PWM current control with FPGA - help with setup

[napatel81]

PWM current control

I have a solenoid with 0-0.8mA current range driven using PWM. The coil resistance is 14ohms and inductance is 14mH. I am using an FPGA to do the closed loop current control, Servoing at 24K. The feedback current is read through a 1 ohm sense resistor. Required pwm resolution is 12bit.

Does anyone have enough experience to help me determine if this is a good configuration??

 

[kender]

PWM current control

I'm probably missing a piece, but usually, the constant current driving is done with an OpAmp. FPGA could provide a setpoint for the OpAmp via PWM. Is that what you were planning to do? Are you planning to run your servo loop at the rate of 24kHz?

 

[napatel81]

Re: PWM current control

Yes i do have an opamp for feedback. The servo rate is 20k and PWM frequency is 24K. Does that sound good?

 

[kender]

PWM current control

I have a feeling that a good PWM frequency would be a multiple of the servo rate (20kHz, 40, 60 etc).
Correct me if I'm wrong, but if you want 12-bit resolution with 20kHz frequency, 80MHz clock.

 

[napatel81]

Re: PWM current control

Yes you are right the clock should be 80MHz. Dont you thing there would be aliasing problems if the PWM frequency is equal to servo rate.

 

[kender]

PWM current control

You're probably right about aliasing (although 24kHz will have some aliasing too). I was thinking that PWM generator and servo engine were synchronized.

 

[FvM]

PWM current control

Do you actually mean 0-0.8mA current range, 1 ohm sense resistor, PWM to control it?

 

[Mr.Cool]

perhaps this will help: http://www.youtube.com/watch?v=aOuI0UlwAkk

Mr.Cool