The circuit below is speed control for a motor drive. If I set the Min. Speed pot (P8) so that the motor turns at 200 rpm I have -0.348Vdc at the non-inverting input (Pin 5) of the op amp. This gives me 0.002Vdc at the inverting input and -9.2Vdc at the output Pin 7. The op amp is powered +/- 12Vdc

If I then remove power from the system and power back up the motor should immediately spin up to 200rpm again. It does not spin, however. I still have -0.348Vdc at the non-inverting input Pin 5 of the op amp so that side seems to stay constant. And now I measure -0.345Vdc at the inverting input Pin 5 and -4.1Vdc at the output Pin 7.

I've discovered that if I momentarily crank up the voltage reference setpoint going into pin 5 and then turn the reference back down to -0.348Vdc that the control will operate as desired and spin up to 200 rpm. I didn't measure the exact voltage I turned pin 5 up to but it's just a quick turn of the reference speed setpoint dial and then back to 0 reference (which gives -0.348Vdc at pin 5).

My thought is that the capacitors on the inverting side of the op amp need to charge up initially. This won't happen if the speed feedback (which is fed into R1) is just a few mV because the motor is not spinning yet.

Thoughts on my diagnosis and a possible solution would be appreciated.

 

I just took off the C1 capacitor and the control works as it should now with 0 reference. However, this gets rid of the derivative action.

Is there a way to move the derivative to the output of the op amp instead of the input? I'm thinking something like this:

 

I have not performed a careful analysis but talk of using 2 millivolt direct coupled signals in an LM358 circuit makes the hair on the back of my neck stand up on end. The A version has a maximum offset of 2 millivolts already, any other offsets or gain errors might prevent your circuit from working.

There are many pin-compatible dual opamps that give a much lower offset and have other benefits that have been developed since the LM358 was developed (about 40 years ago).