VELOCITY SERVO WITH POSITION ERROR CORRECTION

Abstract: A velocity servo comprising a servo drive package (7) including a servo motor (8) and a feed back loop including a tachometer (9) coupled to the servo motor (8). An analog velocity command (6) and a command pulse train (21) are generated by a velocity profile generator (1) and the command pulse train (21) and a feedback pulse train from a pulse generator (10) are connected to a position error counter (11) which compares these pulse trains and generates a digital output representa­ tive of the difference therebetween. This digital output is transformed to an analog position error signal (22) having the correct polarity, and an operation amplifier (13) sums the velocity command and the position error signal to drive the servo drive package (7).

Description: The present invention relates to a servo circuit for a velocity-controlled servo motor as disclosed in document EP-A-0 070 654. Such a servo circuit comprises a velocity profile generator including means for generating an analogue velocity command and a commanded position pulse train, a servo drive package including a servo motor and a feedback loop including a tachometer coupled to said servo motor and supplying a feedback signal to said servo motor, means for generating an analogue position error signal including a pulse generator coupled to said servo motor for detecting the current position of said servo motor, a position error counter to which said commanded position pulse train and a feedback pulse train from said pulse generator are supplied, said position error counter comprising means for comparing the commanded position pulse train and said feedback pulse train for generating a digital output representative of the difference between the commanded and current positions of said servo motor, digital-to-analogue converter means for transforming the digital output of said position error counter to an analogue position error signal, and means for driving said servo drive package including an operational amplifier for summing said analogue velocity command and said analogue position error signal whereby said servo motor will be driven with simultaneously derived velocity commands and position error signals. Whereas conventional velocity control servo motors in particular for use in tool machines such as cylindrical grinding machines speed by ramping up and ramping down through following errors, it is an object of the present invention to drive a velocity-controlled servo motor with position and velocity commands derived simultaneously from a single data source with higher reproduceable accuracy. The velocity servo will be accordingly operated in such a way that it will instantaneously track both velocity and distance commands. A servo circuit for a velocity-controlled servo motor as defined above is characterized in that said commanded position pulse train and said feedback pulse train from said pulse generator are supplied to said position error counter via an anti-coincidence circuit and an input reversal circuit, the output of said position error counter, following digital-to-analogue transformation by said digital-to-analogue converter, is supplied to said operational summing amplifier via a polarity reversal circuit, and in that the switching action of said input reversal circuit and said polarity reversal circuit are controlled by a polarity control circuit supplied at respective inputs thereof with said feedback pulse train and said commanded position pulse train after passing said anti-coincidence circuit. Other objects and advantages of the present invention will become apparent from the accompanying drawings which illustrate a presently preferred embodiment incorporating the principles of the invention. Referring to the drawings: Figure 1 is an electronic block diagram illustrating a velocity servo forming part of a servo circuit in accordance with the teachings of the present invention; Figure 2 is an electronic block diagram illustrating a portion of an embodiment of the invention; Figure 3 is an electronic diagram illustrating the details of the polarity reversal circuit illustrated in Figure 2; and Figure 4 is an electronic block diagram illustrating an alternate embodiment of a portion of another alternate embodiment of the velocity servo with position error correction made in accordance with the teachings of the present invention. The velocity profile generator 1 includes profile generator PROMs 2 which direct command data sets 3 to a divide by N circuit 4 which is driven by the clock frequency (See U.S. Patent No. 4,402,161). The divide by N circuit 4 transforms the command data sets 3 into the command pulse train containing position and instantaneous velocity intelligence which is directed to a frequency to voltage converter (F/V) 5 which extracts velocity information 6. The output voltage or velocity command 6 of the frequency to voltage converter (F/V) 5 is directed to the servo drive package 7 which inlcudes a velocity servo motor 8. The servo drive package 7 also includes a feed back or velocity loop including a tachometer 9 coupled to the velocity servo motor 8 for stabilizing the speed of the servo motor 8. Also coupled to the motor 8 is a pulse generator 10 which supplies a feedback pulse train to the "down-count" side of a position error counter 11. The command pulse train which is directed to the frequency to voltage converter (F/V) 5, is simultaneously connected to the "up-count" side of the position error counter 11. The number of counts in the position error counter 11 will, acccordingly, equal the difference between the position commanded and the actual position of the velocity servo motor 8. When the servo motor 8 has not advanced the number of pulses supplied by the velocity profile generator 1, a nullity will not exist at the position error counter 11. Such a digital difference will be converted to an analog voltage by a digital to analog converter (D/A) 12, and this position error signal will be summed with the velocity command by an operation amplifier 13 to drive the servo motor 8 with velocity and position error commands derived simultaneously from a signal data source. Such an arrangement will also maintain the correct motor position should the velocity be zero. The input to the servo drive package 7 will accordingly be the velocity command and a value proportional to the positional error of the servo motor 8. In Figure 2, the feedback pulse train is directed to an anti-jitter circuit 14 and then to a four input anti-coincidence circuit 15. The command pulse train is also directed to this four input anti-coincidence circuit 15. The outputs (the command and feedback pulse trains) of the four input anti-coincidence circuit 15 are conjointly connected to a polarity control 16 and to an input reversal circuit 17 which supplies up-counts and down-counts to the position error counter 11. In operation, assuming the first pulse to the polarity control 16 and to the input reversal circuit 17 is a command pulse, the command pulses will pass through the input reversal circuit 17 across the upper parallel path to up count the position error counter 11. Feedback pulses will be passed through the lower parallel path to down-count the position error counter 11. A positive difference (the command pulses exceeding the feedback pulses) will be converted to an analog form in the D/A converter 12 and a polarity circuit 18 will define the positive polarity thereby defining the position error signal. The details of the polarity reversal circuit 18 are illustrated in Figure 3. This circuit includes a pair of alternately operable switches 19,20 and a voltage inverter 21. The polarity signal will operate these switches 19,20 to change the polarity of the position error signal as desired. When the count of the position error counter 11 is reduced to zero, the polarity control will be enabled. Should the next pulse be a command pulse, the condition of the input reversal circuit 17 and the polarity reversal circuit 18 will remain unchanged, and the command and feedback pulses will be processed in the same manner. Should, however, this next pulse be a feedback pulse, the condition of these two reversal circuits 17,18 will change, and the feedback pulse will up-count the position error counter 11 while the command pulses will down-count the position error counter 11 until the count is again reduced to zero. The generated position error signal will now have a negative polarity as defined by the polarity reversal circuit 18. This circuitry prevents the position error counter 11 from passing through zero to the negative range (The position error counter 11 is always up-counted). Less circuitry is required and good monotonicity is achieved. When the count of the position error counter 11 is again reduced to zero, the polarity control 16 will again be enabled. Should the next pulse to the polarity control 16 be a feedback pulse, the condition of the input reversal circuit 17 and the polarity reversal circuit 18 will remain unchanged. Should, however, the next pulse be a command pulse, the polarity control will conjointly reverse the input reversal circuit 17 and the polarity reversal circuit 18. The command pulse will accordingly up-count the position error counter 11 and the feedback pulses will down-count the position error counter 11. Another modification is illustrated in Figure 4. Here the command data lines 19 generated by the profile generator PROMs 2 are simultaneously directed to the divide by N circuit 4 and to a digital to analog converter (D/A) 20. The command data lines 19 are connected to the digital to analog converter (D/A) 20 in the normal numerical significance to generate the velocity command of the divide by N circuit 4.

