Electromagnetic gate latch?

[EricH]

Yes. If the electromagnet is causing the motion, then it is considered a solenoid actuator and illegal (<R53>). If anything else is causing the motion, and the electromagnet is acting as a latch, then the electromagnet is considered an electromagnet and legal.

As far as buying/making your own, you'd probably want to ask that in Q&A. It would probably wind up being answered as "both", but you never quite know.

[RRLedford]

Originally Posted by EricH

Yes. If the electromagnet is causing the motion, then it is considered a solenoid actuator and illegal (<R53>). If anything else is causing the motion, and the electromagnet is acting as a latch, then the electromagnet is considered an electromagnet and legal.

As far as buying/making your own, you'd probably want to ask that in Q&A. It would probably wind up being answered as "both", but you never quite know.

So when you say "causing the motion", this would mean initiating a force on an element applied so as to move it over a distance -- i.e. doing work on an element. But a static steel plate stuck to an electromagnet is not in motion, despite the force being applied to hold it in its static position, so, de-energizing the electromagnet merely allows the pin release spring to now do the work of disengaging the pin, and allowing the release of the swing arm.

This seems fairly clear. Just no pushing or pulling MOVEMENT of elements by forces developed within an electromagnetic device are allowed. Only for HOLDING elements in a static position can an electromagnetic device's force be used.
-Dick Ledford

[EricH]

Yes.

Note that the static position seems to be defined relative to the electromagnet--a team asked Q&A about whether a moving electromagnet (moved by a motor) would be legal, and got an affirmative answer.

[ebarker]

Originally Posted by RRLedford

Is it that an electromagnet MOVING an element is forbidden, but RELEASING ITS GRIP on an element is allowed?
-Dick Ledford

The $ 64,000 question !! holding versus moving... need an authoritative definition of 'solenoid'.

[Chuck Glick]

I'm guessing that the "holding" they refer to is the electromagnetic force created by the electromagnet. 272 (2008) physically moved the magnet and used the electromagnetic force to do the "holding".

[Al Skierkiewicz]

Dick,
I know this is confusing, I have yet to understand how Inspectors will be able to make a call on this issue. A second question has been answered on the Q&A here...
http://forums.usfirst.org/showthread.php?t=14470
I am not sure I fully understand that one either. I can explain a device we allowed in a previous year. A clutch was formed by two plates held a set distance apart. One contained a material that would generate a magnetic field in the presence of another field. The other plate contained an electromagnet. The two plates rotated independently one driven by a KOP motor and the other attached to a mechanism. With current the two plates rotated in sync as if attached and the motive force of the KOP motor attached to one was transferred to the other.
So in the question above, a simple plate is used to trigger a moving object, it would seem. If the plate doesn't move simply by the presence of the magnetic field, then it is legal. If the plate moves in the presence of the electric field but does no work, it would appear to be legal. If the plate moves and does some work, i.e. pulling a pin, then it is illegal as it is a solenoid as described by the GDC responses. This would seem to be a valid test that can be used. I am sure I will get some reaction off line but we need to come to meeting of the minds so we can train our inspectors.

[boomergeek]

Is the following interpretation both simple enough and accurate enough to cover all cases?

Movement as a direct result of powering ON electromagnet - BAD

Movement as a direct result of powering OFF electromagnet - GOOD

[Mike Betts]

Originally Posted by Al Skierkiewicz

...we need to come to meeting of the minds so we can train our inspectors.

Al,

Complete agreement.

Some things left undiscussed here (and by the GDC) is how these things are electrically controlled. In this guidance vacuum, I just know we're going to see some real Rube Goldberg (ex: a limit switch actuated by a servo) contraptions.

The custom circuits section of the rules do not seem adequate to allow for switching of relatively high current DC devices.

I really expected the GDC to state that EMs must be controlled via a Spike.

It's gonna be an interesting call next week...

JMHO,

Mike

[Mike Betts]

Originally Posted by Matt H.

For less than 1/2 the price of the gate latch you were looking at you can get an electromagnet rated for 160 lbs from McMaster-Carr part #5698K312...

Matt,

Please read the warning on the McMaster-Carr website for this device:

Warning! "Max. Pull, lbs." ratings are based on ideal conditions. Variations in iron content, thickness, and finish will reduce ratings. In lifting applications, do not use magnets more than 1/4 of their rated capacity.

From am engineering standpoint, this device is only safe for 40 pounds.

Regards,

Mike

[Al Skierkiewicz]

Originally Posted by boomergeek

Is the following interpretation both simple enough and accurate enough to cover all cases?

