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Unread 04-02-2010, 11:08
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Re: Fisher Price Motor Power

Just so we are all on the same page, the thermal protection device is a bi-metal actuated switch. If you solder connections to the FP be careful as it may cause some misalignment of the contacts.
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Unread 04-02-2010, 11:10
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Re: Fisher Price Motor Power

Thanks for the warning. We did use the big soldering iron when soldering wires on an FP motor yesterday, in hopes that it would get soldered quickly enough to not heat up the motor terminal so much that the plastic support would melt.

I guess we need to worry about the thermal limiter too
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Unread 04-02-2010, 13:46
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Re: Fisher Price Motor Power

Quote:
Originally Posted by Joe Johnson View Post
...Some say I am too conservative but my experience with FIRST and with automotive actuators tells me that these numbers are not far from the right ones.
Dr. J.

As always, your numbers will prove to be very close to the mark and a 40% operation point on the FP puts it close to it's optimum efficiency point (a bit more than 10%). The higher the motor efficiency, the less heat you generate...

I would also note that the "chin-up" occurs at the end of the match and, coupled with the current draw of the motors, your battery voltage may not be optimal. Since power falls off as the square of the voltage, I'd be tempted to design for a two FP gearbox and de-populate the second FP after testing.

This should guarantee success...

JMHO,

Mike
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Unread 04-02-2010, 16:37
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Re: Fisher Price Motor Power

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Originally Posted by Mike Betts View Post
Chris,

While I am humbled among the likes of Al and the Good Doctor, I'll give you my 2 cents...

A motor at stall is delivering no mechanical power. All of the electrical power, even constant current, is converted to heat. Most external cooling that teams implement is over the case and not forced into the motor. The heat will build up in accordance with the thermal resistance of the internals of the motor.

This will likely happen very fast.

That little fan inside the FP is directly cooling the motor windings. If that little fan stops, my experience is that failure will occur very quickly.

In my opinion, the PTC added to the FP in recent years is to attempt to save a child's extremity or to mitigate an actual fire in the toy for which the motor was designed. It may not be fast enough to limit damage to the motor.

I would not design a system where the motor could stall.

JMHO,

Mike
Yes, we are all humbled

I think from the discussion, that the FP motor will trip the PTC if it is stalled, although, I have yet to see any test data. I would propose a test where the motor is stalled and voltage increased until the PTC trips to see what the max continuous current is (at least for the lab conditions) , except I don't want to risk a motor for the test. My hair dryer has tripped a few times when the dust builds up over the fan exhaust, but it has always recovered. I would hope the FP would recover too. Has anyone actually tripped an PTC yet?

I really am not sure how to design a fixed roller that sucks up a ball to ensure a motor will not stall. It would seem that this requires limiting the normal force on the roller which is difficult when a ball is squished between a wall and a robot. Even with a frictionless back bar, it seems the ball can deform and push hard against the roller. If the roller is at all sticky getting a normal force equal to the output torque/radius doesn't seem too difficult.
We are using a FP with a banebot 16:1, a 1.6 in dia roller. This can deliver about 30 lb of tangential force to the ball and getting a normal force 30/u_roller seems plausible for any reasonable u_roller.

Squirrel and others seem to be able to spin the ball on the rug which implies u_roller>u_rug. Using a slippery roller can keep from stalling probably , but degrades ball magnet performance.

So, we limit the normal force on the roller and actually shut down the motor when the normal force lifts up the roller. I still would be very happy to know if we do stall that the motor is not damaged.
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Unread 04-02-2010, 16:44
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Re: Fisher Price Motor Power

Quote:
[*]Planetary[list][*] Low ratio (<5:1) 85-90% (depends on how good the bearings are, the grade of the gears, the size of the planets w.r.t. their axles... things like that)[*] High ratio (>7:1) 50%[*] Very High Ratio (>20:1 -- "hunting tooth" stages) 15% <<these are
What would you use for a Banebot 16:1 P60 gearbox? Banebot would not fess up to any number so I've been using 40%.

