Shooter Problem

[ThePlagueDoctor]

My team is doing the semi circle design for the shooter this year with one cim directly driven to a 8 inch high grip wheel. for the arc of the semi circle guard, we cut out a piece of plexiglass and positioned it at the perfect arc. It picks up the Frisbee fine, but it just doesn't fire it far at all. any ideas of what could be wrong?

[ehochstein]

Move the wheel closer to the arc. aka more tension!

[Ether]

My team is doing the semi circle design for the shooter this year with one cim directly driven to a 8 inch high grip wheel. for the arc of the semi circle guard, we cut out a piece of plexiglass and positioned it at the perfect arc. It picks up the Frisbee fine, but it just doesn't fire it far at all. any ideas of what could be wrong?

1) You may have too much or too little compression on the frisbee

2) You may be running the motor too slow or too fast

[MrForbes]

A picture is worth a thousand words...can you post a picture of your setup? You may be doing something wrong that we can't tell from just your words.

Today we got about 35 feet travel from a single 8" kit wheel (from a few years ago) direct driven by a CIM, and less than 90 degrees of arc. We made the arc guide from a piece of 3/4" plywood, cut to the correct radius with a jig saw. We have about 1/8" compression on the frisbee as it travels thru the shooter. We also added a piece of wood on top to keep the frisbee from jumping up and losing grip.

One other problem we encountered is the wheel slipping on the motor shaft, we don't have the correct keyed hub yet.

[ThePlagueDoctor]

A picture is worth a thousand words...can you post a picture of your setup? You may be doing something wrong that we can't tell from just your words.

Today we got about 35 feet travel from a single 8" kit wheel (from a few years ago) direct driven by a CIM, and less than 90 degrees of arc. We made the arc guide from a piece of 3/4" plywood, cut to the correct radius with a jig saw. We have about 1/8" compression on the frisbee as it travels thru the shooter. We also added a piece of wood on top to keep the frisbee from jumping up and losing grip.

One other problem we encountered is the wheel slipping on the motor shaft, we don't have the correct keyed hub yet.

The wheel was slipping a little bit on the motor shaft, but we fixed that. we were using that exact same setup, except for the arc, we used a piece of plexiglass because the arc we cut out of half inch plywood was cut bad. i'll try the amount of compression tomorrow. Thanks

[JosephC]

Try using a softer wheel like a AM pneumatic wheel. It allows more compression on the frisbee without the risk of deformation.

[DonRotolo]

The Plexiglas arc may be too slippery, and the frisbee is slipping instead of rolling along. You may want to add something soft and squishy there, like foam tape.

[ThePlagueDoctor]

The Plexiglas arc may be too slippery, and the frisbee is slipping instead of rolling along. You may want to add something soft and squishy there, like foam tape.

The frisbee rolls and moves in the shooter just fine, its just that it doesn't launch that far, barely at all...

[Bruceb]

Our circular (90) degree shooter uses and AM pneumatic 7.5 inch wheel with a 32 tooth sprocket on it and a 25 tooth sprocket on the CIM. We have it set up so the wheel is hitting just above the widest diameter of the disk and just below the widest diameter of the tire with about 1/4 inch of compression and the tire at maybe 10psi. This ensures the disk does not ride up on the tire. This puppy will hit the top of a 10 foot door at 45 feet. up down variation is about 6 inches and side to side variation is almost nonexistent.
This is just with a plywood POC. You can read that as Proof of Concept or Piece of C&^%. Whatever. Still need to add a guide on the outer rail to keep the disk from riding up on that.
We have also tested a straight shooter bot so far it is not as consistent and much less powerful even with 2 wheel powered by separate CIMs.
Still plan on a little more testing here as I think it would be easier to incorporate to the robot.
Bruce

[Dan Richardson]

One of the neat tricks we learned was to offset an 8" pneumatic wheel up slightly from the frisbee center. The wheels inherent compliance would push down slighlty on the frisbee. This downward force helped resist premature lift and kept the wheel in contact with its track. It adds to the overall friction of the system, but it helped on the Robot in three day build.

Can't be sure how effectively this would work on a stiff wheel.

[ThePlagueDoctor]

One of the neat tricks we learned was to offset an 8" pneumatic wheel up slightly from the frisbee center. The wheels inherent compliance would push down slighlty on the frisbee. This downward force helped resist premature lift and kept the wheel in contact with its track. It adds to the overall friction of the system, but it helped on the Robot in three day build.

Can't be sure how effectively this would work on a stiff wheel.

Thanks. I have to try a lot of different things tomorrow.

[Ether]

"Our circular (90) degree shooter uses and AM pneumatic 7.5 inch wheel with a 32 tooth sprocket on it and a 25 tooth sprocket on the CIM. This puppy will hit the top of a 10 foot door at 45 feet. up down variation is about 6 inches and side to side variation is almost nonexistent.

