JP3570589B2

JP3570589B2 - Tuning fork type load cell and weighing device using the same

Info

Publication number: JP3570589B2
Application number: JP05223396A
Authority: JP
Inventors: 和文内藤; 道人宇都宮; 嘉宏中村; 洋宮本
Original assignee: Ishida Co Ltd
Current assignee: Ishida Co Ltd
Priority date: 1996-02-14
Filing date: 1996-02-14
Publication date: 2004-09-29
Anticipated expiration: 2016-02-14
Also published as: JPH09218080A

Description

【０００１】
【発明の属する技術分野】
この発明は、音叉式ロードセルおよびこれを用いた計量装置に関し、特に、その荷重検出精度向上に関する。
【０００２】
【従来の技術】
一般に、固有振動数で振動する音叉振動子に、外力を加えるとその外力に対応して固有振動数が変化し、この音叉振動子に加えられる外力Ｆと音叉振動子の固有振動数ｆについて、およそ次の式（１）の関係が成立する。
Ｆ＝Ｃ１・ｆ^２−Ｃ２（Ｃ１，Ｃ２；定数）（１）
このように固有振動数が変化するのは、音叉振動子の形状が変化するのではなく、振動する物体の力学的状態（外力付加状態）により変化するものであり、音叉振動子は入力に対して敏感であるので、従来から、この入力に敏感な音叉振動子を利用した荷重検出機構が微量な質量（ｍｇ単位）を高精度に測定する電子てんびん秤等の計量装置に利用されてきた。
【０００３】
ここで、高度な荷重検出精度を達成するためには、音叉振動子に対し所定の一方向にのみ外力が加えられるように、この音叉振動子を備えた荷重検出機構を組み込む荷重付加機構を構成する必要がある。このような荷重付加機構として、２ビーム型の起歪体が考えられる。
【０００４】
【発明が解決しようとする課題】
しかし、この起歪体を利用した計量装置は、起歪体の固定部が基台に固定され、可動部に被計量物を支持する計量皿が固定されるが、２ビーム型起歪体においては、モータの発熱等により固定部から可動部へ、また、高温または低温の被計量物を載せた計量皿を通して可動部から固定部へ熱が伝わり、固定部と可動部間で温度勾配を持つ。このため、音叉振動子をこのような起歪体の固定部と可動部間に取り付けると、温度変化に対してゼロ点変動が大きくゼロ点安定性が劣るため、高精度を確保できないという問題があった。
【０００５】
また、荷重検出機構を起歪体に組み込んだとき、荷重検出機構とこれを取り付ける起歪体との熱膨張係数の差により、荷重検出誤差が生じる問題がある。例えば、固定部、可動部およびビーム部を一体形成した一体型の起歪体は、加工性，材料コスト面を考慮して、通常アルミ合金や鉄鋼を用いているが、音叉振動子を含む荷重検出機構は、温度によってばね定数が変化しないようにヤング率の温度係数の小さいエリンバー等の恒弾性合金を用いている。従って、起歪体の材料の熱膨張係数が音叉振動子の材料の熱膨張係数よりも大きく、起歪体寸法の温度による変化が音叉振動子よりも大きい。例えば、起歪体の材料がアルミ合金の場合、起歪体の熱膨張係数は音叉振動子の約３倍になる。このような起歪体と音叉振動子とでロードセルを構成する場合、周囲温度が変化すると両者の材料の熱膨張係数の違いによって両者のボルト締付部にすべりが生じ、ゼロ点の再現性が悪くなるので、荷重検出精度の確保が困難になる。
【０００６】
この発明は、上記の問題点を解決して、温度変化に影響されることなく高精度化を図ることができる音叉式ロードセルとこれを用いた計量装置を提供することを目的としている。
【０００７】
【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に係る音叉式ロードセルは、左右に固定部と荷重が加えられる可動部とが配置され、可動部が上下のビーム部を介して固定部に支持された起歪体と、この起歪体に装着された荷重検出機構とを有している。前記荷重検出機構は、前記上下のビーム部のいずれか一方に取り付けられた基部と、この基部に支点を介して支持されたてこ部と、前記基部とてこ部との間に取り付けられ、加えられた荷重に応じて固有振動数が変化する音叉振動子とを備え、前記てこ部の荷重点が前記上下のビーム部の他方に連結されている。
【０００８】
上記構成によれば、荷重検出機構は、起歪体の上下のビーム部間に取り付けられている。従って、音叉振動子は、起歪体の固定部と可動部の各々の温度のほぼ中間値であって、互いの温度差も小さい上下ビーム間に固定されるので、固定部と可動部間で温度勾配があっても、固定部および可動部からの熱の影響を低減し、温度変化に対してゼロ点変動が小さくゼロ点安定性に優れるから、荷重検出精度を向上させることができる。
【０００９】
請求項２に係るロードセルは、さらに、上記ビーム部と基部間に、両者の熱膨張係数の間の係数を有する材料からなる中間材を挿入している。従って、両者間の熱履歴によるヒステリシスが減少するので、温度変化に対するすべりを防止し再現性を向上して荷重検出精度を向上することができる。
【００１０】
請求項３に係る計量装置は、上記請求項１または請求項２のロードセルと、このロードセルの荷重検出機構で得られた信号から計量値を求める計量値生成回路とを有し、基台に前記ロードセルの固定部が固定され、可動部に被計量物を支持する計量台が固定されている。従って、計量装置の小型化、低コスト化を図ることができるとともに、微小質量を高精度に計量できる。
【００１１】
請求項４に係るロードセルは、前記起歪体と、この起歪体に装着された第１荷重検出機構および第２荷重検出機構とを有する差動型のロードセルである。すなわち、第１荷重検出機構および第２荷重検出機構のそれぞれは、起歪体の上下のビーム部のいずれか一方に取り付けられた基部と、この基部に支点を介して支持されたてこ部と、前記基部とてこ部との間に取り付けられ、加えられた荷重に応じて固有振動数が変化する音叉振動子とを備え、前記てこ部の荷重点が前記上下のビーム部の他方に連結されており、荷重が可動部に加えられたとき、一方の荷重検出機構の音叉振動子に引張力が作用し、他方の荷重検出機構の音叉振動子に圧縮力が作用するように設定されている。
【００１２】
請求項５に係るロードセルは、請求項４において、前記両荷重検出機構が起歪体の前後面の一方に装着されている。
【００１３】
また、請求項６に係るロードセルは、請求項４において、前記第１の荷重検出機構が起歪体の前面に、前記第２の荷重検出機構が起歪体の後面にそれぞれ装着されている。
【００１４】
上記請求項４ないし請求項６のロードセルによれば、第１および第２荷重検出機構は、起歪体の上下のビーム部間に取り付けられているので、固定部および可動部からの熱の影響を低減して、温度変化に対するゼロ点安定性を向上させるとともに、両荷重検出機構の音叉振動子の一方が引張型で他方が圧縮型となるので、両音叉振動子の周波数変化から荷重を差動型で求めることにより、起歪体の周囲の温度が変化した場合に、起歪体と音叉振動子の熱膨張係数が異なっていても、両者の温度変形が打ち消されるから、温度変化による影響を受けないので高い荷重検出精度を得ることができる。
【００１５】
