You mean a Simpson 360 type meter? At best it might give you average readings for most of the higher throttle values. AT 50% or less, I don’t think you could depend on it to give you any meaningful info other than the polarity and presence. The meter movement varies so much from manufacturer to manufacturer that I don’t think you can make a blanket statement.
I mean an analog voltmeter, with coil movement and a needle pointer.
Any cheap $10 one would do, set to read DC.
By “effective DC-equivalent voltage” in this context I mean the following:
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When the Jag is being commanded to output 100% voltage, the meter would read 12V at the Jag’s output.
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When the Jag is being commanded to output 50% voltage, the meter would read 6V at the Jag’s output.
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When the Jag is being commanded to output 0% voltage, the meter would read 0V at the Jag’s output.
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When the Jag is being commanded to output -50% voltage, the meter would read -6V at the Jag’s output.
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When the Jag is being commanded to output -100% voltage, the meter would read -12V at the Jag’s output.
etc
Haven’t actually tried it though. Anybody got data ?
**
I tested both an analog and a digital multimeter set to DC and got identical responses in the range of 0-5.5v for zero to full throttle on a jag output.
P.S.
I’m a little slow, but I awoke in the middle of the night to the realization that my measurement mistake was because the battery had been drawn low by the motor load probably coupled with a low battery.
The battery I used was drawn down to 7.5v, so apparently ~2v might get dropped by measuring with these type of multimeters.
I’ll check again with a fully charged battery and without a motor load when the real morning comes.
Most interesting. Did you have a motor connected or no ?
Also, at full throttle isn’t the Jag’s output waveform supposed to be essentially a flat-line 100% 12V ?
**
I tested with a stronger battery and it had the same 2v drop through the jag showing on both analog and digital meters.
Jag goes solid green for forward.
I’ll put a scope on it later to see what’s happening to both the power output and to confirm the PWM input signal is at full.
I took the load off for this second test, so the Jag had nothing connected this time.
The original test was done on a competition robot. I moved the second test to a stand-alone cRIO/Jag (quieter that way…).
Mark,
I know I am asking a lot, but when you set the test up with motors, can you evaluate with a CIM connected and window motor connected?
I can add tests with both CIM & window motors.
My scope has a problem so I have to use the one in the robotics lab. I think the one in our lab is a 20MHz model.
I got lazy and brought stuff home where I thought I had a better scope…
Both types (analog/digital) of multimeters do measure Victor full power correctly. I even tested by alternately telling the software it was a Victor/Jaguar just for fun.
Are you saying you don’t see the 2V drop with the Victor, but you do still see it with the Jag? If so, could you put the scope on the Jag’s PWM input and see what pulse width is being commanded at full throttle ?
**
Fixed my scope - broken ground reference wire.
Looks like the PWM signal is correct on both the Victor (smaller range) and Jaguar (larger range) settings.
The solid green/red light on the Jag does indicate full power is being ordered via PWM.
The Jag just reads 2v under at max while the victor reading equals max voltage.
This is all without any load. I created button controls to order full power, in addition to a throttle test.
I’ll pick up motors tomorrow for further testing.
May I echo back to you what I think I hear you saying, so you can correct any misunderstandings?
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you fixed your scope (yea!)
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you scoped the Jag’s input PWM and it had a 2.33ms pulse width at full throttle?
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you scoped the PWM output from the Jag while commanding full throttle and you saw a 100% duty cycle on the output and yet the voltage measured by your meters was 2V lower than the supply???
**
Mark,
Are you looking at the servo signal into the Jag, the waveform on the motor outputs, or both?
I’m curious as to what the waveform on the motor output looks like. Is the peak voltage out of the Jaguar (on the scope) 2V less than the battery voltage?
Can you post pictures of the scope traces?
-David
I mean it’s weirder than I thought it’d be.
The bottom line seems to be that the Jag at 100% provides 2v less power than the Victor.
It has nothing to do with the multimeters, they work fine. They might under report the power based on the output waveforms, but the results are certainly usable and close enough for the average FRC team.
The PWM signals have the appropriate ranges for each device. They also are fine.
The Victor and Jag (via scope) both have the expected designed output waveforms at anything other than full power.
The Jag’s is a lot more vigorous of course.
The Jag does something funny.
The power output waveform begins at lowest duty cycle by oscillating in the range, say ~2v (rather than 0v as expected) to battery voltage. As power is increased, the lower bound rises towards the upper bound which remains steady at battery voltage.
At ~90% the upper bound of the waveform begins collapsing as the last 10% is transistioned. The upper limit of the waveform begins to drop to meet the still rising lower bound. Although the lower bound stops 2v down and waits for the upper bound to fully collapse to meet it.
Eventually, at 100% duty cycle they meet at a point 2v less than the battery voltage.
So peak voltage on the Jag output is 2v less than the battery voltage.
My scope is an old 40MHz Kenwood that doesn’t get used a lot. The multimeters are Craftsman sale specials.
Certainly I’d like someone else to repeat the results before saying I didn’t make some measurement mistake.
For the record, did you observe this same behavior on more than one Jag? And, are we talking Black or Tan Jags here ?
**
Good point.
The odd behavior was on a tan jag.
I just tested a black jag and it did not have the problem.
The multimeters work fine on the Black Jag too.
Weird indeed.
Did you measure the battery voltage when the Jaguar was at full output, to make sure something wasn’t loading it down?
The rising of the low-level voltage is probably happening because there’s no load, but it’ll be a good idea to check that.
I monitored the battery voltage throughout the tests and it didn’t drop.
I will try again with motor loads tomorrow since I’ve come this far.
I don’t know if the tan jags have different rev levels, so that originals have a problem but later revisions do not.
Mark,
I am guessing you are still measuring open circuit on the controllers. The FAN5109 gate driver used in the grey Jags has “adaptive gate drive” and so maybe responding to the lack of a load. The bootstrap cap might be having some effect as well and there is feedback to the micro from each of the bootstrap circuits. (VbootA and VbootB) Are you hanging the scope across the output of the controller or are you connecting the probe ground to the negative power supply input and then connecting the probe to one of the output terminals?
Yes, these measurements were still taken with no load on the speed controller outputs. I’ll apply loads tomorrow to see what that looks like.
Usually I ran with the probe ground to the negative power supply input because I was hopping around testing both the Victor and Jag speed controllers as well as the PWM inputs to both at various times.
But I also tested with the probe ground to one of the output terminals and saw no difference.
The multimeters were applied just across the speed controller outputs.
P.S.
The other caveat is that I could by chance be working with a damaged tan jag. I’ll test with others to see if it’s a broad issue.
Update: Nope, I tested three tan/gray jaguars and they all exhibited the same odd response. They were never-been-used 2010 KOP Jags.
^Reported
I tested with a globe motor load and the oddity disappears, so the odd collapsing waveform and the 2v drop seems to just be a characteristic of the adaptive gate Al brought up and the lack of a load as Alan suggested. It shouldn’t affect normal operation.
There was a .15v drop through the Jag at 100%, but my battery is down a bit so that’s not a full measurement.
The gray/tan jag gives full power at 100% with a load, and 2v less without a load.
With a load the waveform is the typical duty cycle switching between 0 and battery voltage for increasingly longer times as the throttle changes.
The CIM and the window motors show a cleaner 100% than the globe motor for some reason. Up to 100% everything about the power output waveform is identical. At 100% I see the periodic tiny drop to 0v on the CIM and Window motors, but for the globe the scope displays a messier plot.
The CIM also drew the battery down a bit more than the others of course.