Electromagnetic gate latch?

[hans4]

Hey everyone, new to the forums, and i hope this is the right section, my team is looking into using an electromagnetic gate latch to hold back a tension powered ram. We were wondering if it would be legal as long as we powered it from the distribution board.

We were thinking something like this product: http://www.usbuilderstore.com/securitron-1/gl1-gate-lock.html
not this exact model but the same idea.

What do you guys think?

thanks
Team 2002

[EricH]

The latch would be moving under the influence of the electromagnet, wouldn't it?

The Q&A has dealt with several questions on electromagnets.
http://forums.usfirst.org/showthread.php?t=13979
http://forums.usfirst.org/showthread.php?t=14121 Note that this one outlaws any use of electromagnets to provide motion.
http://forums.usfirst.org/showthread.php?t=14093

Due to the second response linked (there are more than 3 out there, trust me), if you're moving the ram with the electromagnet, you are in violation of the rules. By the looks of things, the latch is activated by the electromagnet, making it illegal per <R53> as interpreted by the Q&A.

[hans4]

Thanks for the quick response EricH,

However i dont understand what is illegal about this system. All it does is sit there and close on the incoming object then we supply it power and then releases said object when we are ready to fire...

Clarification please?
Thanks

[pafwl]

The electro magnet cannot move something because it is providing motion. But I think you can use it to hold something with electromagnetic force and then release it when you want by turning off the magnet. That is legal because we did it in 2008 to great effect.

[Vikesrock]

Thanks for the quick response EricH,

However i dont understand what is illegal about this system. All it does is sit there and close on the incoming object then we supply it power and then releases said object when we are ready to fire...

Clarification please?
Thanks

If the electromagnet in the lock moves something to release the latch it is illegal as an electromagnet providing motion has been ruled by the GDC to be a solenoid per the responses Eric linked.

If the electromagnet just holds the part in place, then releases it, with no motion of latch parts then you should be OK.

[hans4]

Thanks guys,
I think we will try the electromagnetic latch, but we'll build a backup system in case we get to regionals and FIRST says we cant.

Thanks Again,
Hans

[jspatz1]

This "electromagnetic latch" is a solenoid actuator, which is expressly prohibited. There is no doubt it will be disallowed at inspection, unless it is not noticed AND you don't reveal it. Not worth the risk. Use a small air cylinder.

[EricH]

It won't pass inspection, unless your inspector doesn't notice it. And with that packaging, I'm fairly certain they'll ask. A number of head inspectors also read CD on a regular basis--if yours is one of those, you're already not going to pass. Don't try to pass with it on your robot.

[ebarker]

[QUOTE=Vikesrock;908225If the electromagnet just holds the part in place, then releases it, with no motion of latch parts then you should be OK.[/QUOTE]

I'd like to see a GDC ruling on this !

Holding is different than actuating.

[EricH]

I'd like to see a GDC ruling on this !

Holding is different than actuating.
The GDC has ruled on holding already--see that second Q&A I linked. The part in question certainly seems to be an actuated device.

[Tom Line]

The GDC has been fairly clear.

Use a magnet to hold your claw to the bar? Fine.

Use a magnet to cause something to move? Not fine.

However, if someone still has a question, post it on the Q&A. None of our answers are official.

[hans4]

haha thanks for the tip, i've been looking into just an electromagnet with no latch instead, but we'll probably go with a piston (more cost effective) ya you're right about inspection too

Thanks again

[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.

All you would have to do is put an iron plate at the end of your kicker and make sure that the plate is pressed firmly against the electromagnet before the magnet is engaged.

However it is important to remember that the iron may become magnetized with unexpected results!

[NerdGuy]

So do we have to buy the electromagnet or can we make our own.

I mean honestly, making an electromagnet is something i did in elementary school.

The only thing i am not sure about is does that violate the custom circuits rule?

Any help would be greatly appreciated.

[RRLedford]

So am I understanding this thread correctly, that if I have an aluminum pivot bar having a pin & a spring at one end, and a steel plate at other end, if my pin engages a bungee powered arm, locking it in place, and uses a spring around the pin that's pushing to disengage it, then if my electromagnet's grip on the steel plate on opposite end of aluminum pivot bar prevents spring from disengaging pin at other end of aluminum bar until the electromagnet is de-energized, allowing bar to now pivot from the spring's force, and disengaging the pin --- is this a violation?
The electromagnet merely releases its grip on a steel plate, allowing the previously overpowered weaker SPRING to now actuate the release of the pin. What element is the real actuator here ? Is it the electromagnet, or is it the spring?
Is it that an electromagnet MOVING an element is forbidden, but RELEASING ITS GRIP on an element is allowed?
-Dick Ledford

[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]

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]

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]

...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]

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]

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]

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 (http://forums.usfirst.org/showthread.php?t=14205)) 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]

Al, Dick, et al,

The GDC appears to consider an EM as a custom circuit (here (http://forums.usfirst.org/showthread.php?t=14205)) 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.

[Al Skierkiewicz]

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.

[Mike Betts]

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.

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

[RRLedford]

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

[boomergeek]

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.

[Daniel_LaFleur]

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

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).

[martin417]

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 (http://www.amazingmagnets.com/sf-plate-magnets.aspx)

[Mike Betts]

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 (http://www.amazingmagnets.com/sf-plate-magnets.aspx)

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 (http://forums.usfirst.org/showthread.php?t=14470) to understand my dilemma.

Regards,

Mike

[martin417]

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 (http://forums.usfirst.org/showthread.php?t=14470) to understand my dilemma.

Regards,

Mike

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.

[Mike Betts]

Wait, what electromagnet? I did say "permanent magnets"...

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

[Alan Anderson]

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

Mount rectangular permanent magnets on a rotatable shaft....

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.

[Jones571]

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.

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

[boomergeek]

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?

[hyperdude]

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?

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.

[Chris is me]

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.

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.

[Al Skierkiewicz]

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.