K8004 PWM how to make speed feedback loop?

Discussion in 'The Projects Forum' started by smilem, Aug 19, 2010.

  1. smilem

    Thread Starter Senior Member

    Jul 23, 2008
    I use velleman K8004 PWM controller card to drive DC motor.

    The voltage from my PSU to the K8004 PWM kit is 12V
    The motor is rated for 12V
    The Amps rating on motor is unknown but I assume no more than 2-2.5Amps

    When I adjust the resistor on K8004 PWM kit (configuration C internal reference voltage 1K POT) the voltage on motor changes.
    At start there is 0V then 8.4V at Max speed. This is the same for forward and back spin direction.

    To change the spin direction I use 3 position switch on K8004 PWM kit that just changes wire polarity connecting the kit to PSU.

    The voltage on the 1K pot changes too, at start there is 0V - at Max speed 4.8V

    Manual: http://www.velleman.be/downloads/0/illustrated/illustrated_assembly_manual_k8004_rev3.pdf

    I have Canon Printer Carriage Motor QK1-1500
    TN425813 12V DC.




    I control the motor via 1K pot. I use internal reference voltage to control the motor speed. But as the load changes so does the motor speed.

    How can I adjust the speed like I do now via POT but be able to lock it?
    Please see manual page 8 for connection options.

    The K8004 is just a motor driver PWM circuit. But I can adjust speed by connecting it like diagram A (Use separate control voltage). I mean it can be driven by another circuit board that adjusts voltage according to the motor speed.

    Back emf might speed control or IR sensor speed control?
    I would like to have another circuit that I can plug-in to the PWM withoud modyfying it because my simulator doesn't have the parts to simulate this and I don;t want to damage the PWM that is working fine :)

    Thank you.
  2. gootee

    Senior Member

    Apr 24, 2007

    You could use a classical "automatic feedback control system". But you would need a feedback signal (voltage) that corresponded to the motor's _actual_ speed.

    The basic idea is: If you had a feedback voltage that indicated the motor's actual speed, and you scaled it to match the amplitude range of the voltage that was used to control the speed, then you could SUBTRACT the feedback voltage from the control voltage to get an "error" voltage, which you could then SUM with the (downstream) control voltage, to get an automatically "corrected" control voltage to apply to the motor.

    That way, whenever the motor's speed tried to change and the control voltage didn't change, the feedback system would try to automatically change the applied control voltage to keep the motor at the speed that the original control voltage was commanding.

    In practice, especially if the motor speed-control voltage might change relatively quickly, you might need to also include error-response-shaping circuitry, to make the system behave well.

    For a basic servo-type control, especially if there is not too much concern about the dynamic behavior, it is often sufficient to use a single op amp configured as a differential integrator, and usually also a low-pass post-filter, to create the correction signal. Examples of the individual sub-circuits for that are shown at http://www.fullnet.com/~tomg/gooteesp.htm , in the section entitled "DC Servo for Audio Amplifier". But they would be configured somewhat differently for your application.

    The basic op amp circuits that you might need are given in Application Notes AN-31 and AN-20, at http://www.national.com .

    Does the motor voltage (or average voltage) change when its speed is changed by a load? If so, then maybe it could be conditioned for use as the feedback voltage. If not, you might need some additional hardware to measure the speed and generate the feedback voltage.

    But note that I didn't look at your circuit very closely. So maybe there's an easier or better way.


  3. smilem

    Thread Starter Senior Member

    Jul 23, 2008
    Well the PWM motor driver supports as it seems 0-5Voltage control signal.

    I do have some nice encoder wheels and IR encoder sensors from canon printers, will post them.

    How can I interface the encoder with my PWM driver circuit, I need to buy some kit that has encoder support and outputs 0.5V control singnal? Only I can't seem to find any.

    Obviously control signal 0-5V is made for a reason as I've seems other kit's to use same 0-5V control signal.