2005 Motor Information

[Paul Copioli]

Please see this thread, post #23:

http://www.chiefdelphi.com/forums/sh...light=Fish er

I took apart the gearbox and counted teeth.

[HPA_Robotics_13]

My AP physics teacher (along with my school's freshman physics teacher) just explained to me that gear reduction does not do anything to change the motors torque. Thus, after the 1:180 reduction of the gearbox and then another 1:10 to get it down to 12 rps, your system still has only .324 Nm at 6V.
We decided that a system using the fisher price motors should have a worm gear to give it the torque that the motor lacks to lift a 1.5 meter 100+ newton arm.

Am I just completely ignorant to have assumed that you would be driving the arm from a direct chain link to the FP gearbox shaft or is my teacher wrong?

I must be missing something becaue many people here, such as jgannon, have explicitly stated that the torque is greater after reduction.

[ChrisH]

Originally Posted by HPA_Robotics_13

My AP physics teacher (along with my school's freshman physics teacher) just explained to me that gear reduction does not do anything to change the motors torque. Thus, after the 1:180 reduction of the gearbox and then another 1:10 to get it down to 12 rps, your system still has only .324 Nm at 6V.
We decided that a system using the fisher price motors should have a worm gear to give it the torque that the motor lacks to lift a 1.5 meter 100+ newton arm.

Am I just completely ignorant to have assumed that you would be driving the arm from a direct chain link to the FP gearbox shaft or is my teacher wrong?

I must be missing something becaue many people here, such as jgannon, have explicitly stated that the torque is greater after reduction.

Your teacher is right when they way that adding gear reduction does nothing to change the motor's torque. A motor with .324 Nm of torque will always put out that much torque, AT THE MOTOR SHAFT.

However adding a gearbox to the system changes the system and therefore the output will change. Power = Torque x omega, where omega = rotational speed in radians/sec. Since the motor is putting out a given amount of power at a certain speed, and we can assume that power in to the gearbox is equal to power out (we'll neglect losses for the moment), then reducing the RPMs will result in a proportional increase in torque.

As I've said numerous times before, design your systems so the motor is operating at no more than 25% of stall torque and at least 75% of free speed and you should be fine. Electric motors like to run fast and this is especially true of the FP motors. By setting up you system this way, the motor actually starts producing more power when it bogs down and is more likely to pull through should it meet a higher than expected load.

Running the motor that fast also means that there is lots of gear reduction. The gear reduction helps reduce the holding torque required to help the arm hold position. If you go far enough with the reduction, then the inherent braking of the speed controller will be enough. If you can't go that far, then you are at least reducing the amount of voltage and current your motor requires to hold position. Either way you are helping your motor to "live long and prosper".

Keep 'em cool, run it fast!

[HPA_Robotics_13]

Thank you!

(PS. so, then, assuming your gear reduction is sufficient, the worm gear stuff isn't necessary, I suppose.)

[HPA_Robotics_13]

My teacher says you're wrong...

I'm going over to my garage right now to do a test, and find out experimentally.

I am inclined to believe the people involved with FIRST in this case, as they have physical experience with the issue.

Thanks, ChrishH

[ChrisH]

Originally Posted by HPA_Robotics_13

My teacher says you're wrong...

I'm going over to my garage right now to do a test, and find out experimentally.

I am inclined to believe the people involved with FIRST in this case, as they have physical experience with the issue.

Thanks, ChrishH

I hope your test works out OK and that you don't burn anything up. One of our former students, now a graduated engineer and mentor in his own right, pointed out recently that the only time we have ever fried an FP motor was in my garage, doing torque vs speed tests. I'd forgotten about that. It must have been back in 99 or so.

Testing is generally the best way to convince yourself that something is or isn't true. However you can't necessarily test everything for yourself, if only because there are too many things to test! In that case you often have to rely on the experience of others. In this case I can prove the answer mathematically as well as practically, and there are generations of engineering and science textbooks to back it up. The case is pretty solid, though your teacher kind of scares me. If this person is teaching Physics, they really should understand the concept of the lever. It is the Basic Machine that is operating here and any Physicist should recognize it as such. I hope it is just a lack of mutual understanding as to what you are really asking.

