Digital Motor Control System

1. Field of the Invention

This invention relates to a motor control system to be used for regulating the speed of a variable speed motor, and is more particularly directed to a system for controlling the speed of a scan motor as used in a variable magnification ratio photocopier.

2. Description of the Prior Art

In conventional motor control systems the measured speed of the motor is compared with the desired speed of the motor and the feedback signal directly proportional to this difference in speed is applied to force the motor to approach the target speed. Although this form of motor control is adequate for most uses, it is incapable of providing the close control of the motor speed which is required for driving a movable scanning system in a variable magnification ratio photocopier.

The typical variable magnification ratio photocopier includes a movable scanning system, a lens system, and a rotable photosensitive drum. The scanning system cooperates with the lens system to transmit an image of an original document to the surface of the photosensitive drum. To obtain an enlarged or reduced copy of this original document, the lens system must first be correctly positioned with respect to the scanning system, and then the speed of the scanning system must be adjusted to the correct speed ratio with respect to the speed of the photosensitive drum. Once the scanning system has been accelerated from rest to the scanning speed, close control over the scanning speed must be maintained to achieve an accurate photocopy.

This is achieved in prior art photocopiers by providing mechanical linkage between the scanning system and the rotable photosensitive drum surface. This prior art approach suffers from two defects. First, it is difficult to provide more than a small number of discrete magnification ratios when utilizing a mechanical system. Secondly, the optical performance of the photocopier deteriorates as the mechanical systems wear which necessitates periodic maintenance.

In contrast to the systems of the prior art the digital motor control system of the present invention utilizes digital techniques to develop the control signals for a variable speed motor which is used to drive the movable scanning system of the photocopier. This motor control system provides at least three distinct advantages over a mechanical system. First, the digital motor control system provides accurate and stable optical performance of the photocopier. Secondly, the system permits automatic compensation for mechanical wear of the mechanical components of the photocopier. And thirdly, the control system permits selection of a large number of magnification ratios.

Information which is processed by the digital motor control system is expressed and manipulated in the form of digital words. The digital motor control system includes a variable speed motor and means for sensing the rotational speed of this motor. The measured rotational speed of the motor is expressed as a motor speed word. Means are provided for producing a target word which is indicative of the desired motor speed. Means for comparing the motor speed with the target word and calculating a motor control word from the data are provided. Means responsive to this motor control word are provided for driving the variable speed motor.

The operation of the comparing means described above is improved by providing means for sequentially comparing the motor speed word with the target word to form a succession of error words. These error words are indicative of the deviation of the measured motor speed from the desired or target motor speed. By, providing means for successively storing sequential values of the error word and subsequently comparing the sequential error words with each other, an acceleration word is produced. In this manner, information concerning the instantaneous velocity and acceleration of the motor is developed. Based upon this information a correction word is calculated, which contains information concerning both the speed of the motor as well as the acceleration rate of the motor. Means are provided to alter the control word which regulates the speed of the motor by an amount proportional to the correction word such that the speed of the variable speed motor converges with the desired motor speed as represented by the target speed word.

In many applications the target speed word which represents the desired speed for the motor may not be constant and may fluctuate with time. In the preferred embodiment the motor control system is utilized to control the speed of a variable speed motor used to drive the movable scanning system of a variable magnification ratio photocopier. As described above, accurate reproduction of enlarged or reduced copies requires the speed of the scanning system be accurately synchronized with the rotational speed of the photosensitive drum. Consequently, the target speed for the scanning system must reflect the measured speed of the photosensitive drum and the desired speed ratio between the photosensitive drum and the scanning system. In the preferred embodiment this requirement is met by providing means for sensing the rotational speed of the photosensitive drum and generating a reference speed word which is indicative of the measured speed of the drum. Further, in this embodiment data reflecting the desired magnification ratio for the photocopy is entered and is represented as a speed ratio constant word. This data is used to calculate a target word which will reflect the desired speed for the scanning system with regard to the desired magnification ratio and the measured speed of the photosensitive drum. Once the target word has been developed the motor control sequence proceeds as described above.

The FIGURE is a combined block and pictorial diagram representing the preferred embodiment of the present invention.

In the FIGURE the digital scan motor control system and associated photocopier components are shown in block diagram form. The photocopier includes a movable photosensitive surface represented by a rotable drum 10 driven by a constant speed drum motor 11. Graphic information conveyed by a system 12 is transmitted to the photosensitive surface by a lens system 9. This scanning system is driven by a variable speed motor 13 which is responsive to the output of an analog amplifier 14. The analog amplifier 14 amplifies an analog signal produced by digital to analog converter 26.

Sensing means 15 are associated with the variable speed motor 13 to provide an indication of scan motor speed. A sensor 15 such as a photoelectric cell is interfaced with a digital counter 16 which in turn is interfaced with a digital latch 27. These components coact to represent the speed of motor 13 in digital form. Likewise, a sensor 17 is associated with drum drive motor 11 to sense the rotational speed of the photosensitive drum 10. Sensor 17 is interfaced with a digital counter 18 and digital latch 28 to develop a digital representation of the speed of the photosensitive drum 10.