Do you use this factor to modify the free current (free speed) on the output of the gear box? I generally don't, but I know the friction load will slow the free speed down.
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Unread 04-02-2010, 18:02
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Re: Fisher Price Motor Power

Chris,
I haven't this year but have in the past. As the device is a bi-metal switch it returns to normal when it cools the same as the breakers.
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Unread 04-02-2010, 18:22
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Re: Fisher Price Motor Power

Quote:
Originally Posted by Joe Johnson View Post
Yes, efficiency is a huge factor and very important for sizing gearboxes and designing mechanisms.

I use the following for designing using torque*:
  • Straight Spur Gear with good bearing condition 95%
  • Straight Spur Gear with funky bearing conditions 90% (look at the final stage of the FP transmission for an example of a funky bearing condition)
  • Planetary
    • Low ratio (<5:1) 85-90% (depends on how good the bearings are, the grade of the gears, the size of the planets w.r.t. their axles... things like that)
    • High ratio (>7:1) 50%
    • Very High Ratio (>20:1 -- "hunting tooth" stages) 15% <<these are great for speed reduction by division, but lousy for torque increases by multiplication
  • Helical Gear, parallel axis 80%
  • Helical Gear, cross axis see Worm Gear
  • Worm Gear -- totally depends on lead angle
    • Best case 50-60% (high lead angles of 40 deg, good bearings, etc.),
    • Worst case 5-15% (lead angles of 10 deg, bad thrust management, etc.)
  • Conical Gears / Bevel Gears depends on bear arrangement and alignment 60-90%
  • Chain 90% (assuming good alignment and tension)
This is PER STAGE.

Example: If you have a 4 stage 4:1 per stage spur gear gearbox with good bearings it would be .95^4 = 81% efficient. So... ...instead of getting a ratio of 256:1 your "effective ratio" (from a torque point of view) would be 207:1.

Continuing with the example, if you put a FP in with a stall torque of .45N-m then you would get 93N-m out of this gearbox, not 115N-m. Now suppose you are trying to lift your robot with this gearbox and you have the output connected to a .17m arm (and assume your robot weight is 600N, then you need 100N-m to lift your robot.

NOTE: You are not going to lift that robot, all you are going to do is turn a lot of electrons into heat.

Continuing, if you put a 3:1 chain stage between the arm and the gearbox, the effective ratio would be 560:1 (207X3*.9). You could put 250N-m of torque on your arm. Now your motor would be loaded at 40% of its stall during your lift (and the motor would be running at 60% of its free speed or the arm would be turning under load conditions at 12RPM = 16,000RPM *.6/(256*3)<<Note: Actual Ratio used for SPEED, Effective Ratio used for Torque).

Now you'd lift in a heartbeat (1/2 turn in 6 seconds -- well... ...kind of a LONG heartbeat ;-) and you have extra torque should another robot get in your way on the way up.

Life is good.... ...always.

Joe J.


*Some say I am too conservative but my experience with FIRST and with automotive actuators tells me that these numbers are not far from the right ones.
Joe,

WOW, this is one of the best and easiest to follow posts on this subject that I have ever read! Being an Electronics Engineer, this mechanical example goes a long way to explain to me how to approach this type of design.

I do have have one question though. The following comment lost me, I would like to know where you came up with the 100N/m value?

Quote:
...and assume your robot weight is 600N, then you need 100N-m to lift your robot.
I'm sure the answer is obvious to some, but for some reason I just can't seem to see how you came up with this value.
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Unread 04-02-2010, 18:27
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Re: Fisher Price Motor Power

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Originally Posted by billbo911 View Post
I do have have one question though. The following comment lost me, I would like to know where you came up with the 100N/m value?

I'm sure the answer is obvious to some, but for some reason I just can't seem to see how you came up with this value.
The assumed use of a .17m arm is the connection between the two numbers.

600N * .17m ~= 100 N-m
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Last edited by Vikesrock : 04-02-2010 at 18:52. Reason: typo
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Unread 04-02-2010, 18:42
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Re: Fisher Price Motor Power

Quote:
Originally Posted by Vikesrock View Post
The assumed the use of a .17m arm is the connection between the two numbers.

600N * .17m ~= 100 N-m
Got it! I should have realized .17~=1/6.
Thanks Vikesrock.

(Dang, if I had just opened my calculator I might have figured that out... )
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Unread 04-02-2010, 19:44
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Re: Fisher Price Motor Power

PTC is wrong. Bi-metal is right.

