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  #31   Spotlight this post!  
Unread 06-03-2013, 19:46
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Re: Shooter consistency

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Originally Posted by Chris is me View Post
We have absolutely no speed control on our direct driven Banebots wheels. My theory is that the two-wheel combination as well as the slip in our system results in a consistent shot. I guess we're not exactly sure WHY it works, but it works.
What is your wheel diameter, what motors are you using, and are you giving 100% voltage to each one?


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Unread 06-03-2013, 19:53
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Re: Shooter consistency

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Originally Posted by Tom Line View Post
Our shooter currently runs up to 7500 RPM. We're using a Us Digital S5 optical encoder with 512 counts per revolution (good to 10k rpm).
How are you decoding the signal ?


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Unread 06-03-2013, 20:06
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Re: Shooter consistency

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Originally Posted by Brandon_L View Post
our theory is that were basically shooting in a straight line and power would only effect how far our disc goes before it drops
In theory the arc changes with changing launch speed. But if the shot distance is short enough and the launch speed is fast enough then the change in the arc may be small enough to allow some variation in speed and still score.


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Unread 06-03-2013, 20:06
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Re: Shooter consistency

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Originally Posted by Ether View Post
What is your wheel diameter, what motors are you using, and are you giving 100% voltage to each one?


Wheel Diameter: 4.875" Orange Banebots
Motors: 1 CIM 1 Mini CIM.
Both get 100% voltage. Minor (~4-5 feet) performance change between 2 mini-CIMs and 1 mini one full like we have now.

We were *shocked* at how easy it was. I was planning on making something very similar to what Jared described and doing a bang-bang controller on the final wheel, but we just don't need it at all.
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Unread 06-03-2013, 20:15
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Re: Shooter consistency

Quote:
Originally Posted by Chris is me View Post
Wheel Diameter: 4.875" Orange Banebots
Motors: 1 CIM 1 Mini CIM.
Both get 100% voltage.
I'd agree with your original assessment. You're probably way past the point of diminishing returns on wheel speed, so slowing it down doesn't have much of a difference.


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Unread 06-03-2013, 22:12
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Re: Shooter consistency

Our discs fly in a very flat trajectory at the ranges we specifically target (all the way from just in front of the goal to about midfield). So flat, in fact, that our auto-aiming function simply aligns our shooter's angle so that the goal is in a constant place in the image frame coming from the arm-mounted camera. I had anticipated nonlinearities requiring a range-to-offset table (e.g. assume shot drop at longer distances and different trajectories at higher angles, etc.), but this turned out to be unnecessary.

Our shot trajectory is not very different with our nominal setup (3000 and 4700RPM) than if we run both wheels at 100% voltage open loop (~6000RPM). The real benefit of speed control/measurement in this application is that we can maintain a high rate of fire with consistent shots (we know exactly when we can fire the next shot), and we manage the wear on our BaneBots wheels by limiting the slip on the disc (and balancing the wear between the front to back wheels).
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Unread 07-03-2013, 16:31
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Re: Shooter consistency

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Originally Posted by Ether View Post
How fast is "that fast"? The OP did not say what speed they are running their shooter at.

A "standard" US Digital E4P 250CPR encoder, if setup properly*, should be able to easily handle speeds up to 5000 rpm.



If the spokes are not wide enough, that detector will have difficulty keeping up at high speeds.


* connect only Channel A. Leave Channel B disconnected. Instantiate it as an up/down counter in the Counter class, and set it to count up only. Set the FPGA sample averaging ring buffer to 125. Use the getPeriod() method in the counter class, and compute rpm = 60/(250*period). See attachment.

You are correct; last year our team had to change from Optical encoders to the US Digital EP4 Encoders but we use 360 count encoders last year and this year.
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Unread 17-03-2013, 10:12
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Re: Shooter consistency

I wanted to update this post with some more info we found yesterday.

While the S5 encoder from US digital states a maximum rpm of 10,000, they only state that is for the bearings. In practice, we're seeing a loss of signal around 8600-8800 rpm. The sensor will begin missing counts and you will see a drop in rpm feedback as you increase the speed of the motor above that.

As a backup, we tried one of the $12 allied electric photo switches (more info on specifics elsewhere - just do a search). They don't have a response time listed so we had low hopes, and it didn't work.

We then switched to allen bradley photo switches from 2011. We have half the wheel colored black, the other half with reflective tape. We have extremely good feedback up to our max speed of around 11,500 rpm.