請求項７に係る計量装置は、上記請求項４ないし請求項６のいずれかのロードセルと、このロードセルの両荷重検出機構のそれぞれで得られた信号の差から計量値を求める計量値生成回路とを有し、基台に前記ロードセルの固定部が固定され、可動部に被計量物を支持する計量台が固定されている。従って、計量装置のゼロ点安定性を向上させて、微小質量を高精度に計量できる。
【００１６】
【発明の実施の形態】
以下、この発明の実施形態を図面に基づいて説明する。
図１に、この発明の一実施形態に係る音叉式ロードセルの斜視図を示し、図２に、図１のロードセルを用いた計量装置の構成図を示す。この装置は、図１に示す起歪体２およびこの起歪体２に装着された荷重検出機構３を有する音叉式ロードセル１と、図２に示す発振回路１４、サイクルカウンタ１６およびディジタル信号処理回路（ＤＳＰ）１８からなる計量値生成回路１５とを備えている。
【００１７】
起歪体２は、左右に固定部２ａと荷重が加えられる可動部２ｂとが配置され、可動部２ｂが上下のビーム部２ｃ，２ｄを介して固定部２ａに支持されている。二点鎖線で示す基台Ｆに固定部２ａが固定され、可動部２ｂに被計量物を支持する同様に二点鎖線で示す計量台Ｋが固定されている。左右の固定部２ａ，可動部２ｂと上下のビーム部２ｃ，２ｄにより囲まれた部分には４隅がそれぞれ半円状の中空部９が形成されている。この中空部９の中央に向かって、上のビーム部２ｃから第１取付フランジ８Ａが延び、下のビーム部２ｄから第２取付フランジ８Ｂが延びており、これら取付フランジ８Ａ，８Ｂは、起歪体２の前面Ｍから凹入した位置に形成されている。これは、荷重検出機構３を起歪体２から前方向に出張ることなくこの凹部に収納するためである。また、起歪体２の上面および底面にそれぞれ中空部９に達する長円状の穴Ｈ１を２つ形成して、この起歪体２全体のバネ定数を小さくしている。
【００１８】
この起歪体２は、例えばアルミ合金材料や鉄系材料で一体型に形成されたパラレログラム（平行四辺形）構造になっている。この起歪体２に荷重検出機構３を装着したロードセル１は、付加される荷重を受けて、音叉振動子６に対し、所定の一方向のみに力を伝達することになる。
【００１９】
前記荷重検出機構３は、その基部４Ａが、この実施形態では下のビーム部２ｄから延びた第２取付フランジ８Ｂに、ねじＢ１のような締結手段で取り付けられ、この基部４Ａの右方に支点Ｐ１を介しててこ部５Ａが支持される。
【００２０】
てこ部５Ａは、鉛直方向に延びており、このてこ部５Ａの荷重点Ｐ２は、水平方向に延びる連結片７Ｂを介して、てこ部５Ａの左方位置で上のビーム部２ｃから延びた第１取付フランジ８ＡにねじＢ２のような締結手段で連結されている。そして、てこ部５Ａの左方に位置する音叉振動子６は、基部４Ａとてこ部５Ａとの間にそれぞれ水平な支持片７Ａを介して取り付けられている。音叉振動子６は、加えられた外力に応じて固有振動数が変化する。
【００２１】
図３に、実際の形状を示す荷重検出機構３の正面図を示す。この図のように、支持片７Ａ（固定端）と支点Ｐ１を形成する各部分（図示α〜γ）、並びに、連結片７Ｂ（負荷端）の一部分（図示δとε）を細く加工して、ロードセル１に外力Ｙが加えられたときの曲げモーメントによる拘束を除去している。
【００２２】
なお、図１の荷重検出機構３を上下逆にして、基部４Ａを第１取付フランジ８Ａに取り付け、かつ荷重点Ｐ２を連結片７Ｂを介して第２取付フランジ８Ｂに連結するようにしてもよいし、または、左右逆にして、基部４Ａの左方にてこ部５Ａを位置させ、かつ音叉振動子６をてこ部５Ａの右方に位置させるようにしてもよい。ただし、これらの場合には、音叉振動子６に対し圧縮力が作用することとなる。
【００２３】
図２において、荷重検出機構３の音叉振動子６は、中心軸に対称にかつ平行して設けられた２枚の振動片１０と、これらの両端部同士をそれぞれ結合する２個の結合部１１とを備えており、結合部１１の両側面にそれぞれ圧電素子１２ａ，１２ｂが接着や蒸着により取り付けられている。各圧電素子１２ａ，１２ｂは外部に設置した発振回路１４に接続されている。外力を加える前に、発振回路１４により圧電素子１２ａを励振させて２枚の振動片１０を固有振動数で発振させ、その固有振動数が発振回路１４に入力される。この状態で外力が加えられると、振動片１０が軸方向に加えられた外力（引張力Ｆｐまたは圧縮力Ｆｃ）に応じて変化した固有振動数で振動し、その固有振動数は圧電素子１２ｂから発振回路１４へ出力されて取り出される。
【００２４】
また、図４に示すように、起歪体２と基部４Ａ間に中間材３６を設けている。この中間材３６には、エリンバーのような恒弾性合金からなる荷重検出機構３とアルミ合金のような起歪体２の熱膨張係数の中間値をもつ、鉄系材料のようなものが用いられる。この中間材３６は、荷重検出機構３の熱膨張係数に近い材質が好ましい。これは、起歪体２と基部４Ａ間に、熱膨張係数が恒温材に近い材料を挟み込むことにより、両者間の熱履歴によるヒステリシスが減り再現性が向上し、温度補正も可能となり荷重検出精度向上に役立つからである。なお、両者間の温度変化に対する影響度によっては、この中間材３６を設けなくともよい。
【００２５】
さらに、第２取付フランジ８Ｂは溝部８Ｃを有している。これにより、中間材３６と第２取付フランジ８Ｂとの接触面積を小さくしている。すなわち、起歪体２のビーム部２ｄから延びた第２取付フランジ８Ｂから荷重検出機構３の基部４Ａへの受熱面積を小さくして、温度変化に対する影響を小さくするようにしている。なお、起歪体２における熱の影響度によっては、この溝部８Ｃを設けなくともよい。
【００２６】
以下、この計量装置の動作を説明する。
図１において、計量台Ｋに被計量物が載せられてその荷重Ｗが加えられると、可動部２ｂは矢印Ｙ方向に変位する。そうすると、上下のビーム部２ｃ，２ｄの相対変位によって、荷重検出機構３の連結片７Ｂが左方向に引っ張られるので、音叉振動子６には、てこの原理で右方向に荷重Ｗによる引張力Ｆｐが作用する。ここで、モータ等の発熱により固定部２ａから可動部２ｂへ、また、高温または低温の被計量物を載せた計量皿を通して可動部２ｂから固定部２ａへ熱が伝わり、起歪体２内の固定部２ａと可動部２ｂ間で温度勾配が生じる。従って、荷重検出機構３の音叉振動子６を固定部２ａと可動部２ｂ間に取り付けると、この間の温度勾配により、温度変化に対してゼロ点変動が大きくゼロ点安定性が劣るため、高精度を確保できない。これに対し、この発明は、荷重検出機構３が、固定部２ａと可動部２ｂのそれぞれの温度のほぼ中間温度であって、しかも上下間で温度差が小さいビーム部２ｃとビーム部２ｄ間に固定されている。このため、音叉振動子６に対する左右の固定部２ａ，可動部２ｂからの熱の影響を低減して、起歪体２の温度変化に対してゼロ点変動が小さくゼロ点安定性に優れるので、荷重Ｗに対応して引張力Ｆｐは正確に作用する。
【００２７】
図２の振動片１０がその引張力Ｆｐに応じて増加した固有振動数ｆ１で振動すると、その固有振動数ｆ１は発振回路１４へ出力されて取り出される。この固有振動数ｆ１は、その周期Ｔ１を測定するため、クロック入力Ｃをサイクルカウンタ１６に入力してカウントし、ＤＳＰ１８に入力される。そして、ＤＳＰ１８の線形化手段２２により、式（１）のＦ＝Ｃ１・ｆ１^２−Ｃ２（ｆ１＝１／Ｔ１）を満たすように固有振動数ｆ１（周期Ｔ１）を２乗したものと引張力Ｆｐとをリニアに関係させ、２段に設けたローパスフィルタのようなフィルタリング手段２４，２５によりロードセル１の固有振動数等の不要分が取り除かれる。こうして、ＤＳＰ１８から被計量物の荷重Ｗに対応した計量値として出力する。
【００２８】