Al has been kindly pointing that out several times, but I have planted a bad seed and it is growing.

PTC is a kind of conducting plastic that has a positive temperature coefficient which is the secret sauce that allows it to be used as resetting breaker for current. Basically the more current it conducts the hotter it gets the hotter it gets, the higher the resistance (the positive coefficient, get it) the this causes more heat build up until you get a cascade where the resistance of the PTC is so high that it only conducts a very small amount of current that (hopefully) allows the fault to clear while the device it protects is kept from catching fire or melting or whatever. PTC's literally revolutionized the automotive electronics business about 20-25 years ago.

BUT... ...at higher currents, PTCs become problematic.

Bi-metal becomes more competitive at currents above 20-25Amps.

Bi-metal is just what it sounds like two types of metal stuck together (usually not literally stuck together but they are more often hot formed together). When the strip of bi-metal gets hot, one metal expands more than the other so the strip bends. This bending action is used to open a switch contact.

SO.... FP uses a bi-metal thermal switch to protect the motor.

Please make a note of it.

Joe J.

P.S. Thanks to Al for his encyclopedic (wikipedic?) knowledge of all things FIRSTian.
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Unread 04-02-2010, 19:51
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Re: Fisher Price Motor Power

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Originally Posted by vamfun View Post
Yes, we are all humbled

I think from the discussion, that the FP motor will trip the PTC if it is stalled, although, I have yet to see any test data. I would propose a test where the motor is stalled and voltage increased until the PTC trips to see what the max continuous current is (at least for the lab conditions) , except I don't want to risk a motor for the test. My hair dryer has tripped a few times when the dust builds up over the fan exhaust, but it has always recovered. I would hope the FP would recover too. Has anyone actually tripped an PTC yet?

I really am not sure how to design a fixed roller that sucks up a ball to ensure a motor will not stall. It would seem that this requires limiting the normal force on the roller which is difficult when a ball is squished between a wall and a robot. Even with a frictionless back bar, it seems the ball can deform and push hard against the roller. If the roller is at all sticky getting a normal force equal to the output torque/radius doesn't seem too difficult.
We are using a FP with a banebot 16:1, a 1.6 in dia roller. This can deliver about 30 lb of tangential force to the ball and getting a normal force 30/u_roller seems plausible for any reasonable u_roller.

Squirrel and others seem to be able to spin the ball on the rug which implies u_roller>u_rug. Using a slippery roller can keep from stalling probably , but degrades ball magnet performance.

So, we limit the normal force on the roller and actually shut down the motor when the normal force lifts up the roller. I still would be very happy to know if we do stall that the motor is not damaged.
If you can use a Jaguar and use CAN, you can easily know the current. There are other ways to measure or infer current (sensors, shut resistors, measure speed & calculate, etc.). Once you have current you can easily build a "how close to tripping the FP bi-metal breaker am I?" model. It will not be too hard. If you are using Labview, I think it may be trivial.

It won't perfect but FIRST almost never requires perfection. Build a simple predictor, tune in some parameters and viola! You can know if your rollers are on the edge of a nervous breakdown ;-)

Cheers,
Joe J.
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Unread 04-02-2010, 23:21
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Re: Fisher Price Motor Power

Quote:
Originally Posted by Joe Johnson View Post
If you can use a Jaguar and use CAN, you can easily know the current. There are other ways to measure or infer current (sensors, shut resistors, measure speed & calculate, etc.). Once you have current you can easily build a "how close to tripping the FP bi-metal breaker am I?" model. It will not be too hard. If you are using Labview, I think it may be trivial.
As a controls engineer, this was my first thought but from this thread it seems the problem requires motor speed to be of any use. When Al and I were pondering the Jaguar linearity vs frequency phenomenon, I had proposed a BackEMF motor speed project but never got around to doing it last summer. I wish I had so we could just use the CAN bus and motor voltage to create a current trip schedule without the use of an encoder.

The trip level could be as simple as a linear function of speed. So when running at speed "w" we set the trip at w/w_free*i_Stall+ delta where the delta is the safety current margin. When stalled , the motor would shut down if current was > delta. Of course there would probably be time elements involved. delta would probably be on the order of a few amps to 10 amps depending on the "allowed time to exceed the scheduled limit" criteria.


