Voltage Clamp behaviour

General discussion of using Roboclaw motor controllers
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Joined: Wed Jun 20, 2018 7:39 am

Voltage Clamp behaviour

Postby rwschumann » Fri Jun 22, 2018 6:38 pm

I am using a switched mode power supply (24V supply @ 3.5 amps) to drive a small 24V motor (peak load observed is around 1.2 amps). Per the documentation I have put in a voltage clamp circuit, but I was still controller errors (appearing as Low logic voltage, but in fact power also goes down to 4 V in some cases.

In trying to debug this I am trying to confirm that the voltage clamp is working correctly. It appears that the S4/S5 clamp circuits are not behaving as I expect.

For debugging I have:
1) Hooked motor to m2
2) Connected S4(also tried S5) via a 4K resistor to ground.
3) Set the S4 mode to Voltage Clamp
4) Hooked up logic analyzer
5) On PWM screen used the slider to drive the motor speed up/down

6) I never see S4 switch states (it stays low but has lots of noise ~ 1V)
7) Eventually the controller errors out with a "Low logic voltage" state.

Any ideas?

Thanks for your help.

Posts: 8
Joined: Wed Jun 20, 2018 7:39 am

Re: Voltage Clamp behaviour

Postby rwschumann » Sun Jun 24, 2018 1:14 pm

Never Mind I figured it out.

Was not an issue with the voltage clamp circuit at all, was an issue with too high of a momentary load on the switching power supply.

I slowed down the acceleration and problem went away. As acceleration was slowed it was easy to watch the dips in the source voltage minimize.

One item which would be very helpful in this regard is a programmatic way to set the max deceleration and max acceleration variables. They are accessible from Ion but do not appear to be accessible using packet commands.

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Basicmicro Support
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Joined: Thu Feb 26, 2015 9:45 pm

Re: Voltage Clamp behaviour

Postby Basicmicro Support » Tue Jun 26, 2018 8:47 am

1. You can set the maximum default accel/decel for each motor in general settings. This applies to Analog, RC and Packet Serial commands without an accel argument.

2. I recommend you set your min main battery voltage to 1v below the switching power supply voltage and the maximum voltage to 1v to 2v above the switching power supply voltage.

By setting the minimum voltage the motor controller will stop accelerating if the voltage dips down to that level.

Setting the Maximum voltage sets the point when the external votlage clamp will be activated(if you are using an external clamp) or the point when the motors will be put into braking mode to disipate the regen energy.

In otherwords, Min Main Battery limits how low the power supply will be drawn under a load and Max Main Battery limits how high the votlage will be allowed to go under regen.

Posts: 8
Joined: Wed Jun 20, 2018 7:39 am

Re: Voltage Clamp behaviour

Postby rwschumann » Tue Jun 26, 2018 12:56 pm

Excellent thank you for the helpful suggestions.

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