]
FIG. 3 is a block diagram showing a part of the electric circuit of the laser printer. In the figure, the control unit 150 is electrically connected to toner

image forming units

6Y, 6M, 6C, and 6K, an optical writing unit 7, a paper feeding cassette 26, a registration roller pair 28, an intermediate transfer unit 15, and a reflective photosensor 40. Control etc. The control unit 150 includes a CPU 150a that controls the arithmetic unit and the like, and a RAM 150b that stores data.
[0019]
The controller 150 controls the image of each toner image forming unit 6 at a predetermined timing such as when a main power supply (not shown) is turned on, when waiting after a predetermined time has elapsed, or when waiting for a predetermined number of prints to be output. It is configured to test imaging performance such as formation performance. Specifically, when this predetermined timing comes, first, the photoreceptors 1Y, 1M, 1C, and 1K are uniformly charged while rotating. This charging is different from the uniform charging (for example, −700 V) during normal printing, and the absolute value of the potential is gradually increased on the negative polarity side. Then, development is performed by the developing devices 5Y, 5M, 5C, and 5K while forming an electrostatic latent image for a reference pattern image by scanning with the laser light. By this development, bias development pattern images of the respective colors are formed on the photoreceptors 1Y, 1M, 1C, and 1K. During development, the control unit 150 controls the developing bias potential applied to the developing roller 51 of each developing unit 5 so that the absolute value is gradually increased on the negative polarity side.
[0020]
The bias development pattern images of these colors are transferred so as to be arranged on the intermediate transfer belt 8 without overlapping. By this transfer, a pattern block constituted by the reference pattern image of each color is formed on the intermediate transfer belt 8. When the reference image of each reference pattern image in the pattern block passes through the position facing the reflective photosensor 40 as the intermediate transfer belt 8 moves endlessly, the amount of reflected light is detected and an electric signal is detected. It is output to the control unit 150. The control unit 150 calculates the light reflectance of each reference image based on the output signals sequentially sent from the reflection type photosensor 40, and stores it in the RAM 150a as density pattern data. The pattern block that has passed through the position facing the reflective photosensor 40 is cleaned by the cleaning device 10.
[0021]
FIG. 4 is a schematic diagram showing the reference pattern image P (Py, Pm, Pc, Pk). The reference pattern image P is composed of three reference images arranged at an interval of 15 mm from each other. In this laser printer, each reference image 101 is 15 mm long × 15 mm wide and is formed through a 15 mm gap. Therefore, the lengths of the reference pattern images Py, Pm, Pc, and Pk on the intermediate transfer belt 8 are L2 = 75 mm, respectively. The reference pattern images Py, Pm, Pc, and Pk are transferred so as to be arranged on the intermediate transfer belt 8 without overlapping, unlike the toner images of the respective colors formed during the printing process. By such transfer, one pattern block PB composed of the reference pattern images Py, Pm, Pc, and Pk of each color is formed on the intermediate transfer belt 8.
[0022]
FIG. 5 is a schematic diagram showing the installation pitch of the photoreceptor 1. As illustrated, the photoreceptors 1Y, 1M, 1C, and 1K are set to L1 = 90 mm, respectively. As described above, the length L2 of each of the reference pattern images Py, Pm, Pc, and Pk is 75 mm, which is shorter than the installation pitch L1 of the photoreceptors 1Y, 1M, 1C, and 1K. For this reason, the reference pattern images Py, Pm, Pc, and Pk can be independently transferred so as not to overlap the respective end portions.
[0023]
