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  #16   Spotlight this post!  
Unread 24-06-2002, 00:20
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Re: Difference in forward/reverse drill motor speed

Posted by Al Skierkiewicz at 2/6/2001 1:46 PM EST


Engineer on team #111, Wildstang, from Wheeling High & Rolling Meadows High and Motorola.


In Reply to: Difference in forward/reverse drill motor speed
Posted by Patrick Dingle on 2/5/2001 11:38 PM EST:



I am having a tough time with this one. I have not heard that there is a difference between forward and reverse. (although I'm old and miss a lot.) The motors are electrically identical in either direction, the transmission is the same, the output shaft is the same and the speed controller is the same. That leaves only two places outside of drive train for the anamoly to occur. 1. The joystick pot is not symetrical (but this should be corrected in the calibration of the speed controller) or 2. the full data is not being sent to the controller (software correctable.) We had a problem a few years ago until we realized that the controller calibrates itself (stores the full swing data points) only when you release the button. I will have to check with Raul on more info on this.


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Re: Difference in forward/reverse drill motor speed

Posted by Matt Leese at 2/6/2001 5:54 PM EST


Other on team #73, Tigerbolt, from Edison Technical HS and Alstom & Rochester Institute of Technology.


In Reply to: Re: Difference in forward/reverse drill motor speed
Posted by Al Skierkiewicz on 2/6/2001 1:46 PM EST:



The whole reason this exists is because of the brush formation. The brushes in the window and seat motors are symmetrical in both directions which means they will go the same speed forward and back. However, in things such as the drill motors and fisher price motors, the brushes are aligned to be more efficient in one direction. I'm not sure why they do this, whether it increases power or not or it's just cheaper, this is the way they are designed. So it's not something that can be worked around.

Matt
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Is this true?

Posted by Joe Johnson at 2/6/2001 9:07 PM EST


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.


In Reply to: Re: Difference in forward/reverse drill motor speed
Posted by Matt Leese on 2/6/2001 5:54 PM EST:



I have looked at these motors for years now and I have
never noticed that the brushes are actually not 180
degrees apart from each other on the Fisher Price and
Drill motors.

Will someone please confirm this for me?

Joe J.



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Re: Is this true?

Posted by Matt Leese at 2/6/2001 9:29 PM EST


Other on team #73, Tigerbolt, from Edison Technical HS and Alstom & Rochester Institute of Technology.


In Reply to: Is this true?
Posted by Joe Johnson on 2/6/2001 9:07 PM EST:



In all honesty I have no idea where I got that bit of information from but I'm fairly certain I picked it up somewhere last year and I seem to remember getting it from a reliable source. So I may just be spouting off random stuff.

Matt



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Re: Is this true?

Posted by PAUL GIANNOSA at 2/6/2001 10:10 PM EST


Engineer on team #27, TEAM RUSH, from OSMTECH and TEXTRON AUTOMOTIVE CO..


In Reply to: Re: Is this true?
Posted by Matt Leese on 2/6/2001 9:29 PM EST:



: In all honesty I have no idea where I got that bit of information from but I'm fairly certain I picked it up somewhere last year and I seem to remember getting it from a reliable source. So I may just be spouting off random stuff.

: Matt
HEY. LAST YEAR, SOMEONE FROM TEAM 33, WHO I WORKED WITH AT WORK, TOLD ME THE MOTORS PRODUCE MORE TORQUE FORWARD THAN IN REVERSE. I TOLD HIM HE HAD BEEN SMOKIN' SOME FUNNY STUFF AND HE HAD MORE THAN A FEW SCREWS LOOSE. WELL, TO MY SUPRISE, WHEN WE WERE AT THE GREAT LAKES REGIONAL, THE BOT PULLED TO THE RIGHT. GUESS WHICH SIDE HAD THE MOTOR RUNNING IN R(FOR RACE? I SMILED, WITH FEATHERS IN MOUTH, AND TRUDGED ON FOR A REASON WHY I NOW HAD EGG ON MY OAKLEYS. TURNS OUT THAT THE WINDINGS IN THE ARMATURE ARE WOUND A PARTICULAR WAY TO PROVIDE MORE TORQUE IN FORWARD THAN IN REVERSE. TAKES MORE GRUNT TO DRIVE A SCREW THAN DOES TO PULL IT FROM ITS HOLE. BEST WAY TO OVER COME THIS"PROBLEM" IS TO RUNN BOTH MOTORS IN SAME DIRECTION. JUST PACKAGE THEM CREATIVLY. PMGRACER


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Unread 24-06-2002, 00:20
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Re: Is this true?

Posted by Michael Betts at 2/7/2001 7:20 AM EST


Engineer on team #177, Bobcat Robotics, from South Windsor High School and International Fuel Cells.


In Reply to: Is this true?
Posted by Joe Johnson on 2/6/2001 9:07 PM EST:



There is no doubt that there is a small difference between forward and reverse on the drill motors. This was also true of the Milwaukee motors we used years ago.

However, I agree with Joe's earlier post. Team 177 has never done anything, mechanically or electrically, to try and "fix" this difference. The drivers learn to compensate almost immediately as soon as we get them practicing.

Build it and drive!

Mike



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Unread 24-06-2002, 00:21
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Re: Is this true?

