EP0763784B1

EP0763784B1 - Method for detecting an end portion of a recording paper in a recording apparatus and end portion detection apparatus

Info

Publication number: EP0763784B1
Application number: EP96306667A
Authority: EP
Inventors: Naoto Yamaguchi; Hiroshi Ishida; Masayuki Kumazaki; Kazuhiko Yamaguchi; Yukihiro Uchiyama
Original assignee: Seiko Epson Corp
Current assignee: Seiko Epson Corp
Priority date: 1995-09-13
Filing date: 1996-09-13
Publication date: 2001-11-21
Anticipated expiration: 2016-09-13
Also published as: JPH09136741A; DE69617146T2; EP0763784A2; EP0763784A3; JP3395822B2; DE69617146D1

Description

The invention relates to technology for detecting a leading edge and a trailing edge of a recording paper, suitable for use in a recording apparatus such as a serial recording apparatus wherein a recording head is moved in the direction of a width of a recording paper and printing is carried out, and a page recording apparatus transferring a toner image formed on a photoconductive drum to a recording paper.
A recording apparatus detects the leading edge of a recording paper loaded from a paper supply port, sets the position of the recording paper leading edge at a constant position with respect to a recording line, carries out what is known as an initial printing position alignment operation and furthermore, if necessary, detects a trailing edge of the recording paper and carries out an operation to stop recording.
Detection of the leading and trailing edges of a recording paper is carried out by a switch disposed within the paper feed path which switches on by abutting with the recording paper and switches off by breaking contact with the end portion of the recording paper.
However, the problem of low detection accuracy exists due to such factors as the choice of mechanical construction for apparatus in which the structure and signal processing is simple.
For this reason, when a recording paper end portion is to be detected with a high level of accuracy, a method is adopted in which a sheet end detector comprising a light-emitting element irradiating a recording paper with light and a light-receiving element receiving reflected light from a recording paper is disposed in a position where it is possible to intercept light reflected from a recording paper, and the level of a signal from the sheet end detector is compared with a previously set standard level.
Because, as shown in FIG. 6(a), the light beam S1 of the light-emitting element which forms the sheet end detector has a slight spread, and in the same way the interception area S5 of the light-receiving element also has a slight spread, when recording paper P moves towards a printing region as shown by the arrow in the drawing, the amount of light reflected by the recording paper changes, and the signal level shown in FIG. 6(b); that is, the detector voltage, changes in proportion to the paper feed amount.
Then, because the amount of reflected light also changes according to the reflectance ratio of the recording paper, the rate of change of the detector voltage, that is, the slope, is also influenced by the recording paper reflectance ratio, finally it changes up to a saturation voltage Vm1 and Vm2 for the recording papers A and B respectively, decided by the respective recording paper reflectance ratio, and afterwards becomes constant.
For these two reasons, the amount of paper feeding up to the point of reaching a standard level standard value Vs determined to be the leading edge is different due to the different reflectance ratios of a recording paper A and a recording paper B. Thus, when the leading edge of a recording paper P is identified an error ΔL occurs in the position where the paper is halted.
Because there are naturally variations in the level of irradiation of the light-emitting element and in the detection sensitivity of the light-receiving element which make up the sheet end detector, in order to reduce the size of this error ΔL, it is necessary to be selective and employ means in which the characteristics are the same, and this causes the problem of increased component costs.
GB 2220742A relates to the detection of the presence of a recording medium in a printer.
In order to achieve the above-mentioned objective, a method for detecting an end portion in a recording apparatus is proposed wherein, regardless of the characteristics of the paper; and moreover without the necessity for a high level of uniformity in a light-emitting element and a light-receiving element which compose a sheet end detector, it is possible to detect an end portion of a recording paper with a high level of accuracy.
Furthermore, another objective of this invention is to provide a recording paper end portion detection apparatus for implementing the above mentioned method.