An input device 19 shown as a keyboard is provided for entering magnification ratio data. This input device 19 as well as the digital latches 27, 28 and the D to A converter 26 are interfaced to an input/output device 21.

This input/output (I/O) device 21 is one component of a microprocessor system generally designated 20. Other elements of the microprocessor system include an arithmetic logic unit (ALU) 22, a random access memory (RAM) 24, a system clock 25 and a programmable read only memory (ROM) 23.

This microprocessor system 20 accepts input data concerning the speeds of the photosensitive drum and the scanning system as well as the magnification ratio data and utilizes this information to calculate a motor control signal which is coupled to the D/A converter to regulate the speed of the variable speed motor.

The data which is utilized by this control system is represented and manipulated in the form of digital words. In the specific embodiment, the data is represented as signed binary numbers grouped into eight bit words. The resolution offered by this encoding scheme has been sufficient to achieve close control over the scanning system speed.

Data developed by the hardware external to the microprocessor system 20 is interfaced with the microprocessor system through the I/O device 21. The I/O device has, in the preferred embodiment, four ports 30-33. This device permits reading and writing data to and from external devices. The reading and writing processes are performed under software control and involve the use of the ALU 22 to fetch data from the I/O device 21 and either manipulate it directly or transfer it to memory for later use. A system clock 25 is provided and is required for both timing sequences used within the microprocessor system itself and for providing an accurate and stable time base for the counters 16 and 18.

The data which is read and manipulated results in the formation of a motor control word which is outputted through the I/O port 33 to the D to A converter 26 which converts the digital information into an analog signal. This analog signal is amplified by an amplifier 14 and is used to drive the variable speed motor 13. The control sequence which results in the calculation of the motor control word is performed and is best understood in a step-wise fashion. However, a brief summary of the operation of the digital motor control system as used in a variable magnification ratio photocopier will aid in an understanding of the step-wise description.

As described before, the accurate reproduction of copies in a variable magnification ratio photocopier requires that the speed of the scanning system be accurately controlled as a function of the speed of the photosensitive drum surface. Thus, for a given magnification ratio there exists unique scanning system velocity which must be achieved and maintained. This unique scanning system speed is referred to as the target speed. The target speed which may be constant in some embodiments, but which in the preferred embodiment, depends upon both the desired magnification ratio and the rotational speed of the drum. At the beginning of a scanning cycle, the scanning system is at rest and the photosensitive drum member is rotating at approximately a constant rotational speed.

The first task for the motor control system during the start-up phase at the initiation of a scanning cycle is to accelerate the scanning system as rapidly as possible from rest to a speed close to the desired scanning speed. This task is accomplished by first inputting a desired magnification ratio data from keyboard 19 into I/O port 30. The ALU 22 transfers the magnification data, which is represented as speed ratio constant word C, into the RAM memory 24. The value of this speed ratio constant word multiplied by an empirically derived constant K.sub.1, is used to decrement sequential values of a control word F.degree.. As shown in control word block 40, F.degree. refers to the start-up phase and the subscripts n, n-1, etc. refer to the successive increments, which are empirically determined to be sufficient to result in both the control word F.degree..sub.n and the starting speed of the scan motor to be within allowed ranges of their respective values for acceptable control. For example, in a preferred embodiment, eleven such increments are used, each at ca. 8 msec. spacings. After a series of decrements of the control word F.degree., the scan motor 13 is accelerated to within an acceptable speed range. At the same time, the control word F.degree. reaches an equally acceptable value, thus completing the start-up phase.

Throughout this description, the direction of the arrows indicates the flow of information through the control system. For example, the calculation of the initial value of the control word previously explained proceeds as follows. Magnification ratio data from keyboard 19 is introduced into I/O port 30 as is indicated by the arrow connecting the two. This data is represented as a speed ratio constant word C, shown coupled to the I/O port 30. This word is transferred through the ALU 22 into random access memory 24 as indicated by the arrow. Next, an initial value for the control word F.degree. is calculated in the ALU 22 by fetching the speed ratio constant word C and multiplying it by an empirically derived constant K.sub.1, which is stored in read only memory (ROM) 23. The operation which multiplies K.sub.1 by C and the arrow which connects the two with F.degree..sub.n, as shown in the diagram, indicates that the quantity F.degree..sub.n-1 and the product K.sub.1 C are combined to the control word F.degree..sub.n. The arrow connecting the control word F.sub.n in the ALU with the control word in the I/O port 33 represents a data transfer from the ALU 22 to the I/O port 33. The arrow connecting the motor control word F.sub.n in the I/O port 33 with the digital to analog converter 26 indicates the transfer of digital information from the I/O port 33 into the D to A converter 26. The arrow connecting the D to A converter 26 with the analog amplifier 14 indicates the transfer of analog information proportional to the digital information. Similarly, the arrow connecting the analog amplifier 14 with the motor indicates that the analog amplifier drives motor 13 to control its speed.

