GB2201259A - Positioning control - Google Patents

Abstract

Positioning apparatus for driving a member (10) to any one of a plurality of predetermined destinations, comprising a bi-directional A.C. electric motor (14), location means (19) associated with said member and having identification means (20) for each of said destinations, sensor means (18) for producing a signal on sensing said identification moans, and a control system for the motor; the control system comprising input means for selecting the desired one of said destinations, means for causing the motor to drive said member towards the selected destination at a first speed, means responsive to the one of said signals which denotes the arrival of the member at the selected destination to stop the motor, means for then causing the motor to drive said member in the opposite direction at a second speed slower than the first speed to compensate for over-run of said member, and means to stop the motor in response to the re-detection by the sensor means of the identification means for the desired destination. The second speed mode of the motor is achieved by removing portions of the A.C. waveform used to energise the motor, switching taking place only during zero crossings of the A.C. waveform. The member (10) may be a frame movable to different positions to receive different sizes of copy sheets in a xerographic copier. <IMAGE>

Description

This invention relates to a positioning apparatus for driving a member to any one of a plurality of predetermined positions. The apparatus of the invention is particularly,. although not exclusively, useful in positioning the finisher module of a xerographic copying machine in accordance with paper size.

In a xerographic copying machine having a finisher module, the finisher module typically collates and staples together sets of copies produced by the copying machine. Most xerographic copying machines nowadays handle a variety of paper sizes, so it is desirable to be able to position a finisher module, transversely of the paper feed direction, in one of a plurality of predetermined positions depending on the size of the copy sheets being fed out of the copying machine. One way of moving the module is to use an electric motor, which may be arranged to drive a carriage of the module through a suitable transmission device for converting rotary motion to linear motion, such as a lead screw system.

A problem encountered with a motor drive system of the kind just outlined is that when the motor is switched off as the module arrives at its desired location, the motor overruns, and coasts to a standstill. This problem is aggrevated in a finisher module when a variable, and possibly large, weight of paper has been collected in the finisher tray. Although servo systems and stepper motors are possible solutions to this problem, these tend to be expensive, especially in view of the need for the motor to be able to drive a module full of paper, possibly weighing a few kilograms, as well as the need for additional electroniccircuitry.

Furthermore, systems using stepper motors or servos usually produce substantial emissions of electromagnetic radiation due to their high switching frequencies at high currents. Such emissions can seriously affect the operation of the low voltage logic circuitry of a typical xerographic machine, and can even produce troublesome fields outside the machine.

The present invention is intended to overcome these problems, and to provide reliable and accurate positioning without undue expense compared with the cost of a standard electric motor, as well as to reduce unwanted emissions of electromagnetic radiation.

According to the present invention, there is provided a positioning apparatus for driving a member to any one of a plurality of predetermined destinations, comprising drive means for the member, location means associated with said member and having identification means for each of said destinations, sensor means for producing a signal on sensing said identification means, and a control system for the drive means, the control system comprising input means for selecting the desired one of said destinations, means for causing the drive means to drive said member towards the selected destination at a first speed, means responsive to the one of said signals which denotes the arrival of the member at the selected destination to stop the drive means, means for then causing the drive means to drive said member in the opposite direction at a second speed slower than the first speed to compensate for over-run of said member, and means to stop the drive means in response to the re-detection by the sensor means of the identification means for the selected destination.

Preferably the drive means is a bi-directional A.C. electric motor, which is driven by a normal A.C. waveform in the first speed mode. In the second speed mode, the duty cycle of the motor is reduced, with portions of the waveform in effect being removed by switching during zero crossings of the A.C. waveform. Because the switching takes place during zero current flow conditions, unwanted emissions of electromagnetic radiation are virtually eliminated.

A positioning apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an end elevation of a finisher module showing only those parts relevant to the present invention, Figure 2 is a schematic circuit diagram for illustrating the operation of the apparatus of Figure 1, and Figure 3 shows how an A.C. waveform is modified to reduce the drive speed of the electric motor of the apparatus.

Referring to Figure 1, the finisher module is carried by a movable frame 10 which is slidably mounted on a fixed frame 11. The fixed frame 11 is secured to a xerographic copying machine (not shown) such that of sheets carrying copies produced by the copying machine are fed out into the finisher module. The movable frame 10 moves to the left or to the right as seen in Figure 1, on suitable sliding tracks mounted between left- and right-hand end members 12 and 13 respectively.

The movable frame 10 is driven by a bi-directional A.C. electric motor 14 mounted on a motor frame 15 which is fixed relative to the fixed frame 11. The output spindle of the motor drives a lead screw 16 which cooperates with a link nut 17 that is mounted on right-hand end member 13 of the movable frame 10. Thus on operation of the motor 14, the lead screw 16 drives the link nut 17 and hence the movable frame 10 to the left or the right.

Also mounted on the motor frame 15 is a sensor 18 which has a U-shaped head the two arms of which carry respectively an infra-red emitter and detector, one each side of an index bar 19. The index bar 19 is mounted between the end members 12 and 13 of the movable frame 10, and has a set of rectangular slots 20 in it. The slots 20 are positioned so that when each slot is directly between the emitter and detector of the sensor 18 (so that the radiation from the emitter is "seen" by the detector) the movable frame 10 is in the desired position for a given size of paper sheet fed out by the copier. The index bar is preferably painted matt black to prevent spurious reflections from being sensed by the sensor. In a typical arrangement, there may be 8 slots 20, each of about 1.7 mm width.Safety limit switches 21 and 22 are also provided for cooperation with the left-hand end member 12 of the movable frame 10, to switch off the motor in the event that the motor is not stopped while driving past the last slot in either direction.

