drv8308: Deceleration under a load causes runaway condition

Dustin Hagstrom

Part Number: DRV8308

Hi all,

I'm certain I'm not seeing the correct setting to make the motor brake correctly. I've implemented a speed control system with the closed loop Speed register supplying a value between 0-4095. Everything works great for speeding up the motors to the desired speed. My issue comes from when the motor is under an external load and I set the speed register to a slower speed, I then see that the motor "runs away" and does not brake at all (or it is not comparable to the load that is forcing it).

Any ideas on what I might be missing? I would expect the system to apply the brakes in order to match the speed set in the speed register. Maybe I have to manually apply the brake pin to force the brake when I detect the speed too high? If that is the case how quickly can I pulse the brakes to ensure smooth braking?

Any help would be very much appreciated.

SPDMODE is set to 2 (Internal Register PWM Mode)

over 8 years ago

0 Dustin Hagstrom over 8 years ago

Prodigy 60 points

Oops I meant Open Loop PWM Speed Control

0 Rick Duncan over 8 years ago in reply to Dustin Hagstrom

TI__Guru** 114905 points

Hi Dustin,

Our experts have been contacted and should reply soon.

Can you provide more details?
What was the sequence of writes to the SPI?
What type of external load is the motor under?

0 Dustin Hagstrom over 8 years ago in reply to Rick Duncan

Prodigy 60 points

Absolutely,

The motor is one of 2 wheels on a mobile device that carries a human (like a wheel chair), I am writing the SPEED register about 10 times per second in accordance to the user's joystick commands. The errors I'm seeing where the motor "runs away" are when the user is going down hill, so the users weight + the device weight will be forcing the motors to turn faster than the SPEED command sent to the register.

I hope this helps.

0 Phil Beard over 8 years ago in reply to Dustin Hagstrom

TI__Expert 4650 points

Hi Dustin,

Thank you for the information.

In the case you outlined, the motor is being forced to spin faster than the speed command by your PWM signals. In this case, I would recommend applying BRAKE commands from your external speed loop to slow the motor down or decrease your SPEED register value when you see that the motor is moving to fast.

0 Dustin Hagstrom over 8 years ago in reply to Phil Beard

Prodigy 60 points

Thank you very much for the reply.

From your answer, it seems that the DRV8308 system will not automatically apply the brake in the case I described above, I'm just trying to make sure I understand your reply as best as I can.

Is it ok to send a PWM signal to the brake pin to apply the brake?

0 Phil Beard over 8 years ago in reply to Dustin Hagstrom

TI__Expert 4650 points

You are correct, the device will not automatically apply a BRAKE behavior in the case described above.

It is okay to send a PWM signal to BRAKE pin to tun on the three low-side MOSFETs.

0 Dustin Hagstrom over 8 years ago in reply to Phil Beard

Prodigy 60 points

Thank you so much! You are amazing.

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drv8308: Deceleration under a load causes runaway condition