このように、この音叉式ロードセル１は、荷重検出機構３の音叉振動子６を起歪体２の上下のビーム間２ｃ，２ｄ間に取り付けるようにしているので、固定部２ａと可動部２ｂの温度変化に対して、ゼロ点変動が小さくなるので、荷重検出精度を向上させることができる。また、中間材３６および溝部８ｃを設けているから、温度変化に対するゼロ点変動がさらに小さくなって、荷重検出精度が一層向上する。
【００２９】
さらに、起歪体２を一体型にしたことにより、つぎの利点がある。すなわち、別部品である２枚の板ばねからなるビームを用い、これらを平行に連結するために２個の連結支持部品を用いて合計４個の部品により荷重付加機構を構成すると、音叉振動子は入力に対して敏感であるので、この敏感な特性を生かすべく、これを組み込んで荷重付加機構を製作するには、上記の構成部品を精密に加工し、正確に組み立てて、微妙な調整を行うといった高度な熟練性を要していた。このため、製作が煩雑となり結合部品も必要となるので、小型化や低コスト化が困難である。これに対し、前記起歪体２は一体型であるから、荷重検出機構との組み立てが容易になり、寸法精度も機械加工時に作り込まれるので、精度の高いロードセル１を低コストで容易に製作できる。また、一体型構造のため結合部品がなくなるので、ロードセル１の小型化も図れる。
【００３０】
次に、第２実施形態の計量装置について説明する。
図５に、第２実施形態に係る音叉式ロードセルの斜視図を示し、図６に、図５のロードセルを用いた計量装置の信号処理部を示す。この計量装置は、前述した起歪体２と音叉振動子６の材料の熱膨張係数の違いによる荷重検出精度低下の問題を解消するために、起歪体２の前後面の一方に２つの荷重検出機構を差動型に設けている。
【００３１】
すなわち、この計量装置は、図５に示す起歪体２およびこの起歪体２に装着された第１，第２荷重検出機構３Ａ，３Ｂからなる音叉式ロードセル１と、図６に示す発振回路１４，カウンタ１６，ディジタル信号処理回路１８および引算器２０からなる計量値生成回路３５とを備えている。さらに、図５の二点鎖線で示す基台Ｆに起歪体の固定部２ａが固定され、可動部２ｂに被計量物を支持する同様に二点鎖線で示す計量台Ｋが固定されている。また、第１，第２荷重検出機構３Ａ，３Ｂは、第１実施形態と同様に、固定部２ａまたは可動部２ｂからの熱の影響を小さくするために、ビーム２ｃ，２ｄ間に配置されている。
【００３２】
この実施形態のロードセルは、第１，第２荷重検出機構３Ａ，３Ｂを同一形状、同一寸法で、同一外力によって同一の周波数変化を生じるように、それぞれ一体構造で形成し、起歪体２の前面Ｍの上方に第１荷重検出機構３Ａを装着し、下方に第２荷重検出機構３Ｂを装着して、両荷重検出機構３Ａ，３Ｂを差動型にすることにより、両者の材料の熱膨張係数差によるゼロ点の温度依存性を補償している。
【００３３】
第１荷重検出機構３Ａは、起歪体２の前面Ｍの下方に装着されており、その基部４Ａが、この実施形態ではビーム部２ｄから延びた第２取付フランジ８ＢにねじＢ１のような締結手段で取り付けられ、この基部４Ａの右方に支点Ｐ１を介して、鉛直方向に延びるてこ部５Ａが支持されている。てこ部５Ａの荷重点Ｐ２が、水平方向に延びる連結片７Ｂを介して、てこ部５Ａの左方位置でビーム部２ｃから延びた第１取付フランジ８ＡにねじＢ２のような締結手段で連結されている。そして、てこ部５Ａの左方に位置する音叉振動子６は、基部４Ａとてこ部５Ａとの間にそれぞれ水平方向に延びる支持片７Ａを介して取り付けられる。音叉振動子６は、加えられた外力（引張力Ｆｐ）に応じて固有振動数が変化する。
【００３４】
第２荷重検出機構３Ｂは、起歪体２の前面Ｍの上方に装着されており、その基部４Ｂが、ビーム部２ｄから延びた第２取付フランジ８ＢにねじＢ１のような締結手段で取り付けられ、この基部４Ｂの左方に支点Ｐ１を介して、鉛直方向に延びるてこ部５Ｂが支持されている。てこ部５Ｂの荷重点Ｐ２が、水平方向に延びる連結片７Ｂを介して、てこ部５Ｂの右方位置でビーム部２ｃから延びた第１取付フランジ８ＡにねじＢ２のような締結手段で連結されている。そして、てこ部５Ｂの右方に位置する音叉振動子６は、基部４Ｂとてこ部５Ｂとの間にそれぞれ水平方向に延びる支持片７Ａを介して取り付けられる。音叉振動子６は、加えられた外力（圧縮力Ｆｃ）に応じて固有振動数が変化する。
【００３５】
なお、第１荷重検出機構３Ａと第２荷重検出機構３Ｂをともに、上下逆に、すなわち、両検出機構の基部４Ａ，４Ｂを、上方のビーム部２ｃの第１取付フランジ８Ａに取り付け、両てこ部５Ａ，５Ｂの荷重点Ｐ２を、連結片７Ｂを介して、下方のビーム部２ｄの第２取付フランジ８Ｂに取り付けるようにしてもよい。
【００３６】
また、第１荷重検出機構３Ａと第２荷重検出機構３Ｂを互いに上下逆に、つまり、第１荷重検出機構３Ａの基部４Ａを下方のビーム部２ｄの第２取付フランジ８Ｂに、第２荷重検出機構３Ｂの基部４Ｂを上方のビーム部２ｃの第１取付フランジ８Ａにそれぞれ取り付けることもできる。その場合、例えば、両てこ部５Ａ，５Ｂをともに、基部４Ａ，４Ｂの右方（左方でもよい）になるように配置して、第１検出機構３Ａの基台４Ａを、ビーム部２ｄから延びた第２取付フランジ８Ｂに取り付け、基台４Ａの右方に位置するてこ部５Ａの荷重点Ｐ２を、連結片７Ｂを介して、ビーム部２ｃから延びた第１取付フランジ８Ａに取り付けるとともに、第２検出機構３Ｂの基台４Ｂを、ビーム部２ｃから延びた第１取付フランジ８Ａに取り付け、基台４Ｂの右方に位置するてこ部５Ｂの荷重点Ｐ２を、連結片７Ｂを介して、ビーム部２ｄから延びた第２取付フランジ８Ｂに取り付けるようにしてもよい。
【００３７】
以下、この計量装置の動作を説明する。
まず、図５の計量台Ｋに被計量物が載せられてその荷重Ｗが加えられたときに、可動部２ｂは矢印方向に変位する。そうすると、起歪体２の前面側に装着された第１荷重検出機構３Ａの音叉振動子６には、てこの原理で上方向に荷重Ｗによる引張力Ｆｐが作用する。一方、図６の圧電素子１２ａは発振回路１４により励振されており、振動片１０がその引張力Ｆｐに応じて増加した固有振動数ｆ１で振動すると、その固有振動数ｆ１は圧電素子１２ｂから発振回路１４へ出力されて取り出される。
【００３８】
この固有振動数ｆ１は、その周期Ｔ１（１／ｆ１）を測定するために、クロック入力Ｃをサイクルカウンタ１６に入力してカウントし、ＤＳＰ１８に入力される。固有振動数ｆ１（周期Ｔ１）と引張力Ｆｐとは、上記の式（１）のＦ＝Ｃ１・ｆ１^２−Ｃ２（ｆ１＝１／Ｔ１）の関係にあるので、ＤＳＰ１８の線形化手段２２により、式（１）を満たすように固有振動数ｆ１（周期Ｔ１）を２乗したものと引張力Ｆｐとをリニアに関係させ、さらに、２段に設けたローパスフィルタのようなフィルタリング手段２４，２５によりロードセル１の固有振動数等の不要分が取り除かれる。そして、音叉振動子６の増加した固有振動数ｆ１（周期Ｔ１）の２乗値に対応した引張力Ｆｐが引算器２０の一方の入力に出力される。
【００３９】
一方、計量台Ｋに被計量物が載せられて可動部２ｂが矢印方向に変位すると、後面側に装着された第２荷重検出機構３Ｂの音叉振動子６には、てこの原理で上方向に荷重Ｗによる圧縮力Ｆｃが作用する。そして、圧電素子１２ａを励振させた状態で、振動片１０がその圧縮力Ｆｃに応じて減少した固有振動数ｆ２で振動すると、その固有振動数ｆ２は圧電素子１２ｂから発振回路１４へ出力されて取り出される。次に、上記と同様に、カウンタ１６およびＤＳＰ１８を介して、音叉振動子６の減少した固有振動数ｆ２（周期Ｔ２）の２乗値に対応した圧縮力−Ｆｃが引算器２０の他方の入力に出力される。
【００４０】