We have it set up so the wheel is hitting just above the widest diameter of the disk and just below the widest diameter of the tire with about 1/4 inch of compression and the tire at maybe 10psi. This ensures the disk does not ride up on the tire. Still need to add a guide on the outer rail to keep the disk from riding up on that."


Useful detail. Wish everyone would follow your example.

At your operating point, do you have any measurements of CIM voltage, current, and speed ?

[hobbes20xxx]

2) You may be running the motor too slow or too fast

Oddly, we found that running the CIM with a PWM input rate of ~50-60% worked the best.

[Aroki]

Oddly, we found that running the CIM with a PWM input rate of ~50-60% worked the best.

What wheel were you guys using? Also did you use direct drive or have gearing (if so what ratio)?

[Ether]

Oddly, we found that running the CIM with a PWM input rate of ~50-60% worked the best.

That gives you a lot of headroom to do closed-loop speed control. A bang-bang controller probably will work very well.

Also, interested in your answer to Aroki's questions (http://www.chiefdelphi.com/forums/showpost.php?p=1213945&postcount=14) about the gearing and the wheel.

[Ian Curtis]

The Plexiglas arc may be too slippery, and the frisbee is slipping instead of rolling along. You may want to add something soft and squishy there, like foam tape.

Having played with a similar setup this is almost certainly the problem. Gaffer's tape on the arc worked really well, if you don't have any of that masking tape would probably work about as well. We're going to try hockey tape tomorrow.

[MrForbes]

We have plans to try several of the tricks mentioned here, and we'll try to report back with our findings.

[roystur44]

Check and make sure the wheel is tighten to the axle and is not slipping

[Jeffy]

We found tread material to give us the most difference in performance. We use wedgetop wheel tread and it works well on a 6 inch wheel.

[DonRotolo]

The frisbee rolls and moves in the shooter just fine, its just that it doesn't launch that far, barely at all...
OK, let's say it is slipping 90% - you would never be able to see this with your 'bare eyes'.

As Ian Curtis mentioned, try some tape on the plexi. Then come back and tell me it didn't work, But not before you try it, all right?

[MrForbes]

We found tread material to give us the most difference in performance. We use wedgetop wheel tread and it works well on a 6 inch wheel.

This is what we're planning to try tomorrow. And we'll set it up so we can play with the speeds, by using chain drive and computer speed control, eventually with a PID loop of some type.

[ThePlagueDoctor]

That gives you a lot of headroom to do closed-loop speed control. A bang-bang controller probably will work very well.

Also, interested in your answer to Aroki's questions (http://www.chiefdelphi.com/forums/showpost.php?p=1213945&postcount=14) about the gearing and the wheel.





We are using a 8 inch high grip wheel directly driven with a cim

[Ether]

This is what we're planning to try tomorrow. And we'll set it up so we can play with the speeds, by using chain drive and computer speed control, eventually with a PID loop of some type.

Try P-only and crank the gain up to the sky*. If you've got enough inertia in the wheel, and you run the loop at 10ms, that's all the tuning you should need.


* this will mimic a bang-bang controller.

[Dinoyan]

We know how we're going to shoot but how we're going to put the frisbees in to the robot or how will the robot going to pick it up????.

[Woolly]

Try P-only and crank the gain up to the sky*. If you've got enough inertia in the wheel, and you run the loop at 10ms, that's all the tuning you should need.


* this will mimic a bang-bang controller.



Yeah, I'd say that you might want to go with a bang-bang controller.

If you get flack from your mechanical/fabricators for "slamming" the motor, do what I did, add a linear cushion within ~3% of your target speed.
Ex. For a shooter wheel capable of 10,000RPM when you're over 300 RPM lower than your setpoint, you go 100% power, when you're 150 RPM Lower, 75%, Dead on 50%, 150RPM Higher 25%, 300RPM Higher 0%.

This will mimic the PID you're trying to use, and unless you really know what you're doing when PID tuning, will be half the headache.

[Ether]

If you get flack from your mechanical/fabricators for "slamming" the motor...

Seriously? Are you saying there was some visible or audible manifestation?

If so, check the following and fix if necessary:

- make sure the motor controller jumper is in the "coast" position (not "brake").

- make sure you are running the control loop fast enough (10 ms is good)

- make sure your sensor is capable of giving an up-to-date speed reading faster than your control loop iteration rate

[Woolly]

Seriously? Are you saying there was some visible or audible manifestation?

If so, check the following and fix if necessary:

- make sure the motor controller jumper is in the "coast" position (not "brake").

- make sure you are running the control loop fast enough (10 ms is good)

- make sure your sensor is capable of giving an up-to-date speed reading faster than your control loop iteration rate






I didn't know what I know now when I made that control loop last year, and I just dropped it in autonomous and teleop. Obviously a case of it not running fast enough. Still, even our poor bang-bang controller was better than a PID for this application.