Electric motor controller

• US 4,622,500 A
• Filed: 07/11/1985
• Issued: 11/11/1986
• Est. Priority Date: 07/11/1985
• Status: Expired due to Fees

• Current Assignee: Fischer Imaging Corporation
• Original Assignee: The Machlett Laboratories Incorporated (Rtx Corporation)
• Inventors: Budelman, Frederick T. Jr.
• Primary Examiner(s): Shoop, Jr., William M.
• Assistant Examiner(s): Ro, Bentsu
• Application Number:
  US06/754,057
• Time in Patent Office: 488 Days
• Field of Search: 318/310, 318/311, 318/318, 318/327, 318/329, 318/341, 318/599, 318/811, 318/254, 318/257, 318/268, 318/293
• US Class Current: 388/811
• CPC Class Codes: H02P 7/2913 whereby the speed is regulated by measuring the motor speed and comparing it with a given physical value
• CPC Class Codes: Y10S 388/902 Compensation
• CPC Class Codes: Y10S 388/908 Frequency to voltage converter

Abstract

Apparatus is provided comprising a motor having a current applied therethrough. The speed of the motor is a function of the current passing through the motor. An error signal is produced representative of the difference between the motor speed and a selected motor speed. The current applied through the motor is modulated in accordance with the speed error signal and a signal representative of the current in order to control motor speed. With such arrangement, motor current variations or ripple due to rectification are detected and compensated for to allow the speed of the motor to be maintained constant despite such ripple. In accordance with a feature of the present invention, the motor has a speed related to the current passed therethrough. An error signal is produced representative of the difference or error between the motor speed and a selected speed. A switch allows current to pass through the motor or inhibits current from passing through the motor, selectively in accordance with a control signal. The control signal is produced in response to the speed error signal and a signal representative of the motor current. The control signal comprises a pulse train having a nominal duty cycle. The duty cycle is modulated in accordance with the difference between the speed error signal and the signal representative of the current. The frequency of the pulse train is selected to be much higher than the mechanical resonant frequency of a load driven by the motor. Thus, current is passed through the switch and the motor at a much faster rate than the mechanical resonant frequency. The vibration in such load, and concomitant hum, associated with pulsing current through the motor at a rate near the mechanical resonant frequency of the load is thus substantially eliminated.

Claims

1. In combination:

a motor having a pair of input terminals adapted for serially coupling the motor to a power supply, said motor having a speed and direction of rotation related to a current passing therethrough and driving a load having a predetermined mechanical resonant frequency;

means for producing an error signal representative of the difference between the speed of the motor and a selected motor speed;

a first switch disposed in series between the power supply and one of the pair of input terminals of the motor, the first switch being responsive to a first control signal;

a second switch connected in parallel across the pair of input terminals of the motor, the second switch being responsive to a second control signal;

means, responsive to the speed error signal and a signal representative of the motor current, for producing a pulse train having a nominal frequency and duty cycle, such nominal frequency being substantially higher than the mechanical resonant frequency, and for modulating said duty cycle in accordance with the difference between the speed error signal and the signal representative of the motor current; and

means for controlling the speed fo the motor at a frequency substantially higher than the mechanical resonant frequency of the load, said speed controlling means comprising means for coupling said modulated pulse train to the first switch as the first control signal when the rotational speed of the motor is less than the selected speed to increase the motor speed, and for alternately coupling said modulated pulse train to the second switch as the second control signal when the rotational speed of the motor exceeds the selected speed to decrease the speed of the motor.

2. The combination of claim 1 further comprising:

power supply means, fed by an Ac power signal, for rectifying said AC power signal and producing a DC output voltage having an amplitude ripple thereon, said DC output voltage being coupled across the motor and the first switch; and

wherein the frequency of the pulse train is substatially greater than the frequency of said amplitude ripple.

3. The combination of claim 1 wherein the coupling means comprises means for delaying the coupling of the modulated pulse train to the second swtich for a predetermined time after the motor exceeds the selected speed.

4. The combination of claim 1 further comprising means for reversing the direction of rotation of the motor.

5. Apparatus for controlling the rate of rotation of a motor driving a load, the load having a predetermined mechanical resonant frequency, such apparatus comprising:

means for generating a first signal having a magnitude corresponding to the rate of rotation of the motor;

means for comparing said first signal with a reference signal, the reference signal having a predetermined magnitude corresponding to the selected speed of rotation of the motor, said comparing means generating a speed difference signal having a level representative of the difference between the magnitudes of the first signal and the reference signal;

means, responsive to the electrical current flowing in said motor, for generating a second signal having a level corresponding to the magnitude of said current;

means, responsive to the speed difference signal and the second signal, for producing an error signal having a level corresponding to the difference between the speed difference signal and the second signal; and

means, responsive to the error signal, for switchably coupling current to and decoupling current from the motor in accordance with the error signal level at a frequency substantially higher than the mechanical resonant frequency of the load, said power coupling and decoupling means comprising;

(a) means for generating a pulse train having a nominal frequency and duty cycle, such nominal frequency being selected to be substantially higher than the mechanical resonant frequency of the load;

(b) means, responsive to said pulse train, for coupling current to the motor during each pulse of the pulse train and decoupling current from the motor between pulses; and

(c) wherein the pulse train generating means modulates the duty cycle of the pulses in accordance with the level of the error signal.

6. Apparatus for controlling the rotational speed of motor, the motor driving a load having a predetermined mechanical resonant frequency, such apparatus comprising:

means for generating a first signal having a level representative of the actual speed of rotation of the motor;

means for generating a reference signal having a level corresponding to a selected speed of rotation of the motor;

means for subtracting the first signal from the reference signal producing a speed difference signal and for integrating said speed difference signal;

means, responsive to electrical current flowing through the motor, for producing a second signal having a level representative of the level of said current flow;

means for comparing the levels of the speed difference signal and the second signal and for generating an error signal having a level corresponding to the difference between the second signal and the speed difference signal;

means, responsive to the error signal, for generating a train of pulses of nominal frequency and pulse width and modulating said pulse width in accordance with the level of the error signal;

wherein such nominal frequency is selected to be substantially higher than the mechanical resonant frequency of the load; and

means, solely responsive to the train of pulses, for coupling voltage to the motor for the duration of each pulse and for decoupling voltage from the motor during the interval between each pulse of the pulse train.