Posted by Brian Cholerton at 2/7/2001 12:12 PM EST


Engineer on team #311, Red Jammers, from East Islip High School and KeySpan Energy, Multiline Corp., Computer Assoc..


In Reply to: Is this true?
Posted by Joe Johnson on 2/6/2001 9:07 PM EST:



The brushes are at 180 degrees from each other. The alignment has to do with the timing of the brushes and the commutator in relation to the magnets. Electric motors are usually set to have the timing advanced by a few degrees so that the motor can make more power (torque) more efficiently in one particular direction. If a motor were set up to be the same in both directions it would definitely affect the torque curve. Similar to an internal combustion having it's timing before top dead center but obviously not for the reason of burning fuel.

Best of luck!


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Re: Difference in forward/reverse drill motor speed

Posted by Al Skierkiewicz at 2/6/2001 1:46 PM EST


Engineer on team #111, Wildstang, from Wheeling High & Rolling Meadows High and Motorola.


In Reply to: Difference in forward/reverse drill motor speed
Posted by Patrick Dingle on 2/5/2001 11:38 PM EST:



I am having a tough time with this one. I have not heard that there is a difference between forward and reverse. (although I'm old and miss a lot.) The motors are electrically identical in either direction, the transmission is the same, the output shaft is the same and the speed controller is the same. That leaves only two places outside of drive train for the anamoly to occur. 1. The joystick pot is not symetrical (but this should be corrected in the calibration of the speed controller) or 2. the full data is not being sent to the controller (software correctable.) We had a problem a few years ago until we realized that the controller calibrates itself (stores the full swing data points) only when you release the button. I will have to check with Raul on more info on this.


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Re: Difference in forward/reverse drill motor speed

Posted by Patrick Dingle at 2/6/2001 3:04 PM EST


Other on team #639, Red B^2, from Ithaca High School and Cornell University.


In Reply to: Re: Difference in forward/reverse drill motor speed
Posted by Al Skierkiewicz on 2/6/2001 1:46 PM EST:



It has to be something with either the motor or the gearbox. When I noticed it, I was hooking up the battery directly to the motor. You can hear the difference (and measure it with proper equipment) in speeds from forward and reverse.

Patrick

: I am having a tough time with this one. I have not heard that there is a difference between forward and reverse. (although I'm old and miss a lot.) The motors are electrically identical in either direction, the transmission is the same, the output shaft is the same and the speed controller is the same. That leaves only two places outside of drive train for the anamoly to occur. 1. The joystick pot is not symetrical (but this should be corrected in the calibration of the speed controller) or 2. the full data is not being sent to the controller (software correctable.) We had a problem a few years ago until we realized that the controller calibrates itself (stores the full swing data points) only when you release the button. I will have to check with Raul on more info on this.


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Yes, there is a difference.

Posted by Al Skierkiewicz at 2/7/2001 9:11 AM EST


Engineer on team #111, Wildstang, from Wheeling High & Rolling Meadows High and Motorola.


In Reply to: Re: Difference in forward/reverse drill motor speed
Posted by Al Skierkiewicz on 2/6/2001 1:46 PM EST:



I have checked with Raul and Jim B and they HAVE experienced a variation in forward/reverse speed. In past years a thorough investigation showed as much as 10% variation in speed from unit to unit and forward/reverse. As Joe has suggested, (we agree) getting drivers to invest time behind the wheel is the best way to overcome the problem. I am still having a problem in analyzing why this occurs. Production variations can explain unit to unit changes but not for/rev. Any other input?
Al


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Unread 24-06-2002, 00:21
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Re: Yes, there is a difference.

Posted by Dave... at 2/8/2001 9:03 PM EST


Engineer on team #5, Robocards, from Melvindale, MI and AFL (Alcoa Fujikura Ltd).


In Reply to: Yes, there is a difference.
Posted by Al Skierkiewicz on 2/7/2001 9:11 AM EST:



DC motors operate by inducing a magnetic field created by the armature within the magnetic field of the permanent magnets in the housing. As any elementary science teacher can attest to, a wire wrapped around a nail with DC current running through it will create a directional magnetic field, and by changing the polarity of the wires, the magnetic field will change 180° (N becomes S and vice versa).

The armature is wrapped with magnet wire from the inside out, and so running current in opposite directions will not only change the direction of the magnetic field from the armature, but will interact differently with the magnetic field from the permanent magnets, which does not get to change its directional field.

I imagine that a DC motor which does not use permanent magnets but rather windings in the housing to create the second magnetic field is not as susceptible to power loss in forward vs reverse. Both fields would change 180° any time the polarity was reversed, keeping roughly the same power in either direction.

We simply limit the power to the stronger drive motor by a trial and error method in the programming. If the robot steers to the right, we decrease the maximum power available to the left drive motor (e.g. p1_y = p1_y MAX 228) until the thing goes straight. Crude, but effective and easy to change.

My two cents worth.

: I have checked with Raul and Jim B and they HAVE experienced a variation in forward/reverse speed. In past years a thorough investigation showed as much as 10% variation in speed from unit to unit and forward/reverse. As Joe has suggested, (we agree) getting drivers to invest time behind the wheel is the best way to overcome the problem. I am still having a problem in analyzing why this occurs. Production variations can explain unit to unit changes but not for/rev. Any other input?
: Al


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