Claims: A servo circuit for a velocity controlled servo motor, comprising a velocity profile generator (1) including means for generating an analogue velocity command (6) and a commanded position pulse train (23), a servo drive package (7) including a servo motor (8) and a feedback loop including a tachometer (9) coupled to said servo motor (8) and supplying a feedback signal to said servo motor (8). means for generating an analogue position error signal (22) including a pulse generator (10) coupled to said servo motor (8) for detecting the current position of said servo motor (8), a position error counter (11) to which said commanded position pulse train (21) and a feedback pulse train from said pulse generator (10) are supplied, said position error counter (11) comprising means for comparing the commanded position pulse train and said feedback pulse train for generating a digital output representative of the difference between the commanded and current positions of said servo motor, digital-to-analogue converter means (12) for transforming the digital output of said position error counter (11) to an analogue position error signal (22), and means for driving said servo drive package (7) including an operational amplifier (13) for summing said analogue velocity command (6) and said analogue position error signal (22) whereby said servo motor (8) will be driven with simultaneously derived velocity commands and position error signals, characterized in that said commanded position pulse train and said feedback pulse train from said pulse generator (10) are supplied to said position error counter (11) via an anti-coincidence circuit (15) and an output reversal circuit (17), the output of said position error counter (11), following digital-to-analogue transformation by said digital-to-analogue converter (12), is supplied to said operational summing amplifier (13) via a polarity reversal circuit (18), and in that the switching action of said input reversal circuit (17) and said polarity reversal circuit (18) are controlled by a polarity control circuit (16) supplied at respective inputs thereof with said feedback pulse train and said commanded position pulse train after passing said anti-coincidence circuit (15).

Inventors: MCCARNEY, DAVID A., NORRIE ANN, NORRIE, GEORGE O.
Applicants: LITTON INDUSTRIAL AUTOMATION SYSTEMS, INC.
Application year: 1987
Main classification: G05B19
IPC codes: G05B19