By way of illustration, if the arrangement so far described were used to position the finisher module, the movable frame 10 might be driven at a speed of about 29 mm. sex , and the mass of the system is such that on switching off the motor, the finisher module would coast to a position such that the sensor would be about 3 - 4 mm past the selected slot 20.

In order to prevent such coasting, in the present invention, the following approach is adopted. After the motor has been switched off following arrival of the module at its desired location, the motor is reversed after a delay (at least 100 msec) and driven at about 25% of its original speed. The motor continues to be driven at this slower speed in a direction opposite to the original direction until the sensor is once again activated as the relevant slot becomes aligned with the sensor 18. At this point the motor is again switched off. Frequently, this will locate the module exactly, there being almost no tendency to coast at the lower speed. In case vibrations move the module slightly backwards, however, the sensor is-again checked after a 100 msec delay. If it is still "seeing" radiation, no further movement is made.If, however, a change in radiation level is sensed, the motor is again driven (at the lower speed and in the same direction as it was previously moving at the lower speed) until the sensor is receiving the appropriate amount of radiation to generate the signal which will again switch off the motor.

The reduced speed mode of the motor may be obtained by using a duty cycle wherein the motor is driven for a half-cycle of the AC current, and is then not driven for the whole of the next following cycle. A suitable driving circuit for the motor is shown in Figure 2. The motor 14 is energised for rotation in either one of its two rotational directions by applying an A.C.

voltage between an A.C. neutral line 25 and either one of two connections 26 and 27. Current is supplied to connections 26 or 27, as required to give the desired direction of rotation, by the respective one of triac drivers 28 and 29. Control pulses for drivers 28 and 29 are supplied by the machine processor (not shown) by way of lines 30 and 31 respectively.

Referring to Figure 3, there is shown a graph of motor current (I) vs. time (t), for a 50Hz A.C. waveform when the motor is operated in the reduced speed mode. The machine processor is programmed to enable the motor drive for 10 msec and then to disable it for 20 msec, as represented by the timing chart below the graph in Figure 3. The triac drivers 28 and 29 have internal zero crossing circuits, so the triac is switched on only during a zero crossing. As can be seen from the example shown in Figure 3, assuming the driver is initially 'off', then after an 'on' pulse has been sent to the driver circuit, the motor is switched 'on' the next time the waveform crosses the zero axis. After 10 msec, an 'off' pulse is sent to the driver, so the next time the waveform crosses the axis, the motor is switched 'off' (i.e. at the end of the positive half-cycle shown shaded ). The next 'on' pulse is sent to the driver 20 msec later, so the motor is switched 'on' again at the next zero crossing, which is at the beginning of the negative half cycle shown shaded. This sequence of events is repeated, with a driving half-cycle occurring at one third of the original waveform's rate. This is found to produce a motor speed of about 25% of that for the full waveform.

In the 50Hz case described, it is found that a suitable 'on' time is 9 1 msec, to allow for timing tolerances of the softvvare. The off' time can have similar tolerances, e.g. 19+1 msec, but is less critical. For a 60Hz motor, the 'on' time should be 7 + 1 msec, and the 'off' time 16 1 msec.

Claims (8)

Claims:

1. Positioning apparatus for driving a member to any one of a plurality of predetermined destinations, comprising drive means for the member, location means associated with said member and having identification means for each of said destinations, sensor means for producing a signal on sensing said identification means, and a control system for the drive means, the control system comprising input means for selecting the desired one of said destinations, means for causing the drive means to drive said member towards the selected destination at a first speed, means responsive to the one of said signals which denotes the arrival of the member at the selected destination to stop the drive means, means for then causing the drive means to drive said member in the opposite direction dt a second speed slower than the first speed to compehsate for over-run of said member, and means to stop the drive means in response to the re-detection by the sensor means of the identification means for the desired destination.

2 The apparatus of claim 1 wherein the drive means comprises a bi-directional A.C.

electric motor.

3. The apparatus of claim 2 wherein the means for causing the drive means to drive said member at the first speed comprises means for energising the motor with a complete A.C.

waveform, and the means for causing the drive means to drive the member at the second speed comprises means for energising the motor with portions of the A.C. waveform.

4. The apparatus of claim 3 wherein the means for energising the motor for the second speed includes a zero-crossing triac driver.

5. The apparatus of claim 4 including means for supplying pulses to the triac driver such that the motor is driven for substantially one half-cycle of the A.C. waveform, and is then not driven for substantially one complete cycle, whereby the successive half-cycles in which the motor is driven are of alternating polarity.

6. The apparatus of any one of claims 1 to 5 including means to introduce a delay between stopping the drive means from driving at the first speed and driving the drive means at the second speed.

7. The apparatus of any one of claims 1 to 6 including means to check for a signal from the sensor after a delay following the stopping of the drive means in response to said detection, and to further drive the drive means at the second speed if no signal is found, the drive means being stopped as soon as the signal re-appears.

8. Positioning apparatus substantially as hereinbefore described with reference to the accompanying drawings.