In a first aspect, this invention provides a method for detecting a recording media end portion in a recording apparatus, wherein a recording media is irradiated with light and the position of an end portion is detected based on an amount of reflected light from said recording media, the method comprising the steps of:
memorizing the amount of reflected light from a recording media guide means;
feeding forward a recording media, detecting the amount of reflected light of said media and calculating the difference between that amount of reflected light and the amount of light reflected from said media guide means; and
setting a standard level, characterised by
the step of setting a standard level including multiplying said difference by a value C which is greater than 0 and less than 1;
pulling said recording media back in an upstream direction to a position where it is possible to detect the amount of light reflected only by said recording media guide means; and
feeding forward said recording media and determining that a leading edge of said paper has reached a standard position at a point in time when the amount of reflected light equals said standard level.
In a second aspect, this invention provides an end portion detection apparatus in a recording apparatus, comprising:
a sheet end detector comprising a light-emitting element and a light-receiving element, detecting changes in the amount of light associated with movement of a recording media;
a standard value calculation means which detects an amount of difference (Vm - Vp) or (Vp - Vm) between said sheet end detector platen voltage Vp resulting from light reflected from a recording media guide means and said sheet end detector media saturation voltage Vm resulting from light reflected from a recording media, and calculates a standard value; and
a paper feed control means for feeding forward a recording media, characterised by
said standard value (Vm - Vp) x C or (Vp - Vm) x C being calculated by multiplying by a constant coefficient C, where C is a numerical value greater than 0 and less than 1;
a comparing means for comparing said standard value and a signal from said sheet end detector and recognizing a point in time when said signal corresponds to said standard value as being indicative of the presence of a leading edge or trailing edge of a recording media; and
said paper feed control means being for feeding forward a recording media before commencing printing to a position where it is possible to detect said recording paper saturation voltage, then pulling said recording media back in an upstream direction to a position where it is possible to detect said platen voltage, and finally feeding forward said recording media to where a leading edge of said recording media is identified by said comparing means.
Thus, in order to solve the above problems, the invention consists of a method of detecting a recording paper end portion in a recording apparatus, wherein a recording paper is irradiated with light and the position of an end portion is detected based on an amount of reflected light from said recording paper, comprising the steps of: a process for recording the amount of reflected light from a recording paper guide means; a process for advancing a recording sheet, detecting the amount of reflected light of said sheet and calculating the difference between that amount of reflected light and the amount of light reflected from said media guide means; a process of setting a standard level by multiplying a value C which is greater than 0 and less than 1 by said difference; a process of pulling said recording paper back in a downstream direction to a position where it is possible to detect the amount of light reflected only by said recording paper guide means; and a process of feeding said recording paper in advance and determining that a leading edge of said paper has reached a standard position at a point in time when the amount of reflected light equals said standard level.
Because the paper feed amount necessary for the amount of reflected light only from a recording paper guide means to reach the amount of reflected light only from a recording paper is constant regardless of the characteristics of the paper, the amount of the difference between the amounts of reflected light of the recording paper guide means and recording paper is constant, and the standard level obtained when this amount of difference is multiplied by a factor C is also constant, the relative position of a recording paper end portion with respect to the sheet end detector at the time when the amount of light has changed up to the standard level is also constant.
Embodiments of the invention will be described, by way of example only, with reference to the accompanying diagrammatic figures, in which:
FIG. 1 (a) and (b) are respectively drawings showing a serial recording apparatus to which the sheet end detection technology of the present invention is applicable, and the construction of the vicinity of a carriage thereof.
FIG. 2 is a block diagram showing one embodiment of a sheet end detection apparatus in the above-mentioned apparatus.
FIG. 3 (a) and (b) are respectively drawings showing the recognition operation of the position of a leading edge of a recording paper in the above-mentioned apparatus when the amount of reflected light from a recording paper guide means is less than that of a recording paper, and when it is higher than that of a recording paper.
FIG. 4 is a flow chart showing mainly the recognition operation of a recording paper leading edge and trailing edge in the above-mentioned apparatus.
FIG. 5(a) and (b) are respectively drawings showing the recognition operation in the above-mentioned apparatus of the position of the leading edges of recording paper and the trailing edges of recording paper when the amount of light reflected from each paper is different in the case where the amount of reflected light from a recording paper guide means is less than that of a recording paper, and where it is higher than that of a recording paper.
FIG. 6 (a) and (b) are respectively drawings showing the detection area of the sheet end detector and errors which occur when recording papers with a different amount of reflected light are aligned for printing.
Below, the present invention is explained based on an embodiment shown in the drawings.
FIG. 1(a) is a drawing showing an embodiment of the present invention, reference numeral 1 in the drawing is a carriage mounted with a recording head 2 (such as an impact wire type recording head, an ink jet type recording head or a thermal transfer type recording head) driven by a drive motor 5 via a timing belt 7 bridging idling rollers 6, and moving reciprocally along a guide 4 forming a recording head guide means disposed parallel to the axis of a paper feed roller 3.
On carriage 1, in the vicinity of recording head 2 as shown in FIG. 1(b), a sheet end detector 10 is disposed in a position spaced at a constant distance from the recording head 2 basic position. This sheet end detector 10, as is widely known, comprises a light-emitting element 11 irradiating paper feed roller 3, and a light-receiving element 12 disposed in a position where it is possible to intercept reflected light from paper feed roller 3 or a recording paper. Furthermore, reference numeral 13 shows a paper holder guiding a printing surface of a recording paper.
FIG. 2 is a drawing showing the sheet end detection apparatus which is a characteristic of the present invention; reference numeral 20 is a platen voltage detection means, detecting an amount of light in the condition before a recording paper is loaded, that is, the amount of reflected light from paper feed roller 3 forming a recording paper guide means, as the voltage (hereinafter referred to as platen voltage Vp) from a light-receiving element 12 composing sheet end detector 10.
Reference numeral 21 is a recording paper saturation voltage detection means, detecting an amount of light reflected by a recording paper itself as the voltage (hereinafter referred to as paper saturation voltage Vm) from a light-receiving element 12 comprising sheet end detector 10. The platen voltage Vp and paper saturation voltage Vm detected by these means 20 and 21 are respectively memorised in platen voltage memorising means 22 and paper saturation voltage memorising means 23.
24 is a standard level calculating means, calculating [(Vm - Vp) x C] +Vp or [(Vp - Vm) x C] + Vm using the difference between platen voltage Vp and paper saturation voltage Vm memorized in platen voltage memorizing means 22 and paper saturation voltage memorizing means 23, and a positive value C, greater than 0 and less than 1, for example 0.3; and producing therefrom a standard determination level Vs for determining if a leading edge and also a trailing edge of a recording paper have reached a specified position. One of these two calculations is selected in dependence on whether Vp or Vm is larger. The calculation for which the value of the term in square brackets is positive is selected.
25 is a comparing means, outputting a signal when a signal from sheet end detector 10 corresponds to the standard determination level Vs, detecting that either the leading edge or the trailing edge of a recording paper has reached a prescribed position. In particular, it is constructed to output a signal which is output after a recording paper is loaded as a forward paper feed halt signal to a paper feed control means 26, described later.
26 is the previously mentioned paper feed control means; as shown in the flow chart described later, it controls the paper feed motor 8, driving it forwardly and in reverse in order to feed a recording paper, it stops feeding of the recording paper at the point in time when a forward paper feed halt signal is input from comparing means 25, and sets the recording paper at a prescribed position in order to commence printing.