引算器２０において、第１荷重検出機構３Ａからの増加した固有振動数ｆ１の２乗値に対応する引張力Ｆｐから、第２荷重検出機構３Ｂからの減少した固有振動数ｆ２の２乗値に対応する圧縮力−Ｆｃを差し引きすれば、（Ｆｐ＋Ｆｃ）となる。従って、被計量物の荷重Ｗに対応した計量値として引算器２０から出力する。
【００４１】
ここで、材料の熱膨張係数の差によって生じる起歪体２と音叉振動子６の温度変化の差は、第１，第２荷重検出機構３Ａ，３Ｂを差動型にしているので、以下のように打ち消され、ロードセル１のゼロ点の温度依存性が補償される。
【００４２】
すなわち、このロードセル１の周囲温度が変化した場合、起歪体２と音叉振動子６の材料の熱膨張係数の差によってロードセル１内に熱応力が発生し、それは２個の起歪体２の前後面に装着された音叉振動子６に外力として伝達される。しかし、その外力は、２個の音叉振動子６に対し、同方向でかつ同じ大きさの力であるので、その力によるそれぞれの音叉振動子６の固有振動数の変化分と変化方向は同じになる。そこで、引算器２０により、一方の第１荷重検出機構３Ａの音叉振動子６からの変化した固有振動数に対応する力から、第２荷重検出機構３Ｂの音叉振動子６からの変化した固有振動数に対応する力を差し引くと、この温度変化による変化分は打ち消されて最終的な出力には現れない。従って、ロードセル１のゼロ点の温度依存性を十分に小さくすることができ、高精度化を図ることができる。
【００４３】
この差動型の他の形態として、図７に、起歪体の両面に２つの荷重検出機構を差動型に設けた第３実施形態の音叉式ロードセルを示す。この場合も、第２実施形態と同様に、可動部２ｂまたは固定部２ａからの熱の影響を小さくするために、ビーム２ｃ，２ｄ間に両荷重検出機構を配置している。
【００４４】
この音叉式ロードセルは、起歪体２が前後に対称で、上下にも対称な形状になっており、第２実施形態と同様に、第１，第２荷重検出機構３Ａ，３Ｂを同一形状、同一寸法で、同一外力によって同一の周波数変化を生じるように、それぞれ一体構造で形成し、起歪体２の前後面の一方に第１荷重検出機構３Ａを装着し、他方にこれと上下逆向きの第２荷重検出機構３Ｂを装着して、両荷重検出機構３Ａ，３Ｂを差動型にすることにより、両者の材料の熱膨張係数差によるゼロ点の温度依存性を補償している。なお、この第３実施形態のロードセルは、両荷重検出機構３Ａ，３Ｂを起歪体２の両面に設けているので、温度変化に対するゼロ点変動が片面に設けたロードセルよりも小さいので、起歪体２における熱の影響は第２実施形態のロードセルよりも小さい。このロードセルからの信号を処理する回路は図６と同様であるので、詳しい説明は省略する。
【００４５】
図７（ａ）において、第１荷重検出機構３Ａは、起歪体２の前面Ｍ側に装着されており、その基部４Ａが、ビーム部２ｄから延びた第２取付フランジ８ＢにネジＢ１のような締結手段で取り付けられ、この基部４Ａの右方に支点Ｐ１を介して、鉛直方向に延びたてこ部５Ａが支持されている。てこ部５Ａの荷重点Ｐ２が、水平方向に延びた連結片７Ｂを介して、てこ部５Ａの左方位置でビーム部２ｃから延びた第１取付フランジ８ＡにネジＢ２のような締結手段で連結されている。そして、てこ部５Ａの左方に位置する音叉振動子６は、基部４Ａとてこ部５Ａとの間にそれぞれ水平方向に延びた支持片７Ａを介して取り付けられる。音叉振動子６は、加えられた外力（引張力Ｆｐ）に応じて固有振動数が変化する。
【００４６】
図７（ｂ）において、第２荷重検出機構３Ｂは、起歪体２の後面Ｎ側に装着されている。基部４Ｂが、上記と同様に第２取付フランジ８Ｂにネジ締結等で、前面のネジＢ１締結位置のほぼ反対側の位置に取り付けられており、この基部４Ｂの右方に支点Ｐ１を介して、鉛直方向に延びたてこ部５Ｂが支持されている。てこ部５Ｂの荷重点Ｐ２が、水平方向に延びた連結片７Ｂを介して、てこ部５Ｂの右方位置で上記と同様に、第１取付フランジ８Ａにネジ締結等で連結されている。そして、てこ部５Ｂの左方に位置する音叉振動子６は、基部４Ｂとてこ部５Ｂとの間にそれぞれ水平方向に延びた支持片７Ａを介して取り付けられる。音叉振動子６は、加えられた外力（圧縮力Ｆｃ）に応じて固有振動数が変化する。
【００４７】
なお、第１荷重検出機構３Ａと第２荷重検出機構３Ｂを、上下逆に、すなわち、両検出機構の基部４Ａ，４Ｂを、ビーム部２ｃから延びた第１取付フランジ８Ａに取り付け、両てこ部５Ａ，５Ｂの荷重点Ｐ２を、連結片７Ｂを介して、ビーム部２ｄから延びた第２取付フランジ８Ｂに取り付けるようにしてもよい。
【００４８】
また、第１荷重検出機構３Ａを図７（ａ）のように配置するとともに、（ｂ）の第２荷重検出機構３Ｂを、てこ部５Ｂが基部４Ｂの左方であって、基台４Ｂをビーム部２ｃから延びた第１取付フランジ８Ａに取り付け、てこ部５Ｂの荷重点Ｐ２を、連結片７Ｂを介して、ビーム部２ｄから延びた第２取付フランジ８Ｂに取り付けるようにしてもよい。
【００４９】
この計量装置により、例えば、数１０〜数１００ｍｇの重量の被計量物を、１００〜３００回／分の高速で、かつ、１／５０００〜１／１００００の高精度で計量することができる。
【００５０】
なお、この発明では、計量装置は被計量物の重量を選別する重量選別機や組合せ計量機に適用されているが、静止計量を行う秤量装置に適用してもよい。
【００５１】
【発明の効果】
この発明の一構成によれば、荷重検出機構は、起歪体の上下のビーム部間に取り付けられている。従って、音叉振動子は、起歪体の固定部と可動部の各々の温度のほぼ中間値であって、互いの温度差も小さい上下ビーム間に固定されるので、固定部と可動部間で温度勾配があっても、固定部および可動部からの熱の影響を低減し、温度変化に対してゼロ点変動が小さくゼロ点安定性に優れるから、荷重検出精度を向上させることができる。
【００５２】
また、他の構成によれば、第１および第２荷重検出機構は、起歪体の上下のビーム部間に取り付けられているので、固定部および可動部からの熱の影響を低減して、温度変化に対するゼロ点安定性を向上させるとともに、両荷重検出機構の音叉振動子の一方が引張型で他方が圧縮型となるので、両音叉振動子の周波数変化から荷重を差動型で求めることにより、起歪体の周囲の温度が変化した場合に、起歪体と音叉振動子の熱膨張係数が異なっていても、両者の温度変形が打ち消されるから、温度変化による影響を受けないので、高い荷重検出精度を得ることができる。
【図面の簡単な説明】
【図１】この発明の一実施形態に係る音叉式ロードセルを示す斜視図である。
【図２】上記のロードセルを用いた計量装置の信号処理部を示す構成図である。
【図３】荷重検出機構を示す正面図である。
【図４】図１の音叉式ロードセルを示す部分拡大図である。
【図５】他の実施形態に係る音叉式ロードセルを示す斜視図である。
【図６】上記のロードセルを用いた計量装置の信号処理部を示す構成図である。
【図７】他の実施形態に係る音叉式ロードセルを示す側面図である。
【符号の説明】
１…音叉式ロードセル、２…起歪体、２ａ…固定部、２ｂ…可動部、２ｃ…ビーム部、３…荷重検出機構、３Ａ…第１荷重検出機構、３Ｂ…第２荷重検出機構、４Ａ，４Ｂ…基部、５Ａ，５Ｂ…てこ部、６…音叉振動子、３６…中間材。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a tuning fork type load cell and a weighing device using the same, and more particularly, to an improvement in load detection accuracy thereof.
