Re: Electromagnetic gate latch?
 

Dick,
One would think that the motor and an electromagnet would have similar reactions, but we must consider that the motor is a series of small windings that are moving at a varying rate and switched by the brush assembly. In the case of the magnet, I am thinking the collapsing field of a single coil at steady state may produce a much higher voltage pulse. Back EMF in the motor, the relative permeability of the coil and the series resistance of the windings are all variables that make this hard to consider. Certainly 350 ma is small compared to the motor, but that might also indicate a relatively high resistance which in turn suggests a lot of wire in the coil. The discharge path is through the diodes in the FETs so things should be fine but it does raise a question in my mind. I would be interested in your testing as to the relative heating of the coil while under control of the Jaguar.

Re: Electromagnetic gate latch?
 

Dick,

I would be extremely careful in checking with the GDC on this use. I feel that the application of force at a distance may be interpreted as being a solenoid by some inspectors.

My thinking is this: If there is any movement while the EM is energized, then the EM implementation would be a solenoid in the GDC's definition.

The GDC did not say that movement away from the EM while the EM was energized would be legal.

As Al stated, the GDC has been very cloudy in this area.

Please realize that I do not like sounding like I am critical of the design. I love it... However, if I were at a competition right now with the guidance that I have right now, I'd likely rule this design illegal.

I personally think that the GDC should have made solenoids legal and given clear guidance on their use weeks ago when they decided that EMs would be legal...

Good Luck,

Mike

Re: Electromagnetic gate latch?
 

When energized, if an EM causes something to MOVE, it is an ACTUATOR => ILLEGAL.
When de-energized, if an EM ALLOWS something to BE MOVED by force from another source, it is NOT an actuator => LEGAL.

The key is to make sure that when EM is energized that NOTHING MOVES from its EM force

-Dick Ledford

Re: Electromagnetic gate latch?
 

Mike,
Thanks for the advice and appreciation of our idea. We've submitted a question on the GDC Q&A.

Al,
Thanks for the analysis. There are clearly several things to be concerned about when trying to use the Jaguar such as voltage spikes and EM radiation.

Dick,
Is a holding force still a holding force if other forces build up against it and exceed it? That is the crux of our question to the GDC.

Our original intent (week one) was to try to do this force thresholding with permanent magnets, but we thought it would be easier to tune to multiple values using electromagnets.

Re: Electromagnetic gate latch?
 

Unless you can point to a rule or GDC decision, this is just your opinion on this.

We are trying to avoid having the inspectors make that decision based on their opinion of how it's supposed to be (else we'll have things deemed illegal at some regionals and legal at others).

Re: Electromagnetic gate latch?
 

For teams wishing to use an electromagnet, here is (I believe) a legal alternative:

Mount rectangular permanent magnets on a rotatable shaft. To hold the load, the magnet surface is positioned parallel to a ferrous plate. when you want to fire, either exceed the holding force, or rotate the shaft. If you desire variable holding strength, you could use different strength magnets that are presented in different orientations of the shaft. Shaft position is somewhat critical here, but cams and micro-switches would do the job of position determination easily enough.

Permanent magnets of truly amazing holding power can be found for a reasonable price at this website

Re: Electromagnetic gate latch?
 

Martin,

Once again, I would seek guidance from the GDC before going too far down this path.

The movement of any system is caused by the sum total of the forces acting on that system.

If you fire by exceeding the holding force, the movement is caused, in part, by the force of the electromagnet. The fact that the movement is away from the electromagnet is, to me, irrelevent.

As I said to Dick (above), knowing what I now right now, I'd have to say that this mechanism would be illegal.

Please read the GDC's post here to understand my dilemma.

Regards,

Mike

Re: Electromagnetic gate latch?
 

Wait, what electromagnet? I did say "permanent magnets". There is NO rule regarding the use of permanent magnets. If the GDC wants to make ALL magnets illegal, they need to make a rule change.

By the way, we are not using magnets in any form (other than what is in the KOP motors) this was just a suggestion for teams are planning to use magnets.

Re: Electromagnetic gate latch?
 

Martin,

My fault. I guess I should take advantage of this snow day and go back to bed...

You are correct. Your suggested design using permanent magnets would be legal.

Regards,

Mike

Re: Electromagnetic gate latch?
 

I was playing with a nifty permanent magnet bearing an on-off switch a couple of weeks ago, thinking about how it could be used instead of an electromagnet. I don't know where it came from; it's just been sitting on a metal workbench at our shop for months. But it's a lot of fun to see how well it holds to the surface, and then flip the lever and find that it barely sticks at all.

Re: Electromagnetic gate latch?
 

Similar to a Manet base for an indicator I would presume... Interesting I would of never thought to use one them on the robot....

Re: Electromagnetic gate latch?
 

The GDC responded to our question regarding EMs used as a holding force that is exceeded by other forces.

http://forums.usfirst.org/showthread.php?t=14599

"We cannot approve/disapprove specific designs. It is noted however that electromagnets that are used to increase the effective inertia of an object are generally acceptable, while electromagnets that are used to increase the momentum of an object are not."

Our interpretation is that our team's use of variable voltage on an EM is consistent with the note the GDC of "used to increase the effective inertia of an object are generally acceptable."

Does anyone want to offer a different interpretation?

Re: Electromagnetic gate latch?
 

I think the GDC is saying that electromagnets used as a latching mechanism would be legal, but using an EM to actually 'move' something (ie. a physical latch) would be illegal.

Re: Electromagnetic gate latch?
 

Basically, if something moves when you turn an EM on, bad.

If the EM makes something NOT move, and when you turn it off, your kicker fires, then alright.

All of this fancy stuff confuses me, so I prefer the pneumatic gate latch route.

Re: Electromagnetic gate latch?
 

OK,
After looking at everything in the mix and coming to an awakening, here is the test I believe the GDC expects inspectors to use.
If you apply electrical power to something and something else moves as a result, it must be a legal motor or servo only.
If you use a legal motor or other legal actuator to push something against an electromagnet and then apply power, and the power merely holds the object in place, then it is a legal use of electromagnets as answered in the Q&A here, http://forums.usfirst.org/showthread.php?t=14595.
If the removal of that power from an electromagnet, causes an object to move because it is acted on with other legal forces, be that electrical, pneumatic or deformed parts, then it is also legal.