Secondary PWM of hobby servo motor

Hi, Its a long story but briefly: I have removed the drive electronics / motor / pot from a large hobby servo and have integrated the electronics to drive a larger motor/ gearbox for a linear actuator and have fitted a linear pot to replace the original. I want to be able to adjust the maximum force that the actuator can supply.
Can I adjust the torque output of the motor by applying secondary PWM between the electronics and the two terminals on the motor?. So to be clear, the drive electronics remain intact and a secondary PWM adjustment occurs as the power goes to the motor.
Thanks for any assistance Rodney

Hello, Rodney.

It is not clear how you plan to add a “secondary PWM adjustment” in between the motor driver and the linear actuator motor, but in general adding electronics in between a motor and the outputs of a motor driver to change the behavior of the motor does not seem too practical or straightforward.

Why do you want to control the torque at the output of the motor to your linear actuator? Do you ultimately want to control how much force your linear actuator is pushing with? If you tell me more about what it is you plan to accomplish, I might be able to make some recommendations.


Hi Jon, The hobby servo internals are 1] the driver with electronics bits and pieced on a board 2] the potentiometer which is normally driven off the gear train and 3] the original DC motor which is connected to the electronics by 2 wires. I removed these parts and fitted a different DC motor/ gearbox driving a ball thread and changed the rotary pot to a slide pot [ of similar value] to record the linear motion of the ball nut to create a linear servo actuator specifically for my application.
Yes, the problem is that it might be too powerful and apply excessive loads to the machine I am making, I want to adjust and experiment with the output thrust of the actuator by adjusting the output torque of the motor. I have PWM adjustment module for DC motors suitable for the motor I am using. So, the specific question is " can I fit the PWM unit in the supply wires that connect power to the DC motor to make the DC motor less powerful?"
I know the obvious answer is " try it" but before I do I wanted to get an opinion about it.
Mechanical engineering is not a problem for me but electronics is.

It is difficult to answer this confidently without knowing more about your system, but it seems unlikely to me that that would work. In particular, what “PWM unit” are you considering adding? How are you connecting the motor driver that you removed from your servo to the motor? It would help clarify things if you link to the product page or datasheet the “PWM unit” you are considering using and post a schematic that shows your proposed connections.

By the way, what are you using to generate your hobby servo signals? Can you link to a product page or datasheet for your linear actuator?


Hi, I am using a " servo tester" to provide the control signals to the Hobby servo electronics but I now see your point about the “secondary PWM speed control of the motor”
The linear servo actuator I have made uses the parts out of a 35kg large hobby servo removed and rearranged except 1] larger motor and the motor works fine via the original servo electronics 2] the original pot replaced with a slide pot to track the travel of the actuator.
Now, as I said I can see the problem you have pointed out… thanks.
So, could I put some form of resistor in the supply wires to the motor to reduce the power output?
Thanks again R

Adding resistance to your system does not seem very practical and it would waste a lot of power. It would be more straightforward to supply a lower voltage to your linear actuator. However, the electronics you removed from that servo probably have a narrow operating voltage range (many standard hobby servos are made to operate nominally from 4.8 - 6V), so you might be limited in how much lower your operating voltage can go (and accordingly, how much less thrust your actuator can produce).

If limiting the maximum force your actuator can push with will work for you, you might consider using one of our Jrk motor controllers and limiting the Max. duty cycle instead of using electronics from the inside of a servo. The Jrks can use the output of a linear potentiometer for position control. If you link to the product page or a datasheet for your linear actuator, I can help you determine which Jrk could be appropriate for your system.