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Tacho feedback to 555 pwm motor speed control

throbscottle

Well-Known Member
It occurred to that using a 555 timer to generate PWM to control the speed of a motor allows a simple feedback mechanism via the 555's CV input. All I need to do is produce a voltage which is proportional to the motor speed, and the 555 will adjust it's output until it matches the set point.

Any thoughts on this idea?
 

schmitt trigger

Well-Known Member
Use a LM2917 frequency to voltage converter.
To actually obtain the pulses, I have used both a photo reflective detector or a hall cell.
On the former, paint a pair of white stripes on opposite sides of the flywheel. Unless you have very strong ambient light, this would be the preferred method.
Otherwise, use the hall cell method. Glue a pair of small magnets on opposite sides of the flywheel. Make sure they are properly aligned and are identical in weight to avoid unbalancing the flywheel. Also the glue should be a strong two-part epoxy, unless you want magnets flying off.
 

Diver300

Well-Known Member
Most Helpful Member
If you are using a brushed motor, the current goes up and down with each brush change. If you can measure fast and accurately enough, and the PWM doesn't upset things, you can read the motor speed from the ripple of the current taken.
 

gophert

Well-Known Member
Most Helpful Member
If you are using a brushed motor, the current goes up and down with each brush change. If you can measure fast and accurately enough, and the PWM doesn't upset things, you can read the motor speed from the ripple of the current taken.
If he can gate it, he can measure back emf voltage during off-time of the duty cycle. Easier with a microcontroller.
 

crutschow

Well-Known Member
Most Helpful Member
that using a 555 timer to generate PWM to control the speed of a motor allows a simple feedback mechanism via the 555's CV input.
The CV 555 input only gives a limited range of PWM duty-cycle change versus voltage.
Much better to use a PWM circuit that allows a full-range adjustment of duty-cycle.
Here's an example of a simple PWM circuit using an LM339 that allows a PWM duty-cycle range of 0-100%.

1608824777650.png
 

throbscottle

Well-Known Member
I love it when simple questions turn up interesting stuff. I merely had it in mind as a way to keep the speed up when the motor is loaded, but phrased it a bit obscurely. My original thought was to use optical feedback to create a speed-related voltage, but I like the idea of getting it from the armature current rate of variation
 

schmitt trigger

Well-Known Member
If you use go for the armature current method, make sure to include a low pass filter to remove commutator noise.

Another good method is back EMF measurement. A Silicon Chip article actually designed a very elegant circuit.
 

schmitt trigger

Well-Known Member
I read that in an actual print magazine perhaps 7 or more years ago.
Silicon Chip actually keeps a good database of all its issues, and you can purchase online a single issue.
 

throbscottle

Well-Known Member
*sigh* And now I am in danger of learning far more about motors than I ever intended to....
I think I'll give that circuit a go, crutschow. Better than playing games with the 555!
Going to be a while though.
Re - gated back emf measurement - would gating even be needed? The pwm could simply be filtered out.
 

Diver300

Well-Known Member
Most Helpful Member
Re - gated back emf measurement - would gating even be needed? The pwm could simply be filtered out.
Part of the idea of gating is that you measure the back EMF when there is no current, or at least a known or consistent current.

If you filter out the PWM, the resistive losses in the motor will need to be compensated for. The average voltage when PWM is applied is just [duty cycle] * [supply voltage] so that doesn't depend on the speed at all.
 

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