Next the operation of an apparatus constructed in this way is explained based on the graphs and flowchart shown in FIG. 3 (a) and (b), and FIG. 4. FIG. 3 (a) shows the changes in a signal from a sheet end detector when a recording paper guide means is constructed from a material such as rubber which has a reflectance ratio lower than that of a recording paper; while FIG. 3 (b) shows the changes in a signal from a sheet end detector when a recording paper guide means is constructed from a base of metal or the like which has a reflectance ratio higher than that of a recording paper, and paper feeding is carried out by a roller disposed in another location. Note that the values in the case of FIG. 3(b) are denoted by apostrophes.
As shown in FIG. 4, when a power source is switched on (Step A), the presence or absence of a recording paper is detected by the sheet end detector and other paper detectors (Step B), and if a recording paper is present, paper feed roller 3 is rotated and the paper is discharged (Step C).
As light from the light-emitting element 11 of sheet end detector 10 irradiates paper feed roller 3 in the condition when a recording paper is absent, light-receiving element 12 intercepts reflected light from paper feed roller 3. The amount of reflected light from paper feed roller 3 detected by platen voltage detection means 20 is memorized in platen voltage memorizing means 22 as platen voltage Vp or Vp' (Step D).
Next, a recording paper is loaded (Step E), and paper feed control means rotates paper feed motor 8 in a forward direction and the recording paper is fed forwardly (Step F). When the leading edge of the recording paper reaches the vicinity of sheet end detector 10 due to the feeding, as light from light-emitting element 11 is reflected from paper feed roller 3 and the recording paper, the amount of light admitted to light receiving element 12 changes at a rate of change governed by the reflectance ratio of the recording paper.
Consequently, as shown in FIG. 3 (a) and (b), the voltage of the signal from sheet end detector 10 gradually changes according to the differences in the reflectance ratios of the recording paper guide means and the recording paper. That is, when the reflectance ratio of the recording paper guide means is lower than that of the recording paper the voltage gradually increases, and when the reflectance ratio of the recording paper guide means is higher, it gradually decreases. When the recording paper is moved as far as a position where all of the light from light-emitting element 11 irradiating the paper feed roller 3 is reflected by the recording paper, the signal from the sheet end detector reaches the level of an amount of light governed by the reflectance ratio of the recording paper, that is a saturation amount of light.
By doing this, all the light admitted to light-receiving element 12 becomes the reflected light from the recording paper, and regardless of the amount of paper feeding, the signal from sheet end detector 10 reaches a voltage decided according to the reflectance ratio of the recording paper; that is, paper saturation voltage Vm or Vm' (Step G), and this becomes thereinafter a constant value. Paper saturation voltage detector 21 detects this paper saturation voltage Vm or Vm' and memorizes it in paper saturation voltage memorizing means 23 (Step H).
Meter paper saturation voltage Vm or Vm' is detected and memorized, paper feed control means 26 rotates paper feed motor 8 in reverse, feeds the recording paper in a reverse direction (Step I), and pauses the leading edge of the recording paper outside the detection region of sheet end detector 10. As this pausing operation continues, light from the light-emitting element 11 of sheet end detector 10 irradiates paper feed roller 3 only, and in the stage where the platen voltage is detected again by platen voltage detection means 20 (Step J) the recording paper is fed in reverse by a predetermined number of pulses only (Step K). Otherwise, Step I is repeated.
Next, paper feed control means 26 rotates paper feed motor 8 forwardly and feeds the recording paper forward towards recording head 2 (Step L). By doing this, as the leading edge of the recording paper enters the detection region of sheet end detector 10, the amount of light from the light-emitting element 11 of sheet end detector 10 admitted to the light-receiving element 12 corresponds to the reflectance ratios of the recording paper guide means and recording paper, and changes in proportion to the amount of paper feeding.