Apologies to AL for not picking up on the bi-metal distinction.
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Unread 04-02-2010, 23:36
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Re: Fisher Price Motor Power

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Originally Posted by vamfun View Post
As a controls engineer, this was my first thought but from this thread it seems the problem requires motor speed to be of any use. When Al and I were pondering the Jaguar linearity vs frequency phenomenon, I had proposed a BackEMF motor speed project but never got around to doing it last summer. I wish I had so we could just use the CAN bus and motor voltage to create a current trip schedule without the use of an encoder.<snip>
Now we have gotten far afield but if you want a project to implement on the Jaguar, consider the following:

This paper (Closed-Loop Motion Control for Mobile Robotics -- by Rich LeGrand available for $1.50) makes reference to the idea of using Back EMF to control a motor. The great thing about it is that you don't need to http://www.chiefdelphi.com/forums/ne...ly&p=913687put any extra stuff on the motor to get PID control. FYI, Rich LeGrand is from Charmed Labs which hosts this example video that dropped my jaw.


If you want to learn more about how it works, look at this link from another cool robot website, Acroname.com. I think they really do a pretty good job of making the concepts understandable. Actually, Acroname.com is part of the way there already in that they offer an H-bridge with velocity feedback built in.

So... I want to see this built in as a feature of the Jaguar. You could command motor speed for any motor on the robot! Think of the well behaved robots you'd see if the default code implemented velocity control as the standard method move a drive wheel? Need a beater bar to move a a standard speed without having the fuss an muss of adding a sensor and implementing a feedback loop? no prob.

This would be a very big deal for FIRST robots.

For your consideration.

Joe J.
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Unread 05-02-2010, 01:38
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Re: Fisher Price Motor Power

Quote:
Originally Posted by Joe Johnson View Post
Now we have gotten far afield but if you want a project to implement on the Jaguar, consider the following:

This paper (Closed-Loop Motion Control for Mobile Robotics -- by Rich LeGrand available for $1.50) makes reference to the idea of using Back EMF to control a motor. The great thing about it is that you don't need to http://www.chiefdelphi.com/forums/ne...ly&p=913687put any extra stuff on the motor to get PID control. FYI, Rich LeGrand is from Charmed Labs which hosts this example video that dropped my jaw.


If you want to learn more about how it works, look at this link from another cool robot website, Acroname.com. I think they really do a pretty good job of making the concepts understandable. Actually, Acroname.com is part of the way there already in that they offer an H-bridge with velocity feedback built in.

So... I want to see this built in as a feature of the Jaguar. You could command motor speed for any motor on the robot! Think of the well behaved robots you'd see if the default code implemented velocity control as the standard method move a drive wheel? Need a beater bar to move a a standard speed without having the fuss an muss of adding a sensor and implementing a feedback loop? no prob.

This would be a very big deal for FIRST robots.

For your consideration.

Joe J.
Maybe you can repost this to

http://www.chiefdelphi.com/forums/sh...light=back+emf

You probably were not hanging around CD when I started this thread else I'm sure you would have contributed then.

Last edited by vamfun : 05-02-2010 at 18:24.
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Unread 05-02-2010, 14:49
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Re: Fisher Price Motor Power

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Originally Posted by vamfun View Post
What would you use for a Banebot 16:1 P60 gearbox? Banebot would not fess up to any number so I've been using 40%.

Do you use this factor to modify the free current (free speed) on the output of the gear box? I generally don't, but I know the friction load will slow the free speed down.
Sorry I missed this. The 16:1 is a planetary with 2 stages. I have not taken the P60's apart so I cannot vouch for the particulars, but generally, the Banebot stuff is pretty solid. I would expect that they are close to the 90% range but if it was a mission critical task, and I just HAD to be sure I had enough torque to do the job, I would drop it down to 85%.

Again, this is PER stage, so... ...general use, I would say 81% effecient from motor shaft to output shaft (effective torque multiplier of 13). Mission Critical (for example, lifting a robot), I would use 72% (effective torque multiplier of 11.5).

Callin' 'em as I see 'em since 1995...

Joe J.
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