[0002]
[Prior art]
In general, when an external force is applied to a tuning fork vibrator vibrating at a natural frequency, the natural frequency changes in accordance with the external force. Regarding the external force F applied to the tuning fork vibrator and the natural frequency f of the tuning fork vibrator, Approximately the following equation (1) holds.
^{F = C1 · f 2 -C2 (} C1, C2; constant) (1)
Such a change in the natural frequency is not due to a change in the shape of the tuning fork vibrator, but to a change in the mechanical state of the vibrating object (external force applied state). Conventionally, a load detection mechanism using a tuning fork vibrator sensitive to this input has been used for a weighing device such as an electronic balance scale for measuring a very small amount of mass (mg unit) with high accuracy. .
[0003]
Here, in order to achieve a high level of load detection accuracy, a load addition mechanism incorporating a load detection mechanism equipped with the tuning fork vibrator is configured so that an external force is applied to the tuning fork vibrator in only one predetermined direction. There is a need to. As such a load applying mechanism, a two-beam type strain body can be considered.
[0004]
[Problems to be solved by the invention]
However, in a weighing device using this flexure element, the fixing portion of the flexure element is fixed to the base, and the weighing pan that supports the object to be weighed is fixed to the movable section. Heat is transmitted from the fixed part to the movable part due to heat generation of the motor, and from the movable part to the fixed part through the weighing pan on which the object to be weighed is placed, and there is a temperature gradient between the fixed part and the movable part. . For this reason, when a tuning fork vibrator is attached between the fixed part and the movable part of such a strain generating element, there is a problem that high accuracy cannot be secured because the zero point fluctuation is large with respect to the temperature change and the zero point stability is inferior. there were.
[0005]
Further, when the load detection mechanism is incorporated in the strain body, there is a problem that a load detection error occurs due to a difference in thermal expansion coefficient between the load detection mechanism and the strain body to which the load detection mechanism is attached. For example, an integrated type flexure element in which a fixed part, a movable part and a beam part are integrally formed is usually made of an aluminum alloy or steel in consideration of workability and material cost, but the load including a tuning fork vibrator is used. The detection mechanism uses a constant elastic alloy such as Elinvar having a small temperature coefficient of Young's modulus so that the spring constant does not change with temperature. Therefore, the thermal expansion coefficient of the material of the strain body is larger than the thermal expansion coefficient of the material of the tuning fork vibrator, and the change in the size of the strain body due to temperature is larger than that of the tuning fork vibrator. For example, when the material of the flexure element is an aluminum alloy, the thermal expansion coefficient of the flexure element is about three times that of the tuning fork vibrator. When a load cell is composed of such a flexure element and a tuning fork vibrator, when the ambient temperature changes, slippage occurs in the bolted portions of both bolts due to the difference in the thermal expansion coefficient of both materials, and the reproducibility of the zero point is reduced. As a result, it becomes difficult to secure load detection accuracy.
[0006]
SUMMARY OF THE INVENTION It is an object of the present invention to provide a tuning fork type load cell capable of solving the above problems and achieving high accuracy without being affected by a temperature change, and a weighing device using the same.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, a tuning fork type load cell according to claim 1 has a fixed part and a movable part to which a load is applied on the left and right, and the movable part is supported by the fixed part via upper and lower beam parts. It has a flexure element and a load detection mechanism attached to the flexure element. The load detecting mechanism is attached to one of the upper and lower beam portions, a lever portion supported on the base portion via a fulcrum, and mounted between the base portion and the lever portion. A tuning fork vibrator whose natural frequency changes in accordance with the applied load, and a load point of the lever portion is connected to the other one of the upper and lower beam portions.
[0008]
According to the above configuration, the load detection mechanism is attached between the upper and lower beam portions of the strain body. Therefore, the tuning fork vibrator is fixed between the upper and lower beams because the temperature of the fixed portion and the movable portion of the flexure element is substantially the middle value of each temperature and the temperature difference between them is small. Even if there is a temperature gradient, the influence of heat from the fixed part and the movable part is reduced, and the zero point fluctuation with respect to the temperature change is small and the zero point stability is excellent, so that the load detection accuracy can be improved.
[0009]
In the load cell according to the second aspect, an intermediate material made of a material having a coefficient between the two coefficients of thermal expansion is inserted between the beam part and the base part. Accordingly, the hysteresis due to the thermal history between the two is reduced, so that the slip with respect to the temperature change can be prevented, the reproducibility can be improved, and the load detection accuracy can be improved.
[0010]
A weighing device according to claim 3 includes the load cell according to claim 1 or 2, and a weighing value generation circuit that obtains a weighing value from a signal obtained by a load detection mechanism of the load cell. A fixed portion of the load cell is fixed, and a weighing table that supports the object to be weighed is fixed to the movable portion. Therefore, the size and cost of the weighing device can be reduced, and the minute mass can be weighed with high accuracy.
[0011]
A load cell according to a fourth aspect is a differential load cell including the strain body, and a first load detection mechanism and a second load detection mechanism mounted on the strain body. That is, each of the first load detection mechanism and the second load detection mechanism has a base attached to one of the upper and lower beam portions of the flexure element, a lever supported by the base via a fulcrum, A tuning fork vibrator attached between the base portion and the lever portion, the natural frequency of which changes according to the applied load, wherein the load point of the lever portion is connected to the other one of the upper and lower beam portions. In this configuration, when a load is applied to the movable portion, a tensile force acts on the tuning fork vibrator of one load detecting mechanism, and a compressive force acts on the tuning fork vibrator of the other load detecting mechanism.
[0012]
A load cell according to a fifth aspect is the load cell according to the fourth aspect, wherein the two load detection mechanisms are mounted on one of the front and rear surfaces of the strain body.
[0013]
According to a sixth aspect of the present invention, in the load cell according to the fourth aspect, the first load detection mechanism is mounted on a front surface of the strain body, and the second load detection mechanism is mounted on a rear surface of the strain body.
[0014]
According to the load cells of the fourth and sixth aspects, the first and second load detecting mechanisms are mounted between the upper and lower beam portions of the strain body, so that the first and second load detecting mechanisms are affected by heat from the fixed portion and the movable portion. To improve the zero-point stability against temperature changes, and because one of the tuning fork vibrators of both load detection mechanisms is of the tension type and the other is of the compression type, the load can be differentiated from the frequency change of both tuning fork vibrators. By calculating the dynamic type, even if the temperature around the flexure element changes, even if the thermal expansion coefficients of the flexure element and the tuning fork vibrator are different, the temperature deformations of both are canceled out. Therefore, high load detection accuracy can be obtained.
[0015]
A weighing device according to claim 7 is a load cell according to any one of claims 4 to 6, and a weighing value generation circuit that obtains a weighing value from a difference between signals obtained by each of the load detection mechanisms of the load cell. The fixed part of the load cell is fixed to the base, and the weighing table that supports the object to be weighed is fixed to the movable part. Therefore, the zero point stability of the weighing device is improved, and the minute mass can be weighed with high accuracy.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a perspective view of a tuning fork type load cell according to one embodiment of the present invention, and FIG. 2 shows a configuration diagram of a weighing device using the load cell of FIG. This device comprises a flexure element 2 shown in FIG. 1 and a tuning fork type load cell 1 having a load detecting mechanism 3 mounted on the flexure element 2, an oscillation circuit 14, a cycle counter 16 and a digital signal processing circuit shown in FIG. (DSP) 18 and a metric value generation circuit 15.
[0017]
The flexure element 2 has a fixed portion 2a and a movable portion 2b to which a load is applied on the left and right, and the movable portion 2b is supported by the fixed portion 2a via upper and lower beam portions 2c and 2d. A fixed portion 2a is fixed to a base F indicated by a two-dot chain line, and a weighing table K similarly supported by a two-dot chain line is fixed to the movable portion 2b. A hollow portion 9 having a semicircular shape at each of four corners is formed in a portion surrounded by the left and right fixed portions 2a and the movable portion 2b and the upper and lower beam portions 2c and 2d. The first mounting flange 8A extends from the upper beam portion 2c toward the center of the hollow portion 9, and the second mounting flange 8B extends from the lower beam portion 2d. It is formed at a position recessed from the front surface M of the body 2. This is because the load detecting mechanism 3 is housed in this recess without traveling forward from the strain body 2. In addition, two elliptical holes H1 reaching the hollow portion 9 are formed on the upper surface and the lower surface of the flexure element 2, respectively, to reduce the spring constant of the entire flexure element 2.
