Re: Voltage vs. PercentVbus
 

Dean -

It would be great to see the traces. I think it becomes more apparent if you scope the current through the motor. Also, the output voltage is better described as "effective output voltage".

Re: Voltage vs. PercentVbus
 

I'd expect a stalled CIM driven by a Jag to show an almost DC current, if the conventional-wisdom 200μH inductance number is correct.

@Dean: while you've got the test setup, could you run locked-rotor traces for some of the other popular motors as well? Nobody seems to have motor inductance specs or data.

Re: Voltage vs. PercentVbus
 

OK. Will try.

What I would love to do is to get a few kids excited to do a decent job in characterizing all these motors/Jags/etc. and publish it for the benefit of all.

I must say I was surprised at the SpiceIV results (also I don't have a model for the motor so was just playing with H values), so I was really surprized how relatively little effect the motor inductance had in the real test rig. (I hope I wasn't doing something wrong :eek: )

Dean

Re: Voltage vs. PercentVbus
 

Here are the tests and results, with scope pictures.
(I am not sue how best to input pictures - my son suggested Imgur - but they say the pics may be deleted if not viewed in 6 months... if anyone knows of a better way to save these let me know)

Summary:
We tested the behavior of two FIRST motors under idle speed (no load) and stalled condition. The Motors are CIM and Fisher Price. All components are standard FIRST KOP. The setup is a 12V battery, cRio, Jaguar CANbus controlled driven in the kVoltage mode, and first a CIM then FP motor tested at various settings of kVoltage from 6V to 12V. The kVoltage mode behaves as predicted but does not seem to fully compensate for the bus voltage drop (battery input voltage). The inductance of the load has remarkably little effect on the waveforms observed. Some odd effects were observed and we speculate on the reasons.


Setup pic:
http://imgur.com/aebA6


First Motor - CIM - pic:
http://imgur.com/7qELk


Scope used:
http://imgur.com/VsFVS

Test 1 - Idle, kVoltage=6V, I=3.23A
Jaguar Current=3.23A -as reported by getOutputCurrent() -This is almost certainly average current. see pic (ignore lines 1 and 3):
http://imgur.com/2Qi2I

Voltage on jag output is a 12V peak-peak 50% duty cycle PWM.
pic:
http://imgur.com/Fnpwi


Test 2 - Stalled, kVoltage=6, I=55A
The PWM duty cycle is now about 55% while the High V is about 10V.
Why does the Jag not make the average Vout closer to 6V? Maybe because it measures the input side Voltage which is higher given that we have about 100A flowing during the high cycle. [ToDo next time - look at input side V.]
pic:
http://imgur.com/0IjKf


Test 3 - Idle, kVoltage=9, I=3
pic:
http://imgur.com/qmp4w


Test 4 - Stalled, kVoltage=9, I=88A
pic:
http://imgur.com/SAn2Y
Jag cut out after a while (thermal cutoff), and restarted a few seconds later.
Pretty decent squarewave for 100A chopped through a large inductor!
I guess its the battery which is a bigger monster than the CIM <smiley>
Our battery measured at 0.02 Ohms internal resistance. Modeling with LTspiceIV confirmed this.


Test 5 - Idle, kVoltage=10, I=3A
pic:
http://imgur.com/oZDfw


Test 6 - Stalled, kVoltage=10, I=98A
Hm... the Jag kept cutting out before I could snap a pic! Also very curiously, it crashed the cRio!! very consistently. Speculation: noise on the Vcc (power input to the cRio) either the voltage drops too far, or the voltage disturbance goes through the input filters and crashed the cRio.

Test 7 & 8 with kVoltage=12 are the same as for 10V. The Current always spikes and settles - it is the settled readings provided here, and there is not much observable diferrence in current between kVoltage of 10 and 12.


The Fisher Price motor date will be uploaded tomorrow - this is taking a long time ...

Re: Voltage vs. PercentVbus
 

Thanks for sharing this, and the time and effort you put into it.

I'm a bit disappointed there's no scope traces of current. I was hoping to get some motor inductance data :(

Re: Voltage vs. PercentVbus
 

Second part of the FIRST motor tests… (see prior posts for the first part.)


Second Motor: Fisher Price
pic:
http://imgur.com/J49vW


Test 1 - Idle, kVoltage=6V, I=2A
http://imgur.com/SPwFd


Test 2 - Stalled, kVoltage=6, I=37A
http://imgur.com/jrqAM


Test 3 - Idle, kVoltage=9, I=1.8A
http://imgur.com/9foYn


Test 4 - Stalled, kVoltage=9, I=55A
http://imgur.com/4xbRV


Test 5 - Idle, kVoltage=10, I=1.6A
http://imgur.com/4XI29


Test 6 - Stalled, kVoltage=10, I=60A
http://imgur.com/a16g6


Test 7 - Idle, kVoltage=11, I=1.5A
http://imgur.com/vVA9q


Test 8 - Stalled, kVoltage=11, I=60A
Jag cutout too soon for pic.


Test 9 - Idle, kVoltage=12, I=0.9A
http://imgur.com/W0QvH


Test 10 - Stalled, kVoltage=12, I=63A
Jag cutout too soon for pic.


This little motor is much smaller than the CIM but not that much weaker.

Stalling the FP never caused the cRio to crash like the CIM did.

A curious observation:
When the FP stalled (at 11V & 12 V), the Jag would cutout giving a 0V scope trace, then in a few seconds it would go ON showing the expected PWM, but the motor would stay dead for another couple of seconds and then spring to action???
Speculation:
The FP has an internal cutoff mechanism. The Jag cuts out first and that somehow(???) causes the motor’s cutout to activate. The motor’s cutout is longer than the Jag’s by a couple of seconds… (this does not seem likely) another possibility is that when warm the motor exhibits a delayed start… Would like to do more tests.

On the Current issue – we didn’t measure it with the scope – we could with a 0.01Ohm length of wire (10AWG is 1mOhm/ft) or some Hall Effect sensor. Playing with LTspice and using our test results, it seems a stalled CIM is ballpark 200uH and 80mOhms – not a bad guess Ether! The current is indeed very flat (just a small sinewavelike ripple of around 1A on top of 88A).

For further testing:
1. Investigate what causes the cRio to crash upon stall at full PWM (see CIM tests). Have other teams experienced this ? We ended up switching back to Victors again this season for this reason… (I believe we are overpowering the CIMs –you can fry an egg on them! And bypassing the Jag’s safety by going to Victors – hey the kids are the boss!)
2. Investigate the FP weird delayed start (see above).
3. plot the Current alongside the Voltage.
4. Test a representative sample of motors. Another mentor was using CIMs to drain test batteries and found that some CIMs drew more Amps than others at idle!
5. We should be able to get real test and performance data from the manufacturers of these motors. Someone must have done this already?

Over and Out – this is hard work!

Re: Voltage vs. PercentVbus
 

When you say the cRIO is crashing, can you tell if it is due to low voltage -- the 8 slot needs 19volts? What does the voltage at the cRIO plug look like?

If it isn't voltage, you can also use the netConsole to see if you get any diagnostic.

Greg McKaskle

Re: Voltage vs. PercentVbus
 

Thanks Greg,

No we did not look at that. Ours is a 8 slot.
Our new robot with another old cRIO but a New PDB draws near 200A with Victors without crashing the cRIO, so it is probably not a voltage drop issue.

So far no one else has responded that they too had cRIO crashes when CIMs stall, so it may be something with our PDB it is an old war scarred PDB so will do more tests and report back when we recover from the post bagging depression :)