If the motor jumps slightly and the FAULT light immediately turns back on, then either the motor is wired backwards or the 
trimpots are misadjusted. Check the trimpot settings. If they seem right then switch the motor leads and try again. If it still
doesn’t work and you followed all the previous steps, call me at the number at the end of this document. 

Now turn on your STEP pulse source and ramp the speed up to see if the motor turns. It should turn clockwise with a 
logical “1” on the DIRECTION input. 

The optimum way to tune the servo is to induce an impulse load on the motor while watching an oscilloscope to see how 
the motor behaves in response, then adjusting the PID co-efficients (GAIN and DAMPING trimpot settings) for optimal 
behavior. 




In all cases the motor must return to the command position, what matters is how it does it. The manner in which the motor 
returns to its command position is called damping. At one extreme called overdamped response, the motor returns to 
position after a long, drawn out delay. At the other extreme called underdamped response, the motor returns to its position 
too rapidly, overshoots, returns and undershoots and so on until it finally settles at its command position. This is also 
caused ringing; when extreme, the over/undershoot builds in amplitude until the motor enters violent oscillation. Between 
the two extremes is the optimal response called critical damping. Here the motor rapidly returns to its position with little or
no overshoot in the minimal amount of time. 

GAIN AND DAMPING 

GAIN and DAMPING settings generally track each other. If you increase GAIN (greater stiffness), then increased 
DAMPING is needed as well to restore critical damping. Be careful, increasing GAIN without increasing DAMPING may 
cause the motor to break out into violent oscillation. 

The higher GAIN is set, the noisier the motor will be when stopped. This is because higher gain causes more vigorous 
dithering between encoder counts at rest. There is a trade-off between high gain (high stiffness) one hand and excessive 
dithering (noise and motor heating) on the other. Use judgment here. 

To see how your servo is compensated it is first necessary to induce a disturbance. The easiest way is to switch the 
DIRECTION input while commanding a constant speed via the STEP input. The abrupt direction change puts just the 
momentary load needed on the motor while you watch how it responds. 

If you are using an oscilloscope, use channel 1 on the test point and channel 2 on the DIRECTION input. Set the trigger to 
“normal”, trigger source to channel 2 and trigger edge to “+”. You should see a single sweep for every clockwise change 
in direction. 

Slowly increase STEP speed until you get a picture similar to one of the three above, and then do the following: 

1) OVERDAMPED: Decrease DAMPING or increase GAIN 
2) CRITICALLY DAMPED: Do nothing; you’re there 
3) UNDERDAMPED: Decrease GAIN or increase DAMPING 

(POSITION ERROR TEST POINT NOTE) 

Don’t confuse the POSITION ERROR with the motor or machine position. The signal is actually the differential position 
error between the command speed and the motor speed. As noted above, sending clockwise STEP pulses moves the 
POSITION ERROR voltage more positive while turning the motor clockwise moves the POSITION ERROR voltage more 
negative. 

When the motor encoder counts match the number of STEP pulses being sent one for one, the POSITION ERROR 
voltage stays at +5VDC. If the motor gets ahead of the STEP pulses such as during very rapid deceleration, the voltage 
will decrease by 0.04 volts for every encoder count the motor is ahead of the STEP pulses sent. The PID algorithm will 
force the motor to match the STEP input over time and restore the POSITION ERROR voltage back to +5 VDC.