Sync Two Jags

[Talguy]

How can I get two jaguars to sync together their PIDs when I only have one encoder. Should I have it that the one jag w/ encoder sens their voltage ramp to the jag without an encoder. How are other teams doing it?

[Joe Ross]

The following thread has some ideas, but no resolution. http://www.chiefdelphi.com/forums/sh...ad.php?t=89282 When you get it working, be sure to let people know how you did it.

[kamocat]

You can try using one in %Vbus, Vcomp, or Current mode, feeding it the value you've gotten from the controlling Jaguar.
For any of these options, you'll want to do some processing to the data to get something acceptable. The trouble is, for low speeds the Jaguars like to return lots of zeros. (though perhaps my PID is tuned a little too high) When you tell a Jaguar in %Vbus or Vcomp to go "0", it doesn't ramp down, it just stops. This leads to very jerky movement. Perhaps a running average of the last 5 samples would work, or perhaps you just want to ignore all the zeros.

In Current Mode, the main issue is simply lack of accuracy. It's +-2 Amps below 8A, and +-1A above. That means you better have a pretty big load in order to use it. However, large loads are usually where the double motor is necessary.

I'm planning on wiring the encoder to both motors, and having the PID independent. Because both Jaguars are subject to the same conditions, I'm hoping I won't get significant integral windup. (Also, the integral is reset when the Jaguar output is "0")

[Talguy]

Originally Posted by kamocat

You can try using one in %Vbus, Vcomp, or Current mode, feeding it the value you've gotten from the controlling Jaguar.
For any of these options, you'll want to do some processing to the data to get something acceptable. The trouble is, for low speeds the Jaguars like to return lots of zeros. (though perhaps my PID is tuned a little too high) When you tell a Jaguar in %Vbus or Vcomp to go "0", it doesn't ramp down, it just stops. This leads to very jerky movement. Perhaps a running average of the last 5 samples would work, or perhaps you just want to ignore all the zeros.

In Current Mode, the main issue is simply lack of accuracy. It's +-2 Amps below 8A, and +-1A above. That means you better have a pretty big load in order to use it. However, large loads are usually where the double motor is necessary.

I'm planning on wiring the encoder to both motors, and having the PID independent. Because both Jaguars are subject to the same conditions, I'm hoping I won't get significant integral windup. (Also, the integral is reset when the Jaguar output is "0")

Is it possible to wire one encoder to two jags?

[kamocat]

Yes, but you have to make a splitter for it.
Make sure to not wire the two 3v power supplies together, but connecting just their - terminal should be fine.

The way I was planning on doing it was taking a prototype board (5x3) and vertical 0.1" headers, and soldering across. That way, I could just take the encoder in its current configuration, plug it into the board, and have two adapter cables, one for each Jaguar.
It's also possible to have a board that plugs directly into one of the Jaguars, but that's a little harder to make.

[Talguy]

ok I could just use an old PWM splitter cable

[Alan Anderson]

Originally Posted by Talguy

ok I could just use an old PWM splitter cable

Not exactly. Quadrature encoders have more than three connections.

[Talguy]

Originally Posted by Alan Anderson

Not exactly. Quadrature encoders have more than three connections.

yea our's has four connections but I can type them both to a common ground and 5v supply. The A&B channels will be split to the jags

[techhelpbb]

We split the encoder like this.

A to both.
B to both.
GND to both.
VSupply from only one.

Don't connect the supplies together unless you diode OR them and if you do that use germanium diodes for the low voltage drop across the junction.

We had a little discussion amongst ourselves about how this shouldn't work, but given we've driven the Victor 884s unmatched into CIM gear boxes with a mere gear dividing the 2 output shafts, and given there's reverse protection on both the Jaguars and Victors (in case something turns into a generator) the risks are low.

There is a chance that one Jaguar might drive more of the mechanism load than the other, as they are able to spin each other's armature like this. It shouldn't be too much of a problem and when the load on the mechanism increases they should both start to contribute. This will make tuning them interesting considering that effectively they are both in a state of semi-permanent runaway.

Only thing I will warn you about...we did this for a velocity set point...not position (haven't thought about it the other way) and I strongly advise you to have a single harness handy in case someone thinks the split harness is causing your issues.

[Mr. Lim]

Hook up your encoder to you "master" Jags only. Put your "slave" Jags into absolute Voltage mode.

In your TeleOp Continuous loop, do:

slaveJag.setX(masterJag.getVoltage());

[kamocat]

Mr. Lim, have you tried this?
I had little success in LabVIEW.
http://www.chiefdelphi.com/forums/sh...5&postcount=10

[Mr. Lim]

Originally Posted by kamocat

Mr. Lim, have you tried this?
I had little success in LabVIEW.
http://www.chiefdelphi.com/forums/sh...5&postcount=10

We have tried this, and anecdotally we are having success, in that we are able to get our robot to drive at the desired speeds, with what APPEAR to be balanced output from both. I haven't scoped, or otherwise recorded, or graphed the values, and our slave Jags appear to run in virtual lock-step with the master Jags.

We are using Java, and I'm wondering if there could be a difference in the implementation between languages, or the fact that we're passing the output voltage (i.e. -12V to 12V) from the master Jaguars to the slaves, and those output voltage values might be filtered to provide a meaningful value (i.e. not an instantaneous %Vbus).

[Hugh Meyer]

Originally Posted by techhelpbb

We split the encoder like this.

A to both.
B to both.
GND to both.
VSupply from only one.

If you connect the two GND wires together you can expect to have ground loop issues. This is only a problem with heavy loads where the motors are drawing significant current. We saw this issue last year in the CAN bus ground wire. We have an opto isolator in the circuit to prevent the ground loop.

So far the motors drive well. We see some unbalance, but not very much. We are still investigating if this unbalance is real, or inherit in our logging scheme.

Attached are two step response graphs from recent testing, one showing current and the other showing speed, both from the same run.

-Hugh

[techhelpbb]

Originally Posted by Hugh Meyer

If you connect the two GND wires together you can expect to have ground loop issues. This is only a problem with heavy loads where the motors are drawing significant current. We saw this issue last year in the CAN bus ground wire. We have an opto isolator in the circuit to prevent the ground loop.

So far the motors drive well. We see some unbalance, but not very much. We are still investigating if this unbalance is real, or inherit in our logging scheme.

Attached are two step response graphs from recent testing, one showing current and the other showing speed, both from the same run.

-Hugh

Interesting. We'll keep that in mind.

Thanks.

[Ether]

Originally Posted by Hugh Meyer

So far the motors drive well. We see some unbalance, but not very much. We are still investigating if this unbalance is real, or inherit in our logging scheme.

Attached are two step response graphs from recent testing, one showing current and the other showing speed, both from the same run.

I didn't see any previous posts in this thread from Team 1741. Could you please provide some context for your post? (What is your drivetrain; What feedback sensor are you using; How do you have it connected; etc) Thanks.