FIG. 6 is a schematic diagram showing the pattern block formed on the intermediate transfer belt 8. On the intermediate transfer belt 8, two pattern blocks PB composed of four reference patterns Pk, Pc, Pm, and Py are formed. Specifically, a pattern block PB1 composed of the reference pattern images Pk1, Pc1, Pm1, and Py1 and a pattern block PB2 composed of the reference pattern images Pk2, Pc2, Pm2, and Py2 are formed. The pattern blocks PB1 and PB2 are formed as follows. That is, the control unit 150 determines that the most upstream reference pattern Py1 is the most downstream from the time when the reference pattern images Pk1, Pc1, Pm1, and Py1 in the first pattern block PB1 have been transferred to the intermediate transfer belt 8. The reference pattern image moves on the intermediate transfer belt 8 until it passes through the transfer nip of the side photoreceptor 1K. Further, the control unit 150 forms the reference pattern images Pk2, Pc2, Pm2, and Py2 of the second pattern block PB2 on the photoconductors 1Y, 1M, 1C, and 1K at a predetermined timing. Specifically, the predetermined timing is a time point when the rear end (reference pattern image Py1) of the first pattern block PB1 moves further by a predetermined amount after passing through the transfer nip of the most downstream photoconductor 1K. The reference pattern images Pk2, Pc2, Pm2, and Py2 of the pattern block PB2 start to be transferred onto the intermediate transfer belt 8.
[0024]
In FIG. 6, a reflection type photosensor 40 as an image detecting unit is disposed on the upper right side of the transfer unit including the intermediate transfer belt 8, and a signal corresponding to the light reflectance on the intermediate transfer belt 8. Is configured to output. In this reflection type photosensor, of the diffused light detection type or the regular reflection light detection type, the difference between the reflected light amount on the surface of the intermediate transfer belt 8 and the reflected light amount of the reference pattern image can be set to a sufficient value. Used. In the present printer, a diffused light detection type is used as the reflective photosensor 40, which is desirable in that it can detect a high density portion of color toner. Each reference pattern image on the intermediate transfer belt 8 is moved with endless movement and detected by the reflective photosensor 40, and then removed by the intermediate transfer belt cleaning device 10 of the transfer unit.
[0025]
The reflective photosensor 40 detects the amount of light reflected from each reference image 101 constituting the reference pattern images Pk1, Pc1, Pm1, and Py1 from the front end to the rear end of the first pattern block PB1 in the following order. To do. That is, in order of three reference images 101 of the reference pattern image Pk1, three reference images 101 of the reference image Pc1, three reference images 101 of the reference image Pm1, and three reference images 101 of the reference image Py1. Detect. At this time, a voltage signal corresponding to the light reflection amount of each reference image 101 is sequentially output to the control unit 150. The control unit 150 sequentially calculates the image density of each reference image 101 based on the voltage signals sequentially sent from the reflective photosensor 40 and stores them in the RAM 150b. Further, the reflection type photosensor 40 receives the amount of light reflected from each reference image 101 constituting the reference pattern images Pk2, Pc2, Pm2, and Py2 from the front end to the rear end of the second pattern block PB2. Detect in the same order as. As in the case of the first pattern block PB1, the control unit 150 sequentially calculates the image density of each reference image 101 based on the voltage signal sequentially sent from the reflective photosensor 40 and stores it in the RAM 150b. I will do it. Each of the pattern blocks PB1 and PB2 includes four reference pattern images Py, Pm, Pc, and Pk, and each of these reference pattern images includes three reference images 101, so that each color (Y, M, The image density of 3 × 2 = 6 reference images 101 is detected for each of C, K).
[0026]
For each color, six reference images 101 are formed on the photoreceptors 1Y, 1M, 1C, and 1K under the image forming conditions shown in Table 1 below. The intensity of the laser beam is set so that the electrostatic latent image for the reference image 101 can be attenuated to −20V regardless of the drum charging potential of the photosensitive member.
[Table 1]