Comparing means 25 compares the signal from sheet end detector 10 and the relevant recording paper determination standard Vs and Vs' calculated by standard level calculation means 24 (Step M), at the point in time when the amount of reflected light corresponds to standard values Vs or Vs', it is identified that the leading edge of the recording paper Ns has reached a standard position Nt (Step N), and furthermore, paper feed control means 26 halts the paper feeding operation and positions the leading edge of the recording paper Ns at prescribed position Nt (Step O).
Namely, as shown in FIG. 5 (a) and (b), the values of the amount of light in the condition where the light from light-emitting element 11 of sheet end detector 10 is reflected only from the recording paper, that is, paper saturation voltage Vm1, Vm2 or Vm1', Vm2', are influenced by the reflectance ratios due to the characteristics of recording papers A and B. However, the values of the amount of light in the condition where the light from light-emitting element 11 of sheet end detector 10 is reflected only from the paper feed roller 3, that is, platen voltage Vp or Vp', become fixed at characteristic values decided by the reflectance ratio of the paper feed roller. Therefore, each difference between the recording paper and paper feed roller 3: (Vm1 - Vp) and Vm2 - Vp), or (Vp' - Vm1') and (Vp' - Vm2') is determined principally by the reflectance ratio of each recording paper loaded at the time of printing.
Then, the paper feeding amount required for the amount of light reflected only from paper feed roller 3 to change only differences (Vm1 - Vp) and (Vm2 - Vp), or (Vp' - Vm1') and (Vp' - Vm2') is principally determined by the optical coefficients of light-emitting element 11 and light-receiving element 12 constructing sheet end detector 10, that is, the diameter of the light-emitting element beam and the area of possible reception of the light-receiving element.
Consequently, at the point in time when signals corresponding to standard determination values or levels Vsa= [(Vm1 - Vp) x C] + Vp, Vsb= [(Vm2 - Vp) x C] +Vp, or Vsa'= [(Vp' - Vm1') x C] +Vm1', Vsb'= [(Vp' - Vm2') x C] + Vm2' (obtained from multiplying these differences (Vm1 - Vp) and (Vm2 - Vp), or (Vp' - Vm1) and (Vp' - Vm2') by a constant ratio and adding the respective lower value Vp, Vm1' or Vm2') are output from sheet end detector 10, the relative position Nt of the leading edge of the recording paper with respect to sheet end detector 10 becomes constant regardless of the characteristics of the recording paper. This relative position Nt is constant for a given value of C and can be altered by selecting a different value of C.
A print command is input (Step P), printing is commenced (Step Q), printing continues up to the vicinity of the trailing edge of the recording paper, and as the recording paper begins to move away from sheet end detector 10, the amount of light admitted to sheet end detector 10 is governed by the paper feed roller 3 reflectance ratio, and is governed by the ratio of the areas of paper feed roller 3 and the sheet of paper within the detection area of leading edge detector 11, and the signal from sheet end detector 10 gradually changes.
Comparing means 25 compares the signal from sheet end detector 10 and the standard level [(Vm1 - Vp) x C] +Vp, [(Vm2 - Vp) x C] +Vp or [(Vp' - Vm1') x C] +Vm1', [(Vp' - Vm2') x C] + Vm2' (Step R), and when they correspond, it identifies that the trailing edge Ne of the recording paper has reached standard position Nb (Step S). When one page of printing is completed, discharging is carried out (Step. T), if printing data still remains, the process moves to Step (D) again (Step U), and the previously described processes are repeated.
Of course, as these standard levels [(Vm1 - Vp) x C] +Vp, [(Vm2 - Vp) x C] + Vp and [(Vp' - Vm1') x C] + Vm1, [(Vp' - Vm2') x C] +Vm2 are set at the time of commencement of printing based on the reflectance ratio of the recording paper in question, the recording paper which has finished being printed has the same nature as at the time of the first sheet end detection, consequently at the time when the signal from sheet end detector 10 corresponds to the standard level value for the second time, the trailing edge Ne of the paper in question reaches a constant position Nb with respect to sheet end detector 10.
Namely, as shown in FIG. 5 (a) and (b), the values of the amount of light in the condition where the light from light-emitting element 11 of sheet end detector 10 is reflected only from the recording paper, that is, paper saturation voltage Vm1 and Vm2, or Vm1' and Vm2', are influenced by the reflectance ratios due to the characteristics (paper roughness etc.) of recording papers A and B. However, the values of the amount of light in the condition where the light from light-emitting element 11 of sheet end detector 10 is reflected only from the paper feed roller 3, that is, platen voltage Vp and Vp', become fixed at characteristic values decided by the reflectance ratio of the paper feed roller. Therefore, each difference between the recording paper and paper feed roller 3: (Vm1 - Vp) and (Vm2 - Vp), or (Vp' - Vm1') and (Vp' - Vm2') is determined principally by the reflectance ratio of each recording paper loaded at the time of printing.
