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  #16   Spotlight this post!  
Unread 05-21-2018, 08:53 AM
wgorgen wgorgen is offline
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Re: Shifting Swerve, Feedback wanted

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Originally Posted by Joe Johnson View Post
Regarding the discussion around using 775Pro for the steering, I think the 775Pro motor (or the AndyMark equiv) should be the default motor for every application on a FIRST robot; If you don't have a good reason not to use it, that's the motor you use.

If you use a Talon SRX it is trivial to limit the current to make it essentially impossible to smoke them. If you gear them properly you don't have to worry about that except in oddball cases anyway.

Finally, more power is almost always better in FIRST. Swerve steering seems like it is a low power application but if you want to go left and your wheels are pointed straight ahead, making that 90 degree turn faster is very desirable and requires more power (doing the same work in a shorter time is the definition of increased power).

Dr. Joe J.
Since you are using absolute position feedback of the module to feed you PID, if the rotation speed is too high, you will get a lot of overshoot and jitter on the steering. You really can't control something that is geared for 1000s of RPMs to make 1 degree movements. You need to gear them down to a slower speed to control them. Making a 90 degree turn faster than your control system can respond is not desirable.

But the bigger issue is when you are driving straight and accelerating or decelerating, the drive motor excerts a torque into the steering turret. The steering motor needs to react that torque to hold the module straight. So, you end up applying some moderate torque loads at 0 RPM (stall). Gearing up the steering reduces those max stall loads. But they are still there.

So, you really need a motor that is suitable for operating at stall. Using the VEX motor testing data, the 775 Pro is not particularly good for operating at Stall. Several other motors that they have tested are very good at operating at stall. So, I think there are better options for steering motors than the 775. The Banebots 550 seems to do pretty well at low to moderate stall torque loads. The Andymark Neverest motor is really good at stalling. We talked about using that motor, but ended up using the 9015 since that is what we have used in the past. But from the data, the Neverest might be a better choice.
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Unread 05-21-2018, 09:12 AM
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Re: Shifting Swerve, Feedback wanted

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Originally Posted by wgorgen View Post

So, you really need a motor that is suitable for operating at stall. Using the VEX motor testing data, the 775 Pro is not particularly good for operating at Stall. Several other motors that they have tested are very good at operating at stall. So, I think there are better options for steering motors than the 775. The Banebots 550 seems to do pretty well at low to moderate stall torque loads. The Andymark Neverest motor is really good at stalling. We talked about using that motor, but ended up using the 9015 since that is what we have used in the past. But from the data, the Neverest might be a better choice.
According to the Vex motor testing data, you can stall a 775pro indefinitely at <4V, and for longer than an FRC match at 6V...
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Unread 05-21-2018, 10:09 AM
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Re: Shifting Swerve, Feedback wanted

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Originally Posted by pkrishna3082 View Post
According to the Vex motor testing data, you can stall a 775pro indefinitely at <4V, and for longer than an FRC match at 6V...
To add on to the 775 vs BAG discussion

Even in a drop in replacement situation (same gearing) the 775 pro lasts longer at stall for a given torque. Say you need the motor to resist a torque of 0.34 N*m, the 775 can do so at just 4V and will last forever doing so, the BAG motor on the other hand requires 8V to do so, after a few seconds 8 volts will not longer be enough... (I chose dummy numbers here, and yes the 775 does see torque fade as well, meaning it would require more voltage after a while, but the basic idea here is that the 775 is hardly working, and therefore happy, while the BAG is working hard and therefore unhappy)

Factor in the additional gearing you would likely see if using a 775 and it only looks better.

6328 looked into swapping our intake motors over to BAGs this year before champs when we decided to apply a small "holding power" to the wheels when we got a cube. Stalling 775s has become such a loudly publicized "bad idea" that we immediately looked at BAG motors (they look like CIMs, they must handle stall like CIMs right?) but because they required so much more power to get us the same "hold" they would have burned up, while the 775 was happy in this application.

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Unread 05-21-2018, 10:16 AM
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Re: Shifting Swerve, Feedback wanted

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Originally Posted by wgorgen View Post
Since you are using absolute position feedback of the module to feed you PID, if the rotation speed is too high, you will get a lot of overshoot and jitter on the steering. You really can't control something that is geared for 1000s of RPMs to make 1 degree movements. You need to gear them down to a slower speed to control them. Making a 90 degree turn faster than your control system can respond is not desirable.

But the bigger issue is when you are driving straight and accelerating or decelerating, the drive motor excerts a torque into the steering turret. The steering motor needs to react that torque to hold the module straight. So, you end up applying some moderate torque loads at 0 RPM (stall). Gearing up the steering reduces those max stall loads. But they are still there.

So, you really need a motor that is suitable for operating at stall. Using the VEX motor testing data, the 775 Pro is not particularly good for operating at Stall. Several other motors that they have tested are very good at operating at stall. So, I think there are better options for steering motors than the 775. The Banebots 550 seems to do pretty well at low to moderate stall torque loads. The Andymark Neverest motor is really good at stalling. We talked about using that motor, but ended up using the 9015 since that is what we have used in the past. But from the data, the Neverest might be a better choice.
We actually used a stalled 775 this year for holding the altitude on our shooter. We used a VP with three stages in a 200:1 ratio and the voltage was sub 4v. we never burned out a motor and it still works now. Also, reference the vex chart in the KOP
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Unread 05-21-2018, 10:48 AM
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Re: Shifting Swerve, Feedback wanted

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Originally Posted by pkrishna3082 View Post
According to the Vex motor testing data, you can stall a 775pro indefinitely at <4V, and for longer than an FRC match at 6V...
Yeah, I am probably not able to see clearly through the haze left over from all the 775pros that we smoked last year.

We swore off those motors this year and had a drama-free season (in sharp contrast to the drama-filled season last year with the 775pros). But, I suspect that if we study our motor uses carefully and really run the numbers, we may decide that we can run 775pros again with little to no risk of drama.
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