Christopher H Husmann, PE (Mechanical)

[HPA_Robotics_13]

I think it was pretty much a lack of understanding.

I played around with the motors, gearboxes, and some sprockets and convinced myself that the reductions I was planning to do to have the FP run our arm will in fact have more than enough torque, with one FP motor, to lift the arm.

Just to make sure I understand: The "stall torque" of a motor is the torque it provides (or the torque it takes to turn it) when the motor is not moving. So if I am to take a motor that is not hooked up to any sort of power source, the torque I apply to force the output shaft to rotate is that motor's "stall torque."

Thus: The reductions on my robot's FP system are such that with a full load (a tetra at the end of the arm) and no power, the system does not move. This means that I am putting no load on my FP motor when my arm is stopped at a 45 degree angle. I am producing no heat! This means that I do not have to worry about overheating my motors, because I have done such a large reduction on my system.

Does this all sound valid?

[petek]

Originally Posted by HPA_Robotics_13

I think it was pretty much a lack of understanding.

Just to make sure I understand: The "stall torque" of a motor is the torque it provides (or the torque it takes to turn it) when the motor is not moving. So if I am to take a motor that is not hooked up to any sort of power source, the torque I apply to force the output shaft to rotate is that motor's "stall torque."

Thus: The reductions on my robot's FP system are such that with a full load (a tetra at the end of the arm) and no power, the system does not move. This means that I am putting no load on my FP motor when my arm is stopped at a 45 degree angle. I am producing no heat! This means that I do not have to worry about overheating my motors, because I have done such a large reduction on my system.

Does this all sound valid?

Sorry, but Stall Torque is the torque developed by the motor at the specified supply voltage with the rotor held stationary. Another way to look at it is the torque load applied to the motor which causes its rotation to stop (stall) when energized.

If you are relying on the de-energized motor magnetic torque (and drive friction) to hold your arm stationary, beware of unexpected loads caused by inertia, other robots, etc. and of changing system losses (gears wearing in, etc.). You may find your arm starts dropping at unpleasant times - like when the buzzer sounds and you just got clear of the tetra you capped with.

[HPA_Robotics_13]

Thank you for the clarification, petek.

How would one normally go about having the arm stopped at a certain height, then? Hopefully you don't keep going up until the torque of the arm is such that the motor reaches stall torque so the system stops there. What is the usual method for this? Obviously, I want to be able to stop at various positions.

[Zanella BR]

Really great job!

Unfortunately, when I discovered this, our robot was almost done...

[sciguy125]

Originally Posted by Tristan Lall

In fact, sciguy125 is mostly right, except that usually, you plot speed vs. torque with stall torque (the max.) at the extreme right of the x-axis, and the torque corresponding to free speed (i.e. 0 torque) at the origin.

I was plotting speed as the independent (x-axis). I should have mentioned that you can just find the inverse to switch the axes.

[MASherry]

if u guys want motor specs or a motor spec clauculator..go to www.montclairrobotics.org and click downloads u can d/l the spec sheet for each motor or the calculator which calcuklates the torque and speed at a given amperage... (at least i think thats what it does)

[Nuttyman54]

Sorry, I didn't explain myself. I needed the CORRECT power curves for the Jideco, seeing as the ones FIRST gave us for it is incorrect. I have double checked and also confirmed with Ken Stafford, an ME Teacher at WPI and mentor of team #190.

[the_short1]

ok.. i have a little issue...
the globe data seems to be off
for the runnign current: 0.58 is right, i tested that b4..

but when i tested the stall current (we were thinking on using a resetable fuse, instead of limit switches for our manipulator instead of running wires etc)

.. it only ammounted to aprox 5amps... not 20.. . i was using a mastercraft multimeter with 10A max . .. i also tested our robot running in the air full throtle at 11A... .so...meter can go a lot higher then 5... is anyone else getting less then 20A for stalling?

also if u know any vehicles (specific) that i could find a Taigene motor out of (at a junk yard etc).< please email me short1@gmail.com. .thx