It appears, however, that this sensor is not widely available anymore. Does anyone have a source with reasonable shipping times? Is there a drop-in replacement that anyone is aware of?
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Unread 17-03-2013, 10:55
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Re: Shooter consistency

Tom,

I personally have had a lot of success with Banner's QS18VN6D (IR) and QS18VN6LV (Visible Light - Red) sensors for these types of FRC applications.

They are close to a drop-in replacement for the ABs, but these only need 12V.

Sourcing them for us has been easy and fast because they used to come in the KoP, and being an older team we've built up a stockpile of them. You could probably call up one of more generous very old MI teams, and there's a good chance they'll have a bunch kicking around.

Else, you can order them direct from Banner at http://www.bannerengineering.com

We're using the QS18VN6D (IR) right now on our 2013 robot.

Sometimes it's easier to prototype with the QS18VN6LV (Visible Light - Red) since you can see the light being emitted, making alignment much easier. Even though the specs say it is supposed to be used w/ retro reflective tape, at close ranges (i.e. < 1ft) a black/white painted/coloured regular diffuse surface will work fine.
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Last edited by Mr. Lim : 17-03-2013 at 10:59.
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Unread 17-03-2013, 11:48
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Re: Shooter consistency

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Originally Posted by Ether View Post
Read the last two paragraphs on page 2

Not sure if this rule exception is valid in 2013.


I don't see anything in the rules about it, so I would say no. It wouldn't hurt to contact FIRST and see if they can add the exception back in.
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Unread 17-03-2013, 12:37
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Re: Shooter consistency

many teams had great accuracy this year. What was really amazing is that both main types of shooters, linear and semicircle, worked great. 1706's shooter was deadly this year, but sadly teams caught on in eliminations and played excellent defense, a well earned victory. http://www.youtube.com/watch?v=U5PgpTQCgBc
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Unread 17-03-2013, 13:14
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Re: Shooter consistency

This is what we learned at our 2nd MAR district and we ended up with the most Autonomous points. We had the same problem. Make 3 in Autonomous and then for some strange reason, frisbees were high. So we said, slow down the speed. Didn't seem to help. Our analysis was that at slower speeds with the frisbee encountering a lot of air resistance, the frisbee had a tendency to "float" and end up high. We cranked up the speed (within reason - there may be a point of diminishing returns) and adjusted our angle. If you can not adjust your angle, then position your robot differently (works great when shooting from the side of the pyramid). This works. Watch how other teams shoot. Keep it simple and good luck!
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Unread 17-03-2013, 17:22
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Re: Shooter consistency

our team was having trouble with encoders as well. Yes different battery voltages do matter when sending a value to a Victor because the value sent to the victor is the equivalent of a percentage of your current voltage. So to try to get a better measurement, I tried a last minute fix.


Wheel Motor Voltage = Value sent to Victor * Battery Voltage.

Therefore, Value Sent to Victor(Or other speed controller) = (End Voltage / Battery Voltage).

All you have to tune is the End Voltage that you want your wheel to have. Batter Voltage can be calculated (in Java) by instantiating a DriverStation object and using the getVoltage() method.

Same method should be in other API's.

It didn't give us 100% consistency but it did significantly improve it. There are of course other variables.
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Unread 17-03-2013, 18:29
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Re: Shooter consistency

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Originally Posted by dheerm View Post
our team was having trouble with encoders as well. Yes different battery voltages do matter when sending a value to a Victor because the value sent to the victor is the equivalent of a percentage of your current voltage. So to try to get a better measurement, I tried a last minute fix.


Wheel Motor Voltage = Value sent to Victor * Battery Voltage.

Therefore, Value Sent to Victor(Or other speed controller) = (End Voltage / Battery Voltage).

All you have to tune is the End Voltage that you want your wheel to have. Batter Voltage can be calculated (in Java) by instantiating a DriverStation object and using the getVoltage() method.

Same method should be in other API's.

It didn't give us 100% consistency but it did significantly improve it. There are of course other variables.
I'd recommend caution with this technique. You can end up with some interesting interactions since you'll be drawing more power, which will drop the voltage more, which will draw more power, etc. etc.
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Unread 17-03-2013, 18:42
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Re: Shooter consistency

You can have the best shooter consistency but you have to feed it first.

195 at Virginia could snipe cross field, but they suffered one flaw. THe hopper into the shooter was poorly fed. They had to jostle the robot to get it down several times, that threw off the aim.

1610 had jam issues twice and two different ways. They could get up close enough to the bar and touch it to line up but if the hopper jammed, it was for naught.

I saw another half dozen such jams, poor feeding issues, etc.

Great shooter or not, consistent or not, your hopper/feeder has got to provide the Frisbee.
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