[0018]
The strain body 2 has a parallelogram (parallelogram) structure integrally formed of, for example, an aluminum alloy material or an iron-based material. The load cell 1 in which the load detecting mechanism 3 is mounted on the flexure element 2 receives an applied load and transmits a force to the tuning fork vibrator 6 in only one predetermined direction.
[0019]
The load detecting mechanism 3 has a base 4A attached to a second mounting flange 8B extending from the lower beam 2d in this embodiment by fastening means such as a screw B1, and a fulcrum is provided on the right side of the base 4A. The lever 5A is supported via P1.
[0020]
The lever 5A extends in the vertical direction, and the load point P2 of the lever 5A extends from the upper beam 2c at the left position of the lever 5A via the connecting piece 7B extending in the horizontal direction. It is connected to one mounting flange 8A by fastening means such as a screw B2. The tuning fork vibrator 6 located to the left of the lever 5A is mounted between the base 4A and the lever 5A via horizontal support pieces 7A. The natural frequency of the tuning fork vibrator 6 changes according to the applied external force.
[0021]
FIG. 3 is a front view of the load detection mechanism 3 showing an actual shape. As shown in this figure, the portions (α to γ in the drawing) forming the support piece 7A (fixed end) and the fulcrum P1 and the portions (δ and ε in the drawing) of the connecting piece 7B (load end) are thinned. The constraint caused by the bending moment when the external force Y is applied to the load cell 1 is removed.
[0022]
The load detecting mechanism 3 of FIG. 1 may be turned upside down so that the base 4A is mounted on the first mounting flange 8A and the load point P2 is connected to the second mounting flange 8B via the connecting piece 7B. Alternatively, the left and right sides may be reversed so that the lever 5A is located on the left of the base 4A and the tuning fork vibrator 6 is located on the right of the lever 5A. However, in these cases, a compressive force acts on the tuning fork vibrator 6.
[0023]
In FIG. 2, the tuning fork vibrator 6 of the load detecting mechanism 3 includes two vibrating reeds 10 provided symmetrically and in parallel with the central axis, and two connecting portions 11 for connecting both ends thereof. The piezoelectric elements 12a and 12b are attached to both side surfaces of the coupling portion 11 by bonding or vapor deposition, respectively. Each of the piezoelectric elements 12a and 12b is connected to an oscillation circuit 14 provided outside. Before applying an external force, the oscillation element 14 excites the piezoelectric element 12a to oscillate the two vibrating pieces 10 at a natural frequency, and the natural frequency is input to the oscillation circuit 14. When an external force is applied in this state, the vibrating piece 10 vibrates at a natural frequency changed according to the external force (tensile force Fp or compressive force Fc) applied in the axial direction, and the natural frequency is changed from the piezoelectric element 12b. The signal is output to the oscillation circuit 14 and taken out.
[0024]
Also, as shown in FIG. 4, an intermediate member 36 is provided between the strain body 2 and the base 4A. As the intermediate member 36, an iron-based material having an intermediate value between the thermal expansion coefficients of the load detecting mechanism 3 made of a constant elastic alloy such as Elinvar and the strain generating element 2 such as an aluminum alloy is used. . This intermediate member 36 is preferably made of a material having a coefficient of thermal expansion close to that of the load detection mechanism 3. This is because, by sandwiching a material having a thermal expansion coefficient close to that of a constant temperature material between the flexure element 2 and the base 4A, the hysteresis due to the thermal history between the two is reduced, the reproducibility is improved, and the temperature can be corrected. This is because it is useful for improvement. The intermediate member 36 may not be provided depending on the degree of influence on the temperature change between the two.
[0025]
Further, the second mounting flange 8B has a groove 8C. This reduces the contact area between the intermediate member 36 and the second mounting flange 8B. That is, the heat receiving area from the second mounting flange 8B extending from the beam portion 2d of the strain body 2 to the base 4A of the load detecting mechanism 3 is reduced, so that the influence on the temperature change is reduced. The groove 8C may not be provided depending on the degree of influence of heat on the strain body 2.
[0026]
Hereinafter, the operation of the weighing device will be described.
In FIG. 1, when the object to be weighed is placed on the weighing table K and the load W is applied, the movable part 2b is displaced in the arrow Y direction. Then, the connecting piece 7B of the load detecting mechanism 3 is pulled leftward by the relative displacement of the upper and lower beam portions 2c and 2d, and the tuning force fork vibrator 6 applies the pulling force Fp due to the load W rightward on the principle of leverage. Works. Here, heat is transmitted from the fixed portion 2a to the movable portion 2b by the heat generated by the motor or the like, or from the movable portion 2b to the fixed portion 2a through a weighing dish on which a high- or low-temperature weighing object is placed. A temperature gradient occurs between the fixed part 2a and the movable part 2b. Therefore, when the tuning fork vibrator 6 of the load detecting mechanism 3 is mounted between the fixed part 2a and the movable part 2b, the zero point fluctuation is large with respect to the temperature change due to the temperature gradient between the fixed part 2a and the movable part 2b. Can not be secured. On the other hand, according to the present invention, the load detecting mechanism 3 is provided between the beam portion 2c and the beam portion 2d, which are substantially at an intermediate temperature between the fixed portion 2a and the movable portion 2b, and have a small temperature difference between the upper and lower portions. Fixed. For this reason, the influence of the heat from the left and right fixed parts 2a and the movable part 2b on the tuning fork vibrator 6 is reduced, and the zero point fluctuation with respect to the temperature change of the strain body 2 is small and the zero point stability is excellent. The tensile force Fp acts accurately in response to the load W.
[0027]
When the vibrating piece 10 of FIG. 2 vibrates at the natural frequency f1 increased according to the tensile force Fp, the natural frequency f1 is output to the oscillation circuit 14 and taken out. The natural frequency f1 is counted by inputting the clock input C to the cycle counter 16 to measure the period T1 and input to the DSP 18. Then, the linearization means 22 of the DSP 18 squares the natural frequency f1 (period T1) so as to satisfy F = C1 · f1 ² −C2 (f1 = 1 / T1) in Expression (1), and the tensile force. Fp is linearly related, and unnecessary parts such as the natural frequency of the load cell 1 are removed by filtering means 24 and 25 such as low-pass filters provided in two stages. Thus, the DSP 18 outputs a weighed value corresponding to the load W of the object to be weighed.
[0028]
As described above, in this tuning fork type load cell 1, the tuning fork vibrator 6 of the load detecting mechanism 3 is attached between the upper and lower beams 2c and 2d of the strain body 2, so that the fixed portion 2a and the movable portion 2b Since the zero point fluctuation becomes small with respect to the temperature change, the load detection accuracy can be improved. Further, since the intermediate member 36 and the groove 8c are provided, the zero point fluctuation with respect to the temperature change is further reduced, and the load detection accuracy is further improved.
[0029]
Furthermore, the following advantages are obtained by forming the strain generating element 2 into an integral type. That is, if a load applying mechanism is constituted by a total of four parts using a beam composed of two leaf springs, which are separate parts, and two connecting support parts for connecting them in parallel, a tuning fork vibrator Is sensitive to input, so in order to take advantage of this sensitive characteristic, to incorporate it into a load applying mechanism, precisely process the above components, assemble them accurately, and make subtle adjustments. It required a high degree of skill such as performing. For this reason, the production is complicated and a coupling component is required, so that it is difficult to reduce the size and cost. On the other hand, since the flexure element 2 is of an integral type, it is easy to assemble with the load detecting mechanism, and the dimensional accuracy is made at the time of machining, so that the load cell 1 with high accuracy can be easily manufactured at low cost. it can. In addition, since there is no connecting part due to the integral structure, the size of the load cell 1 can be reduced.
[0030]
Next, a weighing device according to a second embodiment will be described.
FIG. 5 shows a perspective view of a tuning fork type load cell according to the second embodiment, and FIG. 6 shows a signal processing section of a weighing device using the load cell of FIG. This weighing device is provided with two loads on one of the front and rear surfaces of the flexure element 2 in order to solve the above-described problem of a decrease in load detection accuracy due to a difference in the thermal expansion coefficient of the material of the flexure element 2 and the tuning fork vibrator 6. The detection mechanism is provided in a differential type.