[0027]
In Table 1, (1), (2), (3), (4), (5), and (6) are the first, second, and third from the front end of the pattern block PB1 to the rear end of the pattern block PB2. Reference images 101 formed fourth, fifth, and sixth are shown. That is, the reference images 101 of (1) to (3) exist in the pattern block PB1, and the reference images 101 of (4) to (6) exist in the pattern block PB2. As shown in Table 1, in this laser printer, in each of the toner

image forming units

6Y, 6M, 6C, and 6K, the drum charging potential and the developing bias are gradually switched to high values, respectively (1) to (6). The reference image 101 is formed. As these six reference images 101 are formed later, they are developed with a higher development potential (difference between the electrostatic latent image potential and the development bias), so that the image density becomes higher.
[0028]
The relationship between each development potential shown in Table 1 and the image density of the reference image 101 of (1) to (6) is as shown in the graph of FIG. 7, for example. In other words, if a function (y = ax + b) showing a positive linear graph is used for the development potential and the image density (toner adhesion amount), as shown in Table 2, the development potential of the drum charging potential table is fifteen. And an appropriate drum charging potential.
[Table 2]

[0029]
The control unit 150 selects the table closest to the development gamma a from the image forming condition tables for the toner

image forming units

6Y, 6M, 6C, and 6K, and specifies the drum charging potential associated therewith. The specified drum charging potential is stored in the RAM 150b as correction drum charging potentials for yellow, magenta, cyan, and black. Thereafter, for toner

image forming units

6Y, 6M, 6C, and 6K, yellow is obtained by subtracting the development potential for obtaining a desired image density stored in the RAM 150b from the estimated exposure potential (-20V in the present embodiment). , Magenta, cyan, and black are stored in the RAM 150b as correction development biases. By such correction, the image forming conditions of the toner

image forming units

6Y, 6M, 6C, and 6K during the printout process are conditions that can form toner images having desired image densities, respectively, and the highlight portion is also stable. To be corrected. By periodically executing the above correction, the highlight portion is stabilized over time, so that even if the developer is deteriorated, the background portion is not stained and the image quality is stabilized.
[0030]
As described above, as described in the embodiment, by performing the above-described control, there is an excellent effect that a stable image density can be obtained not only in the solid part but also in the highlight part over a long period of time.
The target development bias potential is determined based on the target charging potential and the target exposure potential so as to obtain the development potential for obtaining the maximum development amount from the development gamma and the development start voltage. Thereby, the target charging potential and the target exposure potential suitable for the development characteristics can be determined.
The target charging potential and the target exposure potential are determined based on a table in which the relationship between the development gamma, the charging potential, and the exposure portion potential is determined in advance. Thereby, a target charging potential and a target exposure potential suitable for development gamma can be selected.
Further, a irregular reflection type photosensor is used as a detection means for detecting the toner adhesion amount.
Further, the above control is performed in an image forming apparatus using an image forming unit in which a photosensitive member and a developing device are integrally formed. Here, since the stain on the image background tends to occur due to the change of the developer with time, when using a process cartridge in which the developing device and the image carrier are integrated, it is not possible to replace only the developing device, Since the carrier had to be replaced, the cost was increased. Therefore, by performing such control, a stable image can be obtained over a long period of time, and the life of the image forming unit can be extended.
[0031]
【The invention's effect】
According to the first to ninth aspects of the invention, there is an excellent effect that a stable image density can be obtained over a long period of time not only in the solid portion but also in the highlight portion.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment.
FIG. 2 is a schematic configuration diagram of an image forming unit.
FIG. 3 is a block diagram of a control unit of the image forming apparatus according to the present embodiment.
FIG. 4 is a schematic diagram showing a reference pattern image.
FIG. 5 is a schematic diagram showing the installation pitch of a photoconductor.
FIG. 6 is a schematic diagram showing a pattern block formed on an intermediate transfer belt.
FIG. 7 is a linear approximation formula regarding development potential and image density (toner adhesion amount).
FIG. 8 is a graph showing variations in developing ability.
FIG. 9 is a bright graph showing the relationship between the exposure amount and the latent image potential.
FIG. 10 is a graph showing the relationship between the exposure amount and the toner adhesion amount.
FIG. 11 is a diagram showing a latent image potential distribution when 1 dot optical writing is performed on the photosensitive member.
FIG. 12 is a flowchart of control.
[Explanation of symbols]
1 Photoconductor
2 Cleaning device
4 Charging device
5 Development device
6 Toner image forming section
7 Exposure equipment
8 Intermediate transfer belt
9 Primary transfer bias roller
10 Intermediate transfer belt cleaning device
12 Secondary transfer backup roller
13 Cleaning backup roller
14 Tension roller
15 Intermediate transfer unit
19 Secondary transfer roller
20 Fixing unit
26 Paper storage cassette
27 Paper feed roller
28 Registration roller pair
29 Paper discharge roller pair
40 reflective photosensors
100 printer
150 Control unit
150a CPU
150b RAM

