Hello, I am a mentor for a FIRST Robotics team. I was
wondering if I could pick your brains about some Labview VI's
We have a lift on our team's robot this year. Picture if you
will a fork-truck type lift. The lift is connected via a chain to a gearbox,
which is turned by an electrical 12 volt motor. The gear box is 12:1 ratio. The
motor has plenty of power to lift the lift up and down. The load is about 5 lbs
plus the weight of the lift which is 10 lbs. So about 15 lbs total on a 12:1
ratio gear box.
When we energize the motor full speed (+12 volts applied) to
raise the load, the speed of the lift is about 30 inches per second going
UPWARD. There is no strain on the motor and the motor is rated for the load.
When we energize the motor full speed (-12 volts applied) to
lower the load, the speed of the lift is about 60 inches per second going
DOWNWARD, because it's got gravity helping it...
When the motor is not energize, no voltage and the lift is
raised to the top, the lift will "creep" slowly downward. So it's not
a "free fall" lift with no power. But the weight of the lift is back
feeding the chain, to the gear box and the motor. The 12:1 ratio isn't enough
to keep the lift still when the motor has no power. This is not a big deal to
us in our design and not a safety concern. So it doesn't take much motor power
to make the lift come down.... 5-7 volts?
We have a 1000 PPR encoder on the output shaft of the gear
box (12:1) and a speed controller controlling a 12 VOLT motor. We are using a
National Instruments cRIO controller running a Lab view app with a PID VI in
position mode control.
Now that you have all that mechanical details.
I've done a few PID tunes, most have been horizontal loads.
I assume and we are expecting to have a different tune for the UP direction of
the lift, verses the DOWN direction of the lift.
While helping the programming team last night, we were able
to tune the UP DIRECTION of the PID with no problems. It actually looks pretty
sweet. But we struggled with the downward direction.
P = 0.8
I = .03
D = .002
(values I remember)
Up direction looks nice…
The lift is position based. We send a command of 30 inches
while the lift is at 0 inches, and the lift moves up to position and hold 30
inches. When we are up at 30 inches and command a downward direction of 0
inches, the lift drops violently, no matter what kind of a tune we place in the
PID. Either P only, PD or PI tune...
Again, we expect the up direction will have a different tune
that the down direction because of the free fall load. How can we slow down the
lowering? So it does not reach 60 inches per second free fall with motor
assist?
One thing on the speed controller command output works like
this:
-1 = -12 volts to the motor
0 = 0 volts to the motor
1 = +12 volts to the motor
The motor has no brake.
We did not try and "limit" the downward output of
the speed controller, we may try this tonight.
Will this HELP in keep the lift from dropping like a dead
weight to limit our output of the speed controller to say something like -0.4
(5 volts)?
Or do you have any other suggestions we can use for a dead
weight PID tune for our downward direction?
Another thing I was wondering if there was a VI someone had
written or built in that we can add a "gravity" constant to the PID?
Or an advance PID setting somewhere that can point us in the right direction.
Any helpful thoughts would be appreciated.
