PWM offset voltage?

PWM offset voltage?

Postby mwm » Tue Aug 30, 2016 3:57 am

As noted elsewhere, I'm trying to use the FG085 as a servo testing tool. It actually works very well - or at least has a great set of features for that. It will do everything a high end servo tester will do, and more, for not a lot more money.

However, I have a problem with a winch servo, which rotates 4 full rotations from end to end. This servo came off my bench, and isn't very good - but it works fine when attached to a receiver. However, when plugged into the FG085 it goes into continuous rotation mode. This normally means the servo is shot - except that this one works on an Rx.

The only difference between the two signals is that the FG085 base signal is slightly negative (- about 0.15 volts). The PWM modes don't seem to have any way to set the offset, and changing the offset in CW mode doesn't change it. That may be the issue, but I can't really tell without being able to offset it.

This could be an issue with my build - I've noticed that the amplitude and offset voltages tend to be inaccurate, off by as much as 10%, but not in a predictable way. I couldn't find any documentation on expected accuracy, and I can deal with it if it really matters, so I haven't worried about it. But if so, a suggested fix would be nice.
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Re: PWM offset voltage?

Postby jye1 » Tue Aug 30, 2016 2:15 pm

The amplitude and offset can be set by pressing ADJ or Mode button (I don't remember clearly and I'm out of office, sorry). Please see manual or try out those buttons.
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Re: PWM offset voltage?

Postby mwm » Wed Aug 31, 2016 2:18 am

I started by looking at the manual. Note that we're talking about the two PWM modes here, not either CW or sweep mode.

In the PWM modes, as in the CW and sweep modes, just pressing the AMP button will let you change the amplitude, just like it does in the CW and sweep modes. This is documented in the manual.

To change the offset in CW and sweep modes, you use the OFS button. Again, documented in the manual. This does not work in the PWM modes, as pressing the OFS button doesn't do anything. The manual doesn't mention setting the offset in either PWM mode.

Pressing the Mode button in either PWM mode brings up mode selection, which can't be used to set the offset.

Pressing the ADJ button in either PWM mode brings up the PWM settings menu. This lets you set a bunch of things: the min, max and midpoint of the pulse length. The increment for the ADJ knob when dialing the pulse length in PWM position mode. The step size and interval for the PWM run mode. The cycle length and maximum amplitude for the PWM signal. Offset is nowhere on the list, either in the manual or in that menu.

So again, I tried all three things you suggested for setting the offset in the PWM modes. None of the provided an answer to the question.

While I'm at it, why is the midpoint settable? As far as I can tell, nothing actually uses this value. Might I suggest replacing that menu entry with an offset setting? Or better yet, just make the OFS button work in the PWM modes.
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