Over travel engine kill switch for braking system

[daveyh]

Does anyone know of a suitable overtravel switch to use to cut the engine off in case of brake failure

[daveyh]

Does anyone know of a suitable overtravel switch to use to cut the engine off in case of brake failure

[Denny Trimble]

We use a simple toggle switch (radio shack, etc) mounted about .5 inches past the max bias bar travel. If the brake system were to lose pressure, the bias bar would move forward and flip the switch.

University of Washington Formula SAE ('98, '99, '03, '04)

[Beast]

We also used a simple toggle switch located in front of the balance bar in 2003. FL Tech.
Not sure of the plan for this year's car, probably something similar.

[jack]

i was just looking at getting this switch for our car, but am confused by the rule. according to the rule, the switch cannot be turned back on again...easily. http://fsae.com/groupee_common/emoticons/icon_confused.gif it seemed to me like i needed a switch like a snowmobile kill switch or something. in the past, WWU has used toggle switches with shields (like on space shuttles), this way, the shield is pressed, flips the switch, and then nothing car happen untill the shield is re-opened. i just looked at some pictures or other cars (including UW's), and they just have toggle switches which "could" get switch back on...

i guess my question is, everyone else passed tech with a simple toggle switch, so no need for a fancy switch?

jack @ WWU
http://www.etec.wwu.edu/

[Jarrod]

from memory, the rule reads that the switch can't be reset and return to live if the pedal is pressed a second time, or something to that effect. So that if you were to lose brakes, and pump the pedal, it won't kill, then refire the car. In Oz last year, we had a switch hooked up to a relay, which was wired through the ignition on button on the dash. Had a bit of grief in tech over the possibility that in a brake failure, the car would switch off, then the driver would simply hit the button, reset the relay, and restart. They grudgingly let us through because the rules do not mention this situation, despite them adamantly stating that it was illegal. I can see where they are coming from, and I think the rule may be changed in future years.

[Denny Trimble]

I just re-read the rules, and it's not in there, but I thought the intent was a switch that turns off and can't be turned back on by the driver while he's sitting in the car. Ours can't.

University of Washington Formula SAE ('98, '99, '03, '04)

[Frank]

we just have a "reset" button on the dash..

excuse the poor attempt at explaining the logic

if: pedal goes down
switch becomes "on"
system gets disabled

when reset pressed
system enabled

if: switch is "on"
system gets disabled

the switch is not too easilly accessable to the driver, and it means you have to consistantly "enable" the system if the brakes are failing

[jack]

i think your right denny, i guess the way i interpreted the rules i thought you would need a switch that "locked" off, but as long as the pedal cant turn it back on, i guess its ok.

i didnt even think of a dash reset button...excuse my lack of eletrical knowledge, but if you have such a divice, can you have a toggle switch remain off, but still connect the circuit with the reset button? therefore bypassing the toggle switch because its still off, or do you use a push-button instead of toggle? it does seem to be pushing the rules, but i would hate to have to retire from an event because a stupid unreachable switch was accidently flipped...

jack @ WWU
http://www.etec.wwu.edu/

[Dan Deussen @ Weber Motor]

Frank-

If your brakes fail then your car should not be driven anymore; ergo, the driver should NOT have the option to reset this safety feature with the intent to finish an event.

If you are afraid of the switch being tripped accidentally without having an actual brake failure, then design the switch actuation properly and you won't have to worry about it.

The safety rules are written with the intention to ensure the future existence of the competition. I am sure that if a serious injury would occur at an event due to a car being able to continue to run without brakes, many schools would retract their participation in fear of lawsuits and bad publicity.

I am completely in favor of trying to find the "gray areas" of the rules to make your car faster, but don't ever jeopardize safety.

Daniel Deussen
www.walbro-italy.com (http://www.walbro-italy.com)

[Jarrod]

the intention of our system was not to allow the driver to reset the system, or 'bypass the rules' it was not even thought of until the tech guys brought it up. It was just the way our ignition etc. was that it ended up that way. We did not have enough room for a decent toggle switch on our pedal box, so we had a microswitch type thing, so it is only a momentary contact. hopefully in the event of a brake failure the driver would have enough common sense not attempt restarting. (at least in a perfect world) I completely agree that safety should be a priority.

