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Remotely powering a mechanical foot.

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Stiiiiiv, Mar 13, 2018.

  1. Stiiiiiv

    Stiiiiiv New Member

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    Hi,

    I'm new here and my level of experience and knowledge in electronics is limited (That's why I'm here - to get help). I'm better at mechanical things.

    I'm planning a project whereby I can remotely apply the brakes in a car I'm towing on an a-frame towing bar. I plan on having a sensor (a sliding potentiometer) that's about 10 - 12 cm long (4 - 5 inches) attached between the brake pedal and the floor of my car. This will be connected by wire to the cigarette lighter, and then on to a temporary assembly (a sort of mechanical foot) attached to the brake pedal of the car I'm towing. Within this assembly I want to have a piston in an electromagnetic cylinder which will push down on the brakes. The piston would ideally be a permanent magnet. It will have to exert sufficient force to engage the brakes (at least part way) when the motor is off. It can't consume more than 10 amps of power since I have a 10 amp fuse on the cigarette lighter. It will be grounded to the trailer lights outlet on my car. The whole assembly will be easily removed and installed.

    I would prefer not to use a linear servo to push the brake pedal because the reaction time is too slow, and the pressure it exerts may be hard to control, potentially causing damage to the parts of the car against which it will be stabilized (the under sides of the driving column and the driver's seat. If there's no other viable alternative, I may look into a linear servo, but I'm not thrilled by the idea. The more complexity I add to this the more things can go wrong, and a linear servo can't be controlled by a simple sliding potentiometer.

    I've been looking for a 12v electromagnetic-driven piston on the internet but can't find any for sale. The cylinder has to be able to push the piston out at least 12 cm (5 inches). I'd rather not have to make one myself if there are any available to buy, but if there aren't, I'll need help with instructions. I also need an appropriate slide potentiometer. Can anyone help me find these two parts?

    If you see any faults with my reasoning, please feel free to point them out for me.

    Steve
     
  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Welcome to ETO!
    I think even with a 10A electromagnet you will struggle to get the force needed to operate the brake pedal satisfactorily over the range of movement needed. When the electromagnet is in its extended position its pulling force is a lot less than when in the retracted position. A linear actuator would have the oomph but, as you say, is likely to be rather slow.
     
  3. Stiiiiiv

    Stiiiiiv New Member

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    That may be so, in which case I may need an electromagnet which extends longer than what I need. It should still exert the most force within its effective range when it has the maximum amount of power going through it, shouldn't it?
     
  4. dave miyares

    Dave New Member

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  5. Stiiiiiv

    Stiiiiiv New Member

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    By the way, thanks for the welcome.
    :)

    If I can get enough pressure to achieve half the normal braking power I'll be pleased.
     
    Last edited: Mar 13, 2018
  6. tomizett

    tomizett Active Member

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    Rather than actuating the brakes of the towed vehicle with the pedal of the towing vehicle, I think you should consider applying them based upon the compression force in the drawbar - that's the way braked trailers do it. That way, just enough braking force is applied at the towed vehicle to match its speed to the vehicle towing.

    I agree with Alec. A piston-in-electromagnet (solenoid) probably won't do what you want... It's more complex, but do you think pheumatics would be a viable option for you?
     
  7. shortbus=

    shortbus= Well-Known Member

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    From some one who used to tow a car with a car, using a tow bar, to me it sounds like the tow vehicle is under powered, brake wise for the car being towed. The insurance liabilities of a "home made" fix like this would be scary in case of an accident.
     
  8. dave miyares

    Dave New Member

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  9. AnalogKid

    AnalogKid Well-Known Member

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    Nothing is going to get you more force for less electrical power than a motor with a worm gear on the shaft. And, you get constant force over any length from inches to yards. After decades of development for disk drives, a stepper motor and worm gear or linear band should have the reaction time and controllability you need.

    ak
     
  10. Stiiiiiv

    Stiiiiiv New Member

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    Tomizett: That solution, while logical, will probably not meet my needs.

    Shortbus=: The towing vehicle has enough braking power for the weight it's towing. The problem is it doesn't fulfill the legal requirements to be considered a trailer, which is what I'm trying to achieve.

    Let me explain. I live in Sweden, and if I tow using a towbar, I can travel at maximum 30 km/h. That's because the towed vehicle doesn't have the proper lights and reflectors to be considered a trailer (easily remedied) but more importantly, it has brakes but they are not functioning "normally". "Normal" for a real trailer can be everything from no bakes at all, if it doesn't come equipped with any, to an emergency brake that is released in case the trailer gets detached from the towing vehicle, to pneumatic brakes like on a semi-trailer (this is overkill for my needs since I won't be towing anything that heavy). In this case, since it's a car I want to make into a temporary trailer, "normal" means that every time the driver of the towing vehicle steps on the brakes, the towed vehicle's brakes should also engage. I don't need 100% of its braking power. I know there are expensive systems (like BrakeBuddy) that will do this, but they are mounted permanently in both vehicles and actuate the towed vehicle's own pneumatic system (I think). I want something I can quickly install in any vehicle I want to tow with or tow. Thus I've chosen to try to mirror the driver's braking on the towed vehicle's brake pedal.