Then, the paper feeding amount required for the amount of light reflected only from the recording paper to change only differences [(Vm1 - Vp) x C] +Vp and [(Vm2 - Vp) x C] +Vp, or [(Vp' - Vm1') x C] +Vm1' and [(Vp' - Vm2') x C] +Vm2' is principally determined by the optical coefficients of light-emitting element 11 and light-receiving element 12 constructing sheet end detector 10, that is, the diameter of the light-emitting element beam and the area of possible reception of the light-receiving element.
Consequently, at the point in time when signals corresponding to determination standard values Vsa= [(Vm1 - Vp) x C] +Vp, Vsb= [(Vm2 - Vp) x C] +Vp, or Vsa'= [(Vp' - Vm1') x C] +Vm1', Vsb'= [(Vp' - Vm2') x C] +Vm2', are output from sheet end detector 10, the relative position Nb of the trailing edge Ne of the recording paper with respect to sheet end detector 10 becomes constant regardless of the nature of the recording paper. This relative position Nb is constant for a given value of C an can be altered by selecting a different value of C.
Thus, even with the variations in strength of light emission and area of irradiation in light-emitting elements 11, and sensitivity and range of possible interception of light-receiving elements 12 composing sheet end detectors 10 disposed in a plurality of recording apparatus; these are peculiar to each sheet end detector 11, that is, as they are factors determined unrelated to the type of recording paper, even if there are variations between recording apparatus in the position where a leading edge or trailing edge is identified, the position where the leading edge or trailing edge is identified in each recording apparatus is constant, irrespective of the nature of the paper. Therefore, as variations between recording apparatus in the position where a leading edge or trailing edge is identified can be easily corrected by adjusting the position of the sheet end detector 11, as a result it is possible to detect the leading edge and trailing edge with a constant level of accuracy. Alternatively, variations can be corrected by altering the value of C used.
Furthermore, in the embodiment described above, it is explained in the case where it is applied to a serial type apparatus, but even if used as a recording paper detection means for a page recording apparatus transferring a toner image formed on a photoconductive drum to a recording paper, it is clear that a similar effect can be obtained.
Yet further, in the embodiment described above, the leading edge and trailing edge of a recording paper are detected by the same detector, but if a second sheet end detector having the same construction for detecting a trailing edge is disposed in a position suitable for detecting the trailing edge, even if the amount of light detected by this second sheet end detector is compared to a standard level determined when printing first commences, if there is at least no influence due to changes in the nature of the paper, the same position can be detected as the trailing edge.
As explained above, the present invention comprises a process for recording the amount of reflected light from a recording paper guide means; a process for advancing a recording sheet, detecting the amount of reflected light of said sheet and calculating the difference between that amount of reflected light and the amount of light reflected from said media guide means; a process of setting a standard level by multiplying a value C which is greater than 0 and less than 1 by said difference; a process of pulling said recording paper back in an upstream direction to a position where it is possible to detect the amount of light reflected only by said recording paper guide means; and a process of advancing said recording paper and determining that a leading edge of said paper has reached a standard position at a point in time when the amount of reflected light equals said standard level; wherein the leading edge and trailing edge of a recording paper can be identified at a high level of accuracy regardless of the nature of the recording paper and the detection region and sensitivity of the sheet end detector, and the standard for components making up the sheet end detector can be relaxed, and the cost can be reduced.
Although the above description is given in terms of a recording paper, the present invention can be applied to any recording media.
The aforegoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention.

