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Author Topic: Unexpected waveforms with ST VNH2SP30 H Bridge  (Read 1130 times)
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Hi all,

Until recently I was using an LMD18200T H Bridge for small motor control (~5A max) in a project of mine, but have come across the VNH2SP30 bridge from ST. So, I've made up a little SMD to DIP adaptor to be able to prototype with the device, as its a MultiPowerSO30 package only. Here's the datasheet for the device.

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/CD00043711.pdf

I've got the device hooked up as per the application diagram on page 20.

I've got a piece of test code running, which just sweeps from 0 to max duty, back down, reverses direction and repeats. Powering up, all is as expected - the motor sweeps no problem, at ~.5A no load the IC and motor etc is stone cold. Fine.

So, just thought I'd hook my scope up to check out what's going on - and the waveform across the motor is more than a little strange.

The following shot is two scope channels, each one on each motor terminal



This shot is one motor terminal, the other is the PWM output from my Uno



So, just to check something isn't majorly wrong - I drove a 56ohm resistive load (lowest value resistor I had in the house tonight) and got this...



Any thoughts as to why the waveform is like this, it's not the typical inductive waveforms I've seen before?

EDIT - Fixed image links.
« Last Edit: March 07, 2013, 06:10:58 pm by jtw11 » Logged

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You gave the link to the data sheet instead of the wave forms from your scope
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Just fixed that before I read your post - I normally use OS X, but had booted into Windows and by habit was trying to copy and paste using Cmd, not Cntl - my bad!
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I presume this is a brushed DC motor you are driving - in which the waveform looks perfectly plausible.
The regular spikey bits are the commutator switching transients, the undulations in the back EMF from the
armature are due to the geometry of the motor.

The 0V segments are when the PWM is on, the high segments are the inductive kick-back taking the
bridge into fast-decay mode at the end of an on period.  Only after fast-decay is finished does the
back EMF signal become visible (and the switching transients which presumably are inter-winding
currents being interrupted as the brush passes over a gap in the commutator)

You might try spinning the motor by hand while looking at its output on the scope.
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Quote
I presume this is a brushed DC motor you are driving

It is, yes.

Quote
in which the waveform looks perfectly plausible.
The regular spikey bits are the commutator switching transients, the undulations in the back EMF from the
armature are due to the geometry of the motor.

The 0V segments are when the PWM is on, the high segments are the inductive kick-back taking the
bridge into fast-decay mode at the end of an on period.  Only after fast-decay is finished does the
back EMF signal become visible (and the switching transients which presumably are inter-winding
currents being interrupted as the brush passes over a gap in the commutator)

I see, however - the very same motor produced the following waveform when being driven by an LMD18200 device, as opposed to the VNH2SP30. Admittedly, one is an analog scope - the other is digital, not sure if that should make a difference?



I drove another larger motor today that draws roughly twice the current, and got a very similar waveform.



...and if I stall the motor, I get a very squarewave like waveform.



If I enable the recorded noise band on my digital scope, it looks like there is a serious amount of noise being thrown around - now, I'm not sure if this is the sort of noise you'd visibly see on an analog scope? So everything I've seen in the past may have had these noise components, I've just never seen them.



Quote
You might try spinning the motor by hand while looking at its output on the scope.



Perhaps I can't quite spin it long enough or fast enough by hand...

So, any thoughts on; a) why the different waveforms with the same motor, at the same frequency between two different devices? The only thing I can think of, is the LMD18200 was being run in locked anti-phase PWM mode... and b) regarding noise, how to reduce this? I've got a HV 10uF cap across the motor terminals, and 220uF at the 12V supply to the IC.


« Last Edit: March 08, 2013, 04:05:38 pm by jtw11 » Logged

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The LMD18200 was being driven in fast decay mode only (maybe because a higher PWM rate), so the waveform is rectangular.
The back=EMF signal only shows up when at least one of the arms of the bridge is floating and the PWM off time is long enough
for the winding current to decay to zero.

And I mean spin the motor open-circuit, not connected to the bridge - you'll get a wavey DC output possibly with switching glitches.
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