[0031]
That is, the weighing device includes a tuning fork type load cell 1 including a strain body 2 shown in FIG. 5 and first and second load detecting mechanisms 3A and 3B mounted on the strain body 2, and an oscillation circuit shown in FIG. 14, a counter 16, a digital signal processing circuit 18, and a metric value generating circuit 35 including a subtractor 20. Further, a fixed portion 2a of the strain body is fixed to a base F indicated by a two-dot chain line in FIG. 5, and a weighing table K similarly supported by the two-dot chain line is fixed to the movable portion 2b. . Similarly to the first embodiment, the first and second load detecting mechanisms 3A and 3B are arranged between the beams 2c and 2d in order to reduce the influence of heat from the fixed part 2a or the movable part 2b. I have.
[0032]
In the load cell of this embodiment, the first and second load detecting mechanisms 3A and 3B are formed in the same shape and the same size, and are formed in an integrated structure so that the same frequency change is caused by the same external force. The first load detecting mechanism 3A is mounted above the front surface M, the second load detecting mechanism 3B is mounted below, and the two load detecting mechanisms 3A and 3B are of a differential type. The temperature dependency of the zero point due to the coefficient difference is compensated.
[0033]
The first load detecting mechanism 3A is mounted below the front surface M of the strain body 2, and the base 4A of the first load detecting mechanism 3A is fastened to the second mounting flange 8B extending from the beam portion 2d with a screw B1 in this embodiment. A lever 5A extending in the vertical direction is supported to the right of the base 4A via a fulcrum P1. The load point P2 of the lever 5A is connected to the first mounting flange 8A extending from the beam 2c at the left position of the lever 5A by a fastening means such as a screw B2 via a connecting piece 7B extending in the horizontal direction. ing. The tuning fork vibrator 6 located to the left of the lever 5A is mounted between the base 4A and the lever 5A via support pieces 7A extending in the horizontal direction. The natural frequency of the tuning fork vibrator 6 changes according to the applied external force (tensile force Fp).
[0034]
The second load detecting mechanism 3B is mounted above the front surface M of the strain body 2, and its base 4B is attached to the second mounting flange 8B extending from the beam 2d by fastening means such as a screw B1. A lever 5B extending in the vertical direction is supported to the left of the base 4B via a fulcrum P1. The load point P2 of the lever portion 5B is connected to the first mounting flange 8A extending from the beam portion 2c at the right position of the lever portion 5B via a connecting piece 7B extending in the horizontal direction by a fastening means such as a screw B2. ing. The tuning fork vibrator 6 located to the right of the lever 5B is attached between the base 4B and the lever 5B via support pieces 7A extending in the horizontal direction. The natural frequency of the tuning fork vibrator 6 changes according to the applied external force (compression force Fc).
[0035]
Note that both the first load detecting mechanism 3A and the second load detecting mechanism 3B are turned upside down, that is, the bases 4A and 4B of the two detecting mechanisms are mounted on the first mounting flange 8A of the upper beam part 2c, and the levers are both The load point P2 of the portions 5A and 5B may be attached to the second attachment flange 8B of the lower beam portion 2d via the connecting piece 7B.
[0036]
The first load detection mechanism 3A and the second load detection mechanism 3B are turned upside down, that is, the base 4A of the first load detection mechanism 3A is attached to the second mounting flange 8B of the lower beam 2d, and the second load detection mechanism The base 4B of the mechanism 3B can be attached to the first attachment flange 8A of the upper beam portion 2c. In this case, for example, both the levers 5A and 5B are arranged so as to be to the right (or to the left) of the bases 4A and 4B, and the base 4A of the first detection mechanism 3A is moved from the beam 2d. Attached to the extended second mounting flange 8B, the load point P2 of the lever 5A located to the right of the base 4A is attached to the first mounting flange 8A extended from the beam 2c via the connecting piece 7B, The base 4B of the second detection mechanism 3B is mounted on the first mounting flange 8A extending from the beam portion 2c, and the load point P2 of the lever 5B located on the right side of the base 4B is connected via the connecting piece 7B. You may make it attach to the 2nd attachment flange 8B extended from the beam part 2d.
[0037]
Hereinafter, the operation of the weighing device will be described.
First, when the object to be weighed is placed on the weighing table K in FIG. 5 and the load W is applied, the movable portion 2b is displaced in the direction of the arrow. Then, the pulling force Fp due to the load W acts upward on the tuning fork vibrator 6 of the first load detecting mechanism 3A mounted on the front side of the flexure element 2 by the leverage principle. On the other hand, the piezoelectric element 12a in FIG. 6 is excited by the oscillation circuit 14, and when the vibrating piece 10 vibrates at the natural frequency f1 increased according to the tensile force Fp, the natural frequency f1 is oscillated from the piezoelectric element 12b. It is output to the circuit 14 and taken out.
[0038]
The natural frequency f1 is counted by inputting the clock input C to the cycle counter 16 to measure the period T1 (1 / f1), and is input to the DSP 18. And tensile force Fp is the natural frequency f1 (cycle T1), since the relationship of ^{F = C1 · f1 2 -C2 (} f1 = 1 / T1) of the above formula (1), the linearization means 22 of DSP18 , The tensile force Fp is linearly related to the square of the natural frequency f1 (period T1) so as to satisfy Expression (1), and further, filtering means 24 and 25 such as low-pass filters provided in two stages. Thus, unnecessary components such as the natural frequency of the load cell 1 are removed. Then, a tensile force Fp corresponding to the square value of the increased natural frequency f1 (period T1) of the tuning fork vibrator 6 is output to one input of the subtracter 20.
[0039]
On the other hand, when the object to be weighed is placed on the weighing table K and the movable part 2b is displaced in the direction of the arrow, the tuning fork vibrator 6 of the second load detecting mechanism 3B mounted on the rear side is moved upward by the principle of leverage. A compressive force Fc due to the load W acts. Then, when the vibrating reed 10 vibrates at the natural frequency f2 reduced according to the compressive force Fc in a state where the piezoelectric element 12a is excited, the natural frequency f2 is output from the piezoelectric element 12b to the oscillation circuit 14. Taken out. Next, in the same manner as described above, the compression force -Fc corresponding to the square value of the reduced natural frequency f2 (period T2) of the tuning fork vibrator 6 is calculated via the counter 16 and the DSP 18 on the other side of the subtracter 20. Output to input.
[0040]
In the subtractor 20, the square value of the reduced natural frequency f2 from the second load detection mechanism 3B is calculated from the tensile force Fp corresponding to the square value of the increased natural frequency f1 from the first load detection mechanism 3A. (Fp + Fc) by subtracting the compression force -Fc corresponding to. Therefore, the subtractor 20 outputs the measured value corresponding to the load W of the object to be weighed.
[0041]
Here, the difference in temperature change between the flexure element 2 and the tuning fork vibrator 6 caused by the difference in the coefficient of thermal expansion of the material is as follows because the first and second load detecting mechanisms 3A and 3B are of a differential type. Thus, the temperature dependence of the zero point of the load cell 1 is compensated.
[0042]
That is, when the ambient temperature of the load cell 1 changes, a thermal stress is generated in the load cell 1 due to a difference in the thermal expansion coefficient between the material of the flexure element 2 and the material of the tuning fork vibrator 6. It is transmitted as an external force to the tuning fork vibrator 6 mounted on the front and rear surfaces. However, since the external force is a force in the same direction and the same magnitude with respect to the two tuning fork vibrators 6, the change amount and the change direction of the natural frequency of each tuning fork vibrator 6 due to the force are the same. become. Therefore, the subtractor 20 uses the force corresponding to the changed natural frequency from the tuning fork vibrator 6 of the first load detecting mechanism 3A to change the changed natural frequency from the tuning fork vibrator 6 of the second load detecting mechanism 3B. If the force corresponding to the frequency is subtracted, the change due to the temperature change is canceled out and does not appear in the final output. Therefore, the temperature dependency of the zero point of the load cell 1 can be sufficiently reduced, and high accuracy can be achieved.
[0043]
As another form of this differential type, FIG. 7 shows a tuning fork type load cell according to a third embodiment in which two load detecting mechanisms are provided on both surfaces of a flexure element in a differential type. In this case, as in the second embodiment, both load detecting mechanisms are arranged between the beams 2c and 2d in order to reduce the influence of heat from the movable portion 2b or the fixed portion 2a.