    A pneumatic system might work, providing it is inexpensive and simple enough to install.
     
  11. Stiiiiiv

    Stiiiiiv New Member

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    AnaloguKid: I don't know enough about stepper motors and worm gears to know if that would serve my needs. I imagine it would require the same sort of electronic controls that a linear servo has.
     
  12. gophert

    gophert Active Member

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    You are going to need some control logic beyond proportional control. With only proportional control, you can get into some terrible oscillations of jerk/go as the slack in the system comes and goes when you least expect it (hills, advancing at stop/go traffic (especially on hills)).
     
  13. Stiiiiiv

    Stiiiiiv New Member

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    I've read more of the law texts, and the requirements for braking power to get the towed vehicle classed as a trailer are such that:

    "the combined braking power att the wheels' periferi must at least equal a deceleration of 5.8m/s2 while braking from 60km/h to a standstill".

    The only easy way I can see guaranteeing that much braking power in the towed vehicle is to tow it while its motor is running. I seriously doubt if that is legal in Sweden.
    Also, the law states that all the wheels on a trailer must have working brakes, so I can't just engage the handbrake. Unless someone was a brilliant idea for a workaround, I'll just have to satisfy myself with driving 30km/h while towing.
     
  14. gophert

    gophert Active Member

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    And dis-satisfy every driver stuck behind you.
     
  15. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Most cars have power brake systems. It takes a lot more force to activate a brake with that system off.
     
    • Agree Agree x 1
  16. tomizett

    tomizett Active Member

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    I think that was Stiiiiv's point - hence thinking about towing with the engine running. On the other hand, I'm sure it should be possible to brake a car to a stop effectively without the servo (ie, with the engine stopped). I wouldn't be surprised if it was a legal requirement of the design.
    The problem is that it could require a *lot* of force on the brake pedal, and given that you'd be towing a variety of different cars, and wouldn't have any force feedback, there might be a chance of damaging the brake pedal assembly or anchor points. Or, you may just end up locking up the towed vehicle and having it jack-knife on you.
     
    • Agree Agree x 1
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    But, you could always supply vacuum from a pump to the booster, just not likely from the towed car.

    You could likely activate with a double-acting air cylinder. So, now a compressor and a vacuum pump on board.
     
  18. Stiiiiiv

    Stiiiiiv New Member

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    I'll be dissatisfying a lot of people driving 30 with a towed vehicle behind me, but all the alternatives I've seen so far have something that disqualifies them.

    1. Solonoid - not strong enough with 12 volts and 10 amps
    2. linear servo - too slow reaction time and hard to control the pressure + more electonic complexity which reduces its reliability.
    3. stepper motor and worm gear or linear band - don't know enough about this setup, but likely the same difficulties as with the linear servo, except the reaction time might be better (probably at the cost of pressure).
    4. cable mimicing of pedal movement - requirtes the towing vehicle's engine to be runniing or at least for the pneumatics to be running, which will either be illegal (motor running) or expensive (complex in as much as it must be attached to the towed vehicle's brake system, and that will be different for each vehicle I tow, and possibly illegal too).
    5. using the compression force in the towbar to actuate the brakes - involves me taking the towed vehicle through MOT (yearly inspection) to check that the vehicle's brakes match the towbar's compression mechinism (unreasonable for the sake of towing a vehicle).

    Unless someone has some brilliant suggestion for a workaround that is inexpensive and simple to set up (constructing it may be more complex, but setting it up in the vehicles must be easy, simple AND legal) I feel I must put this idea back on the back burner for a while. I don't want to give it up entirely, because I believe the concept has merit, but at this time I don't have any ideas how to get any closer to my goal.

    Stiiiiiv
     
    Last edited: Mar 14, 2018
  19. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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  20. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Agreeing with DerStrom8, Pneumatics which the TS/OP missed in the list. Agreed, two pumps (vacuum, compressor) one for the air cylinder and one for the booster or possibly both activated by vacuum. You could just add a 3-way valve at the booster and a port(s) on the front of the car.

    I don't like it either.

    Here's something odd: https://en.wikipedia.org/wiki/Electro-hydraulic_actuator An Electro-Hydraulic actuator.
     
  21. tomizett

    tomizett Active Member

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    If you could do it with just pressure, you could probably get quite a lot of braking cycles from a cylinder of CO2. You'd need a pressure monitor with feedback to the driver to alert you when it was running low, but it might be worth considdering.
     
  22. Mickster

    Mickster Well-Known Member

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    One of my concerns with this scenario is brake-proportioning.
    If the braking effort on the rear axle of a vehicle is greater than that of the front, that vehicle is likely to want to swap ends.
    One vehicle is enough of a handful in that situation, wouldn't want to be trying to recover control of two.
     
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