Jaguars and FAST PID Looptimes

[Andrew Schreiber]

Originally Posted by Joe Johnson

I have a question about the speeds you discuss. You say you have a 9:1 ratio and I think you guys are a 4" wheel shop these days (which roughly checks with your 12 fps top speed). You happy with that? Do you guys wish you had a bit more metal in the pedal when zipping back and forth across the field? Do tell.

6" shop.

[Ether]

Originally Posted by Joe Johnson

Can you provide details on the ratios and motors involved on that swerve or do I have to go pester Anthony?

Hi Joe,

I'd like to help but you'll have to ask Anthony for those details. He contacted me because he needed help implementing integer math. He had coded the swerve inverse kinematics in floating point, and the floating-point emulation library was chewing up all the processing power on the ATmega2560. I sent him an integer-math implementation scaled for his inputs and outputs.

[Ether]

Originally Posted by Joe Johnson

do the Jaguar's really have 1ms update periods when in PID mode?

Follow the link for the full text:

Originally Posted by dyanoshak

...Which is called by the Controller Interrupt Handler which is triggered every 1ms by the H-Bridge.... When the PID is updated, the output is converted to a voltage command to the H-Bridge.

[Ether]

Originally Posted by Joe Johnson

what is the minimum steering angle speed you'd recommend before the vehicle kinematics would be compromised?

As you no doubt are aware, pedantically speaking any steering response (or drive motor response) less than infinite causes some compromise in the kinematics (in the form of wheel scrubbing).

As a practical matter, as the steering response gets slower and slower, you get more and more wheel scrub, especially during rapid evasive maneuvers. Eventually I suppose you'll reach a point where the vehicle starts exhibiting undesired or unpredictable "macro" behaviors.

Calling all teams who have deployed successful "Unicorn" swerves with 3DoF driver interface: please share your experience/data concerning steering dynamic response.

[Gdeaver]

What is the rational for very fast steering response and why is there a need to expend allot of power to accomplish this?
This is a link to a page showing what we have done with swerve.

http://wiki.team1640.com/index.php?title=Swerve_Central

We use a RS540 and banes bot 1 : 133 trans. Plenty for power and speed. Other designs and drive wheels may need more power but this solution has worked well for us.
I would caution on the use of Jags for positional control. We burned up several on the steering motors. We have used Victors and Talons for the steering motors with out failures.
For auto navigation, Tight PID tuning, well calibrated and accurate steering angle sensor and a good stable gyro - IMU are needed. Swerve does not like to go straight.
For teleop, a looser PID can actually be better. This is especially true if the driver is the button smasher gamer type. Next year we are planning on a very tight PID values for Auto and looser PID values for teleop. This year we tuned it to a value between the 2 ideal settings.
For 2015 we are looking forward to the power distribution boards ability to log current draw of all motors. The excel spread sheet and code TOM LINE posted looks very promising for swerve power analysis under battle field conditions. We don't know what our swerve consumes on the field and we should know this.
So again I ask why is there a need to bash the steering for very fast and precise angle?

[Ether]

Originally Posted by Gdeaver

So again I ask why is there a need to bash the steering for very fast and precise angle?

I think the question here is how fast is "fast enough" to accomplish the desired tasks.

The answer depends on what you want to accomplish with the swerve. Rapid evasive maneuvers require faster response than a gently sweeping curve.

The steering doesn't have to be perfect or instantaneous. But if the steering is way too slow or inaccurate, the vehicle will not behave as desired.

Your team seems to have found an acceptable compromise. Can you link to some videos showing rapid evasive maneuvers?

[RyanShoff]

Originally Posted by Ether

Calling all teams who have deployed successful "Unicorn" swerves with 3DoF driver interface: please share your experience/data concerning steering dynamic response.

Bag motor 90:1 to wheel through versaplanetary.

This loop was minimally tuned to stop any shaking. We didn't spend a lot of time optimizing it. Response of wheels turning 90 degrees wasn't timed. I would guess around 250ms. As tuned, it visibly overshoots a little and comes back on a full speed 90 degree move.

This was all copied from Bombsquad. Looking back on their original code (2013), it looks like we took D from 0 to .2 and increased the STEERPOW from .75 to 1. I don't know what motor or reduction they used.

Next year we'll be looking to decrease the loop period and the tolerance if we do a swerve again. The speed and response was just fine but it could have driven straighter.

PID loops setup like this:

#define CONTINUOUS true
#define P 1.0
#define I 0.0
#define D 0.2
#define F 0.0
#define POTMIN 0.2
#define POTMAX 4.8
#define STEERPOW  1.0
#define TOLERANCE 0.2
#define PERIOD .02
#define RATIO 1

driveTrainFrontLeftDrive = new Talon(1, FLD);
driveTrainFrontLeftPos = new AnalogChannelVolt(1, FLP, true, RATIO);
driveTrainFrontLeftSteer = new Talon(1, FLS);
driveTrainFrontLeft = new PIDController(P, I, D, F,  driveTrainFrontLeftPos,
driveTrainFrontLeftSteer, PERIOD);
driveTrainFrontLeft->SetContinuous(CONTINUOUS); 
driveTrainFrontLeft->SetAbsoluteTolerance(TOLERANCE); 
driveTrainFrontLeft->SetInputRange(POTMIN, POTMAX);
driveTrainFrontLeft->SetOutputRange(-STEERPOW, STEERPOW);

[Ether]

Originally Posted by RyanShoff

The speed and response was just fine but it could have driven straighter.

Thanks Ryan.

When you commanded rapid evasive maneuvers, did it consistently behave as expected¹? Do you have some videos?


¹ i.e. "as expected" according to kinematic calculations.

[RyanShoff]

http://www.thebluealliance.com/match/2014wimi_qf3m1
This is high def match video from Wisconsin. It ended up 3 vs 1 so we have lots of room to drive around at the end.