[Eric]

We have a latching circuit on our car this year also. In my opinion, this enables the use of a momentary limit switch, which is actually made for this kind of application. Most of the toggles I see are likely to be destroyed in the event of a brake overtravel. If you are using the switch to complete your ground, loose switch parts may or may not make contact with the chassis. If you are using it to connect your +12v, I suppose this is safer, but still...I am surprised that the ability for the driver to reset upset the tech guys so much. In industrial controls, an operator can hit the reset button on an emergency stop. It is assumed the driver/operator doesn't have a death wish I guess. Frank, are you using a latch circuit this year or the toggle switch?

[Eric]

Another note Frank: When you make an emergency stop button, always use a normally on. You said that if reset, switch goes on. Remember that it is easier to break a circuit than it is to make one, so never rely on making a circuit to stop something in an emergency.

[flybywire]

Seems to me they beefed-up the over-travel switch rule for this year. Something about cannot resort to logic or programming. In other words no latching circuit. Ergo, no reset switch.

Just about any latching circuit I can think of will be reset by the driver toggling the dash kill switch off and then back on. Definately withing reason. ("sh*t, sh*t, it's died on me. Won't restart. That damn ECU! I know toggle power and try again....")

We had a problem in 2001 when we used a latching relay and a reset switch. I wasn't there but apparently the judges didn't like it.

2003 we used a push-pull type switch. One you push in to turn off and pull back out to turn on. It was just wired in series with the dash kill switch and killed the main relay. (The dash kill switch can operate through a relay.) This means that someone has to reach down behind the brake pedal and pull the switch back out--something the driver definately cannot do while seated in the car.

I think the point of the rule is that over travel is indicative of fluid loss therefore the car should not be allowed to get back up to speed--even if the driver doesn't realize the brakes are shot. As in, "the engine just quit so I restarted it, got around the corner and onto the straight; then I figured out why the engine died..."

[Ryan Schoffer]

exactly.

i use a bat handle toggle that is flipped by a bar attached to the rods coming out of the master cylinders - it is wired in series with the dash kill

so if the brake goes all the way back, the switch will get flipped off, and you need to take the nose off (or get someone with really long arms) to flip the switch back on

Vehicle electronics leader

www.ucalgary.ca/fsae (http://www.ucalgary.ca/fsae)

[Eric]

Flybywire, when you say no logic means no latch, aren't you (or the judges) forgetting that any circuit with a switch is a logic circuit? I don't see any difference between a toggle or momentary switch cutting power to a relay coil. The rule is vague, not robust. Anyway, I guess we're going to revert to a toggle to avoid confrontation. I wish this sort of thing were spelled out in the rules though.

[flybywire]

Good point Eric.

There are a lot of rules I wish were spelled out. Although the 2004 rules do spell out more than the previuos rules.

I just re-read the 2004 rule and the way it's worded a latching circuit is not prohibited. It just needs to be implemented using analog components. I guess that means relay logic or even discrete logic ICs would be okay. Although, the judges could argue that such a setup fell under "a simalar functioning digital controller" but I don't think that's what is meant.

I personally wouldn't use any system with an electronic memory element though. Only because of the reset on power cycle scenario--but that isn't stipulated in the rules.

[This message was edited by flybywire on March 08, 2004 at 04:25 AM.]

[Eric]

I agree with your analysis flybywire. Here is the actual paragraph:
3.2.5.1 Brake Over Travel Switch
A brake pedal over-travel switch must be installed on all cars.
This switch shall be installed so that in the event of brake system
failure such that the brake pedal over travels, a switch must be
activated which will stop the engine from running. This switch
must kill the ignition and cut the power to any electrical fuel
pumps. Repeated actuation of the switch must not restore power
to these components. The switch must be implemented with
analog components, and not through recourse to programmable
logic controllers, engine control units, or similar functioning
digital controllers.

[Frank]

oops,

i am wrong

the judges made us "relocate" the switch behind the dash

we had it on the dash for testing, while setting up the "overtravel switch"

sorry about that