Movement as a direct result of powering ON electromagnet - BAD

Movement as a direct result of powering OFF electromagnet - GOOD

Dick,
I think both are BAD but I am still awaiting some advice.

[boomergeek]

We have a design that we would like to use that uses an electromagnet.
A retract stroke moves the leg back and touches steel to a powered off electromagnet.
This is the normal position of the kicker.

For the fire command, the electromagnet is powered on to temporarily hold the leg in its current position.
The solenoid for the retract stroke is then released and the leg holds in place due to the electromagnet.
The solenoid for the extension stroke is then powered: Due to cambered leverage, the magnet does not need to hold much holding force- maybe 10 pounds-
and within 1 second the software control automatically removes power from the electromagnet and the leg moves forward for the kick.

The electromagnet is only used to hold the leg in place and is only used temporarily as part of the sequence of a "fire" command.

It is a low power 12V magnet using less than 350 milliamps.

I have asked Luminary Micro if it is safe to control the voltage of such an electromagnet with a Jaguar. We believe we have found a quite novel way to adjust the power of the kick by adjusting the voltage to the electromagnet. (Other teams are welcome to borrow this idea especially if you let us and the judges know that we helped inspire your use of it.)

The team will redesign if the GDC/inspectors find such an arrangement does not have enough safety, either electrically or mechanically. But I must admit I do not see how such a system would be any more dangerous than a latch powered with a solenoid on a pneumatic actuator.

We would appreciate sage advice from safety experts or experienced FIRSTers as soon as possible.

Thanks!

[Al Skierkiewicz]

Dick,
I have waited to see what some teams have decided to use before I weigh in on the speed controller issue. Certainly with any current that exceeds the abilities of the Spike, you would have no choice. On your coil though, the Spike would not have a problem. The PWM nature of the Jaguar output might cause some issues with the inductance of the coil. It is possible that this interaction could produce voltage peaks in excess of the breakdown of the Power FETs in the Jaguar. did you possibly address this question with Luminary? Most designs using a coil in a DC circuit would contain a diode snubber across the coil to prevent the inductive "kick" from damaging the drive components and producing significant RF interference.

[Mike Betts]

Originally Posted by Al Skierkiewicz

Dick,
I have waited to see what some teams have decided to use before I weigh in on the speed controller issue. Certainly with any current that exceeds the abilities of the Spike, you would have no choice. On your coil though, the Spike would not have a problem. The PWM nature of the Jaguar output might cause some issues with the inductance of the coil. It is possible that this interaction could produce voltage peaks in excess of the breakdown of the Power FETs in the Jaguar. did you possibly address this question with Luminary? Most designs using a coil in a DC circuit would contain a diode snubber across the coil to prevent the inductive "kick" from damaging the drive components and producing significant RF interference.

Al, Dick, et al,

The GDC appears to consider an EM as a custom circuit (here) and as such, it must be protected by a 20A circuit breaker (<R46> part F).

There is no scenario I can see where a Spike should not be an adequate and, in my opinion, the preferred method of controlling an EM.

Regards,

Mike

[boomergeek]

Hi Al.

Thanks for the response. Yes, we can use a Spike especially if we only want one force pull on the electromagnet. We could even set up a rheostat to provide a second voltage for a second release force on the electromagnet. It would just be simpler to adjust if we could just wire a single path with a Jaguar.

I having trouble getting my head around the difference in the inductance created in a relatively large electromagnetic motor like the CIM versus that created by a relatively small electromagnet designed for use as a trailer brake and why a Jaguar would have more trouble with the inductance from the electric brake than from the CIM.

I'm sure there is a formula that I forgot 30 years ago that would put it all in to context- but it's escaping me at the moment.

I'm still waiting from a response from Luminary Micro.

[boomergeek]

Originally Posted by Mike Betts

Al, Dick, et al,

The GDC appears to consider an EM as a custom circuit (here) and as such, it must be protected by a 20A circuit breaker (<R46> part F).

There is no scenario I can see where a Spike should not be an adequate and, in my opinion, the preferred method of controlling an EM.

Regards,

Mike

Mike-

Thanks for pointing to a key GDC decision and associated rule- I found them quite helpful.

We are still investigating the idea of powering our EM with multiple voltages for different release forces which may become slightly cumbersome using Spikes. One team member suggested a servo mechanically turning a potentiometer.
That's why we decided to check with Luminary Micro to see if they would give the OK for our specific application.