# Overcoming cogging/ripple torque in gimbal motor

So I'm trying to figure out how to spin a camera on a brushless gimbal motor. I am feeding each of the terminals a sinusoidal pwm signal from a via 3 half bridges on the SN754410. Each of the pwm signals is 120 degrees apart in phase.

When this signal is fed into the motor I get a kind of cogging (almost like a clock) and not smooth motion. see this https://www.youtube.com/watch?v=tDNnhxuPoEQ

I am wondering how normal gimbal drivers (for quadcopters and stuff) accomplish smooth motion. From what I understand, you can create acceleration maps with a magnetic position sensor and then do compensation (inverse and apply) to get smooth motion but it seems like those gimbal motor drivers don't use magnetic postion sensors.

How do those gimbal drivers work?

• Am am also interested if any insight into the method used in the youtube video can be given. I'm not sure exactly how he accomplishes compensation. Apparently he uses a DDS (direct digital synthesizer) – Daniel Caoili Feb 25 '18 at 23:02
• Torque is proportional to current. youtube.com/… You need to define your design and test results in the context of what does electrical part does not meet capability – Sunnyskyguy EE75 Feb 25 '18 at 23:09
• Cogging can be a sign that the PID loop is too aggressive in correcting known position to desired position. Either the derivative is too much (for rapid short-term changes), or the integral is too soft, allowing overshoots in both directions. There are mechanical shock absorbers, but the PID needs to be finely tuned for a given motor and mass. – Sparky256 Feb 25 '18 at 23:26
• Cogging is also a characteristic of PMSM with BLDC exhibiting the highest amount of cogging. – JonRB Feb 26 '18 at 0:01
• @DanielCaoili Take a look at the current that goes into the BLDC, it should be almost perfectly sinusoidal when the BLDC is continuously rotating. Maybe your SPWM has a modulator frequency (i.e. the triangular wave) so low that it's not filtered by the RL low pass filter (i.e. each BLDC phase), thus leading to torque ripple. – Chi Feb 26 '18 at 3:15