Claims

A method for detecting a recording media end portion in a recording apparatus, wherein a recording media is irradiated with light and the position of an end portion is detected based on an amount of reflected light from said recording media, the method comprising the steps of:

memorizing the amount of reflected light from a recording media guide means (3);

feeding forward a recording media, detecting the amount of reflected light of said media and calculating the difference between that amount of reflected light and the amount of light reflected from said media guide means; and

setting a standard level, characterised by

the step of setting a standard level including multiplying said difference by a value C which is greater than 0 and less than 1;

pulling said recording media back in an upstream direction to a position where it is possible to detect the amount of light reflected only by said recording media guide means; and

feeding forward said recording media and determining that a leading edge of said paper has reached a standard position at a point in time when the amount of reflected light equals said standard level.
The method for detecting a recording media end portion in a recording apparatus of claim 1, further comprising:
a process wherein during the conveying process of said recording media, a second point in time when said amount of reflected light again corresponds to said standard level is determined as being indicative of the presence of a trailing edge of a recording media.
The method for detecting a recording media end portion in a recording apparatus of claim 1, further comprising:
selecting a value C in such a way that a standard level can be set between the reflectance ratio of said recording media guide means and a reflectance ratio of a recording media closest to the reflectance ratio of said recording media guide means.
An end portion detection apparatus in a recording apparatus, comprising:

a sheet end detector (10) comprising a light-emitting element (11) and a light-receiving element (12), detecting changes in the amount of light associated with movement of a recording media;

a standard value calculation means (24) which detects an amount of difference (Vm - Vp) or (Vp - Vm) between said sheet end detector platen voltage Vp resulting from light reflected from a recording media guide means (3) and said sheet end detector media saturation voltage Vm resulting from light reflected from a recording media, and calculates a standard value; and

a paper feed control means (26) for feeding forward a recording media, characterised by

said standard value (Vm - Vp) x C or (Vp - Vm) x C being calculated by multiplying by a constant coefficient C, where C is a numerical value greater than 0 and less than 1;

a comparing means (25) for comparing said standard value and a signal from said sheet end detector and recognizing a point in time when said signal corresponds to said standard value as being indicative of the presence of a leading edge or trailing edge of a recording media; and

said paper feed control means (26) being for feeding forward a recording media before commencing printing to a position where it is possible to detect said recording paper saturation voltage, then pulling said recording media back in an upstream direction to a position where it is possible to detect said platen voltage, and finally feeding forward said recording media to where a leading edge of said recording media is identified by said comparing means.
The end portion detection apparatus as claimed in claim 4, wherein:
when the sheet end detector platen voltage Vp is greater than the sheet end detector media saturation voltage Vm, said standard value is [(Vp - Vm) x C] + Vm; and when the sheet end detector media saturation voltage Vm is greater than the sheet end detector platen voltage Vp, said standard value is [(Vm - Vp) x C] + Vp.
The end portion detection apparatus in a recording apparatus of claim 4 or claim 5, wherein:
said comparing means outputs a signal at a point in time during the printing process when a signal from said media end detector corresponds to said standard value.
The end portion detection apparatus in a recording apparatus of any one of claims 4 to 6, wherein:
a value C is selected in such a way that a standard level can be set between the reflectance ratio of said recording paper guide means and a reflectance ratio of a recording paper closest to the reflectance ratio of said recording paper guide means.
The end portion detection apparatus in a recording apparatus of any one of claims 4 to 7, wherein:
said paper feed control means stops a paper feeding operation in response to a first signal from said comparing means.
The end portion detection apparatus in a recording apparatus of any one of claims 4 to 8, wherein:
said paper feed control means feeds forward a recording paper before commencing printing to a position where it is possible to detect said recording paper saturation voltage, then pulls said recording paper back towards a downstream side by a further constant amount to beyond a position where it is possible to detect said platen voltage, and finally feeds forward said recording paper to where a leading edge of said recording paper is identified by said comparing means.
The end portion detection apparatus in a recording apparatus of any one of claims 4 to 9, wherein:
said sheet end detector is arranged in such a way that a light-emitting element and a light-receiving element are optically opposed by said recording paper guide means.