Matching Drill Motors and Fischer-Price Motors

Posted by Tim Gates at 2/14/2001 9:33 AM EST

Engineer on team #288, RoboDawgs, from Grandville High School and X-Rite/Delphi.

A 4 to 1 gear ratio brings the no load RPM range of the two motors within reasonable range of each other. A 3.6 to 1 might be better, but we used a 4 to 1. This leaves the FP motor with a slightly higher RPM than the drill motor. We ran both motors individually on the same load and measured the RPM over the entire range of the speed controller. Unfortunately the load wasn’t enough to keep the RPM under the inherent balance of Counter Electromotive Force(CEMF) and Electromotive Force(EMF), the balance between the volatge applied to the armature and the voltage induced into the armature as it spins through the flux from the field magnets. However, if you apply excessive load to the drive train and lock rotor, there is no CEMF and both motors are contributing the maximum torque possible to the system. This torque is limited by the DC resistance of all current paths involved from the battery to the armature. As you lessen the load towards the baseline friction of the two motors, the two motors continue to contribute torque to the system until one of them hits the RPM where its CEMF opposes EMF enough to limit rpm. At that point if you increase RPM without increasing the current through the armature, the CEMF will exceed EMF and the motor becomes a load on the system. I haven’t completed enough test yet to prove it, but I intuitively believe that a 130lbm on carpet is enough load to keep the RPM below the situation described above. The other consideration is regenerative braking. When the speed controller output drops to nothing and the robot is coasting, the motors become generators. I believe the jumper on the speed controller determines how that current is handled.

Posted by Tim Gates at 2/14/2001 9:36 AM EST

Engineer on team #288, RoboDawgs, from Grandville High School and X-Rite/Delphi.

In Reply to: Matching Drill Motors and Fischer-Price Motors
Posted by Tim Gates on 2/14/2001 9:33 AM EST:

I forgot to mention that the 4 to 1 ratio was for the FP motor and 1 to 1 on the drill motor.

Posted by Andy Baker at 2/14/2001 10:15 AM EST

Engineer on team #45, TechnoKats, from Kokomo High School and Delphi Automotive Systems.

In Reply to: Re: Matching Drill Motors and Fischer-Price Motors
Posted by Tim Gates on 2/14/2001 9:36 AM EST:

We are using both the F-P motor and the Bosch Drill motor to drive our tank treads.

But, we are simply using a 1:1 ratio.

It seems to work so far.

Andy B.

Posted by Jason Rudolph at 2/14/2001 3:29 PM EST

Other on team #459, Rampage, from University of Florida/Eastside High School and .

In Reply to: We’re using both motors also
Posted by Andy Baker on 2/14/2001 10:15 AM EST:

We are using the FP and the drill motors as well. We have a 3:1 on the drill motors, and a 1:1 on the FP motors, and everything seems to work fine for us as well.

Jason

Posted by Shawn McMahon at 2/15/2001 7:04 PM EST

Student on team #263, Aftershock, from Sachem H.S. and Symbol, Citycorp., Apoge, CA …

In Reply to: Re: We’re using both motors also
Posted by Jason Rudolph on 2/14/2001 3:29 PM EST:

I designed the drive system and I believe (have so many #'s in my head can’t remember quite right anymore) we have a 3:1 from the fp to drill, then a 1:2 from the drill to the drive axle. I calculated it all out and at that ratio, running 50% rpm on both motors (most power) and 70% efficiency we should be able to pull our robot +130lbs up the 15-degree bridge at 4ft/s. We haven’t tested it. We’re testing it tomorrow. It should work well. If not we can just switch out a sprocket or two.

Posted by Tim Gates at 2/15/2001 10:23 PM EST

Engineer on team #288, RoboDawgs, from Grandville High School and X-Rite/Delphi.

In Reply to: We’re using both motors also
Posted by Andy Baker on 2/14/2001 10:15 AM EST:

With a 1:1 ratio, how hot are the FP motors getting after a couple minutes of running up and down the ramp? I’m always looking for a better way.

Tim Gates