Once the variable speed scan motor 13 is traveling at approximately the correct scanning speed, the second phase begins and error correction procedures of the motor control system are initiated. The first step in the second phase is to sense the rotational speed of the drum drive motor 11 by sensor 17. In preferred embodiment, sensor 17 is a conventional magnetic pickup. The output of the sensor 17 is a pulse signal whose repetition rate is inversely proportional to the rotational speed of motor 11. The output of sensor 17 is applied to the counter 18 which is a free-running binary counter. This counter counts clock pulses generated by the system clock 25 for a time period determined by the pulse repetition rate of sensor 17. Consequently, the number of counts between successive increments m is inversely proportional to the speed of the photosensitive drum 10. The value of the binary number contained within the counter 18 is stored within latch 28. The latch is interfaced with the I/O port 31 and when that I/O port location is read by the ALU 22, the value of the latch, referred to as the reference speed word B.sub.m, is transferred through the ALU 22 and stored in RAM 24. Since the drum motor 11 is not synchronously driven with the scan motor 13, the increments n and m are not necessarily the same. Successive values of the reference speed word B.sub.m are obtained by subtracting sequential values of the latch 28, such that at a constant drum speed, the reference speed word B.sub.m is similarly constant. At this point, sufficient information has been introduced into the microprocessor 20 to permit the calculation of a target word D.sub.m. The product of the reference speed word B.sub.m1, the speed ratio constant word C and a constant K.sub.4 are multiplied to form the target word D.sub.m as shown in target block 42. The constant K.sub.4 is a normalization factor between the drum motor and scan motor speeds, and represents the various gear ratios and the like used in coupling the scan motor 13 to the scanning system 12. The target word D.sub.m thus represents the desired speed for the scan motor 13.

The speed of the scan motor 13 is measured by a sensor 15 coupled to a counter 16 and latch 27. These elements cooperate as described with respect to the drum motor system to generate a motor speed word A.sub.n which may be read from the I/O port 32. Successive values of the motor speed word A.sub.n are likewise derived from sequential values of the latch 27 such that as the actual speed of the scan motor 13 changes, the value of the speed word A.sub.n is changed. The successive values of A.sub.n are then transferred by the ALU 22 into RAM 24.

An error word E.sub.n representing the difference between the previously described motor speed word A.sub.n and the target speed word D.sub.n is next derived as shown in error word block 41. Sequential values of the error word E.sub.n, E.sub.n-1 and E.sub.n-4 are transferred to RAM 24. Once sequential values have thus been stored in memory, the acceleration word H is calculated by subtracting the error word E.sub.n from the error word E.sub.n-4, i.e., four increments earlier, as shown within acceleration word block 44. This ensures that a sufficiently large time span is included such that the scanning system respond to changes in the control word F.sub.n. Finally, the correction word G.sub.n is calculated by multiplying the acceleration word H.sub.n by an empirically derived constant K.sub.3 and adding this to the product of the error word E.sub.n multiplied by another empirically derived constant K.sub.2 as shown in correction word block 43. The correction word G.sub.n, which is the sum of these two products thus contains both acceleration and velocity information.

This correction word is used to alter successive values of the control word F.sub.n by taking the algebrative sum of the previous value F.sub.n-1 and the correction word G.sub.n as shown in block 40. The successive values of F.sub.n are outputted through the I/O port 33 to alter the speed of scan motor 13 as described above.

This error correction process is repeated periodically at a rate matching the pulse rate of the scan motor sensor 15 and provides updated information to the scan motor 13 throughout the scan cycle.

In a specific embodiment, the control word is updated at a rate which corresponds to approximately 0.030 inch (0.76 cm) movement of the scanning system. The frequent updating of the control word achieves close control and accurate synchronization between the scanning system 12 and the rotation of the photosensitive drum 10.

It is important to note that the value of the control word F.sub.n "floats" and its magnitude depends on the drag that the scan motor must overcome as well as on any change in speed of the drum 10. As a consequence, the average value of the scan motor word F.sub.n may vary between ostensibly identical machines. Also, the value of the control word may vary over the course of a single scan cycle. Accordingly, variations of the control word may be used as a diagnostic tool to monitor the mechanical condition of the scanning system. The automatic compensation feature provided by the "floating" value of the control word also permits the control system to adapt for the mechanical wear of the photocopier as it ages. This feature also obviates the need to make final adjustments of the mechanical systems of photocopiers on the production line, thus reducing the manufacturing costs for the photocopiers.

The digital motor control system is preferably implemented with the 8085 microcomputer system manufactured by and available from Intel Corp., 3065 Bowers Ave., Santa Clara, California. A partially documented assembly language listing of software used in the present invention is filed with the patent application. ##SPC1## ##SPC2## ##SPC3## ##SPC4## ##SPC5## ##SPC6##