Need help to (re) design a motor speed control

Thread Starter

Thanacity

Joined May 12, 2021
7
I am looking for someone who can help me (re) design / build a motor speed control for one of my hobby projects. The small motor, with coupled tachometer, should run at some preset speeds based on the feedback from the DC tachometer (up to about 12vdc).

The tacho voltage is compared to a reference voltage (preset speed) and then gives a pulse to the motor until the preset speed is reached.

I have a description of the system as well as some diagrams. The diagrams are for an automatic system with an number of inputs. I only want a manual system with a speed selector switch (5 speeds).

Thanks!
 

Attachments

dendad

Joined Feb 20, 2016
4,168
I would use an Arduino. Easy to change as required.
But, first, what is the specs of the motor?
You need to be able to handle the stall current, not just the running curret so as not to blow the driver up in a fault cindition.
Is it only one direction or 2?
 

Thread Starter

Thanacity

Joined May 12, 2021
7
I would use an Arduino. Easy to change as required.
But, first, what is the specs of the motor?
You need to be able to handle the stall current, not just the running curret so as not to blow the driver up in a fault cindition.
Is it only one direction or 2?
The motor unit is already taken care off and has it's own power supply. There are only three connections to run/control the motor 1. signal to run the motor (off or full speed)
2. tacho output and
3. ground
 

dendad

Joined Feb 20, 2016
4,168
What voltage levels are the control signals?
and is the run actually just an on/off or speed as well? PWM?
It is strange to have a motor with a tacho that does not support speed control.
Have you tried feeding a PWM signal into the input to see if the speed varies?
 

Thread Starter

Thanacity

Joined May 12, 2021
7
What voltage levels are the control signals?
and is the run actually just an on/off or speed as well? PWM?
It is strange to have a motor with a tacho that does not support speed control.
Have you tried feeding a PWM signal into the input to see if the speed varies?
The reference DC voltages are 1.15V / 2.3V / 4.6V / 9V and 18V (just checked today). To be generated by a 28VDC power supply (already in place).
The motor speed is regulated by just witching on / off. I guess this was a modification from an old design (1960's). At first I thought it would be continuously variable but this is not the case. At first I tried to feed signals into the motor input but later I found in the description that the motor speed is controlled by an external rate selector (see schematic that i posted / note: this schematic might have a combination of circuits to run 5 separate motors).
The way I think it works is as follows....
The reference speed (voltage) is selected and the motor runs until the reference voltage is reached (by the tachometer), then switches off. Once the tacho voltage drops 0.2 volt (my guesstimate) the motor starts again. So the relays are switching on and off all the time.
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
Normally now it is done with PWM speed control using a small microprocessor that reads the Opto feedback and compares it to the set/required speed in order to control the RPM.
.
 

Thread Starter

Thanacity

Joined May 12, 2021
7
Normally now it is done with PWM speed control using a small microprocessor that reads the Opto feedback and compares it to the set/required speed in order to control the RPM.
.
Good enough for me but how do i go f
Normally now it is done with PWM speed control using a small microprocessor that reads the Opto feedback and compares it to the set/required speed in order to control the RPM.
.
You are probably correct but i want to stick to the original design (to avoid having to modify the instrument) which means that the motor control must supply the run/stop signal as described. Thanks
 
Top