[0044]
In this tuning fork type load cell, the flexure element 2 is symmetrical in the front-rear direction and symmetrical in the up-and-down direction. Like the second embodiment, the first and second load detecting mechanisms 3A and 3B have the same shape. They are formed in an integral structure so as to produce the same frequency change by the same external force with the same dimensions, and the first load detecting mechanism 3A is mounted on one of the front and rear surfaces of the flexure element 2 and the other is turned upside down. By mounting the second load detecting mechanism 3B, the load detecting mechanisms 3A and 3B are of a differential type, thereby compensating for the temperature dependence of the zero point due to the difference in thermal expansion coefficient between the two materials. In the load cell of the third embodiment, since both load detecting mechanisms 3A and 3B are provided on both sides of the strain body 2, the zero point variation with respect to temperature change is smaller than that of the load cell provided on one side. The effect of heat on the body 2 is smaller than in the load cell of the second embodiment. The circuit for processing the signal from the load cell is the same as that in FIG. 6, and a detailed description thereof will be omitted.
[0045]
In FIG. 7A, the first load detecting mechanism 3A is mounted on the front surface M side of the flexure element 2, and its base 4A is attached to the second mounting flange 8B extending from the beam 2d like a screw B1. A lever portion 5A extending in the vertical direction is supported to the right of the base portion 4A via a fulcrum P1. The load point P2 of the lever 5A is connected to the first mounting flange 8A extending from the beam 2c at the left position of the lever 5A by a fastening means such as a screw B2 via a connecting piece 7B extending in the horizontal direction. Have been. The tuning fork vibrator 6 located to the left of the lever 5A is mounted between the base 4A and the lever 5A via support pieces 7A extending in the horizontal direction. The natural frequency of the tuning fork vibrator 6 changes according to the applied external force (tensile force Fp).
[0046]
In FIG. 7B, the second load detection mechanism 3B is mounted on the rear surface N side of the flexure element 2. The base 4B is attached to the second mounting flange 8B in the same manner as described above by screwing or the like at a position substantially opposite to the fastening position of the screw B1 on the front surface, and to the right of the base 4B via a fulcrum P1. A lever 5B extending in the vertical direction is supported. The load point P2 of the lever portion 5B is connected to the first mounting flange 8A at the right position of the lever portion 5B via a connecting piece 7B extending in the horizontal direction by screwing or the like in the same manner as described above. The tuning fork vibrator 6 located to the left of the lever 5B is mounted between the base 4B and the lever 5B via support pieces 7A extending in the horizontal direction. The natural frequency of the tuning fork vibrator 6 changes according to the applied external force (compression force Fc).
[0047]
The first load detecting mechanism 3A and the second load detecting mechanism 3B are turned upside down, that is, the bases 4A and 4B of the two detecting mechanisms are mounted on the first mounting flange 8A extending from the beam 2c. The load points P2 of 5A and 5B may be attached to the second attachment flange 8B extending from the beam portion 2d via the connecting piece 7B.
[0048]
In addition, the first load detecting mechanism 3A is arranged as shown in FIG. 7A, and the second load detecting mechanism 3B shown in FIG. 7B is moved to a position where the lever 5B is on the left of the base 4B and the base 4B is The attachment point may be attached to the first attachment flange 8A extending from the beam portion 2c, and the load point P2 of the lever portion 5B may be attached to the second attachment flange 8B extending from the beam portion 2d via the connecting piece 7B.
[0049]
With this weighing device, for example, an object to be weighed having a weight of several tens to several hundreds mg can be weighed at a high speed of 100 to 300 times / minute and with a high precision of 1/5000 to 1/10000.
[0050]
In the present invention, the weighing device is applied to a weight sorter or a combination weigher that sorts the weight of an object to be weighed, but may be applied to a weighing device that performs static weighing.
[0051]
【The invention's effect】
According to one configuration of the present invention, the load detecting mechanism is attached between the upper and lower beam portions of the strain body. Therefore, the tuning fork vibrator is fixed between the upper and lower beams because the temperature of the fixed portion and the movable portion of the flexure element is substantially the middle value of each temperature and the temperature difference between them is small. Even if there is a temperature gradient, the influence of heat from the fixed part and the movable part is reduced, and the zero point fluctuation with respect to the temperature change is small and the zero point stability is excellent, so that the load detection accuracy can be improved.
[0052]
Further, according to another configuration, the first and second load detection mechanisms are attached between the upper and lower beam portions of the flexure element, so that the influence of heat from the fixed portion and the movable portion is reduced, In addition to improving the zero-point stability against temperature changes, one of the tuning fork vibrators of both load detection mechanisms is of the tension type and the other is of the compression type. Thereby, when the temperature around the flexure element changes, even if the thermal expansion coefficients of the flexure element and the tuning fork vibrator are different, the temperature deformations of both are canceled out, so they are not affected by the temperature change. High load detection accuracy can be obtained.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a tuning fork type load cell according to an embodiment of the present invention.
FIG. 2 is a configuration diagram showing a signal processing unit of a weighing device using the above load cell.
FIG. 3 is a front view showing a load detection mechanism.
FIG. 4 is a partially enlarged view showing the tuning fork type load cell of FIG. 1;
FIG. 5 is a perspective view showing a tuning fork type load cell according to another embodiment.
FIG. 6 is a configuration diagram illustrating a signal processing unit of a weighing device using the load cell.
FIG. 7 is a side view showing a tuning fork type load cell according to another embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Tuning fork type load cell, 2 ... Distortion body, 2a ... Fixed part, 2b ... Movable part, 2c ... Beam part, 3 ... Load detection mechanism, 3A ... 1st load detection mechanism, 3B ... 2nd load detection mechanism, 4A , 4B: base, 5A, 5B: lever, 6: tuning fork vibrator, 36: intermediate material.

Claims

A fixed part and a movable part to which a load is applied are arranged on the left and right, and a flexure element in which the movable part is supported by the fixed part via upper and lower beam parts, and a load detection mechanism attached to this flexure element A load cell having
The load detection mechanism,
A base portion attached to one of the upper and lower beam portions, a lever portion supported on the base portion via a fulcrum, and a base portion mounted between the base portion and the lever portion, and each of which is unique in accordance with an applied load. A tuning fork vibrator whose frequency changes,
A tuning fork type load cell, wherein a load point of the lever portion is connected to the other of the upper and lower beam portions.

In claim 1,
A tuning fork type load cell in which an intermediate material made of a material having a coefficient between the two thermal expansion coefficients is inserted between the beam portion and the base portion.

A load cell according to claim 1 or 2, and a weighing value generation circuit for obtaining a weighing value from a signal obtained by a load detection mechanism of the load cell, wherein a fixed portion of the load cell is fixed to a base and covered by a movable portion. A weighing device to which a weighing table that supports weighing objects is fixed.

A fixed part and a movable part to which a load is applied are disposed on the left and right sides, and a flexure element in which the movable part is supported by the fixed part via upper and lower beams, and a first load detection mechanism mounted on the flexure element And a load cell having a second load detection mechanism,
Each of the first load detection mechanism and the second load detection mechanism
A base attached to one of the upper and lower beams of the flexure element, a lever supported on the base via a fulcrum, and a base attached to the base and the lever, and applied to the applied load. A tuning fork vibrator whose natural frequency changes according to
The load point of the lever portion is connected to the other of the upper and lower beam portions,
A tuning fork type load cell which is set so that when a load is applied to a movable portion, a tensile force acts on the tuning fork vibrator of one load detecting mechanism and a compressive force acts on the tuning fork vibrator of the other load detecting mechanism. .

In claim 4,
A tuning fork type load cell in which the two load detecting mechanisms are mounted on one of the front and rear surfaces of a strain body.

In claim 4,
A tuning fork type load cell wherein the first load detecting mechanism is mounted on a front surface of a strain body and the second load detecting mechanism is mounted on a rear surface of the strain body.

7. A load cell according to claim 4, further comprising a weighing value generation circuit for obtaining a weighing value from a difference between signals obtained by both load detection mechanisms of said load cell, wherein a fixed portion of said load cell is fixed to a base. A weighing device in which a weighing table that supports an object to be weighed is fixed to a movable portion.