Claims

The image carrier is uniformly charged and then exposed to form an electrostatic latent image, and the electrostatic latent image on the image carrier is developed by attaching toner from a developing device and developing it. Forming an electrostatic latent image of a plurality of reference patterns on the image carrier, developing the reference pattern with the developing device, detecting the toner adhesion amount of the developed reference pattern, In the image forming method for correcting the image forming condition based on the detection result,
A linear approximation expression of the development potential and the toner adhesion amount is obtained from the latent image potentials of the plurality of reference patterns and the detection result of the toner adhesion amount, and a development gamma and a development start voltage, which are inclinations of the linear approximation expression, are obtained. The target charging potential and the target exposure part potential are calculated from the development gamma, and the target development bias potential is determined from the target charging potential and the target exposure part potential, the development gamma and the development start voltage, and the image forming conditions are determined. An image forming method comprising correcting the image.

2. The image forming method according to claim 1, wherein the target development bias potential is obtained from a development gamma and the development start voltage to obtain a development potential for obtaining a maximum development amount, and the target charging potential and the target development potential are obtained so as to obtain the development potential. An image forming method characterized in that it is determined based on a target exposure potential.

3. The image forming method according to claim 1, wherein the target charging potential and the target exposure potential are determined based on a table in which a relationship between the development gamma, the charging potential, and the exposure portion potential is determined in advance. Forming method.

4. The image forming method according to claim 1, wherein an irregular reflection type photosensor is used as the detecting means for detecting the toner adhesion amount.

An image carrier that carries an electrostatic latent image, latent image forming means that forms an electrostatic latent image on the image carrier, a developing device that converts the electrostatic latent image on the image carrier into a toner image, and Patch pattern writing means for forming an electrostatic latent image of a plurality of reference patch patterns on an image carrier, image density detecting means for detecting the image density of the patch pattern developed by the developing device, and the image density In an image forming apparatus including an image forming condition correcting unit that corrects an image forming condition based on a detection result by the detecting unit,
The linear approximation formula relating to the developing potential and the toner adhesion amount is obtained from the latent image potentials of the plurality of patch patterns formed on the image carrier and the detection result of the image density detection means, and the slope of the linear approximation formula is obtained. Development characteristic calculation means for calculating development gamma and development start voltage, latent image characteristic calculation means for determining a target charging potential and target exposure potential based on the development gamma calculated by the development characteristic calculation means, and the latent image characteristic calculation means An image forming apparatus comprising: a developing bias potential calculating means for determining a target developing bias potential from the target charging potential and the target exposure potential determined in step 1, the developing gamma calculated by the developing characteristic calculating means and the developing start voltage. .

6. The image forming apparatus according to claim 5, wherein the developing bias potential calculating means obtains a developing potential for obtaining a maximum developing amount from the developing gamma calculated by the developing characteristic calculating means and the developing start voltage, and obtains the developing potential. An image forming apparatus characterized in that a target developing bias potential is determined based on a target charging potential and a target exposure potential determined by the latent image characteristic calculating means.

7. The image forming apparatus according to claim 5 or 6, wherein the target charging potential and the target exposure potential calculated by the latent image characteristic calculating unit are preliminarily related to a relationship between the developing gamma calculated by the developing characteristic calculating unit, the charging potential, and the exposed portion potential. An image forming apparatus, characterized in that the image forming apparatus is determined based on a determined table.

8. The image forming apparatus according to claim 5, wherein an irregular reflection type photosensor is used as the image density detecting means.

9. The image forming apparatus according to claim 5, wherein an image forming unit in which the image carrier and the developing device are integrally formed is used.