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Simple circuit provides motor-feed control

-July 25, 2002

Because we needed a small grinding machine, we modified an old milling machine that lacked a control system. The table of the grinding machine needed only to move back and forth with adjustable feed. Using an existing dc servoamplifier, a servo motor, and limit switches, we devised the circuit shown in Figure 1. Because the motor had no tachometer, we used part of the motor voltage as feedback. We reduced the feedback voltage from the motor to approximately 8V by using the resistors R1 to R3. (Motor voltage=60V, maximum amplifier input=10V, R1=R2=33 kΩ, and R3=10 kΩ.) This feedback voltage feeds back to the speed-command differential-voltage input. You must be careful with the feedback-signal polarity to avoid an uncontrolled runaway of the motor. The actual command voltage connects to the tachometer input, which is not differential. Using the appropriate gain and control-loop adjustments available on most drivers, you can obtain good motor response. This design uses the ±15V the driver supplies to power the control circuit. Switch S1 enables manual and automatic modes. For both modes, potentiometer P1 reduces the control voltage, and C1 filters it. Two LEDs, D1 and D2, show in which direction the axis moves. This indication can be especially useful in automatic mode if the potentiometer is turned to its zero position. The driver becomes disabled in one direction when the inputs +Limit or –Limit no longer connect to 15V; that is, when the limit switches LS+ or LS– (located at each end of travel) become activated. The following describes the operation of the two modes:

  • In manual mode, the momentary switch S2, selects 15V or –15V. If you use two separate switches, take care to avoid shorting both power supplies together.

  • In automatic mode, the polarity of the voltage depends on the setting of relay K1. Upon power-up, K1 is off; thus, a positive voltage goes to the driver. The motor moves in the positive direction until limit switch LS+ activates. At that instant, the driver is disabled (for the positive direction), and the relay, K1, energizes. K1 holds itself on through its contact, diode D3, and the limit switch, LS–. You need the diode to avoid feeding voltage to the –Limit input through the activated LS+ limit switch. The motor now runs in the opposite direction until the limit switch LS– activates. At that instant, the driver is disabled (for the negative direction), and relay K1 turns off. The cycle begins anew.

The values of the components the circuit uses depend principally on the selected servoamplifier and motor, thus Figure 1 shows no values. The machine has worked satisfactorily in two shifts for more than two years. During this period, contrary to our expectations, we never readjusted the servoamplifier.

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