I want the cheapest, most basic method of turning off the motor while an electronically assisted vehicle is moving at over a certain fixed speed.

I don't need to measure the speed other than it being 'slower or faster' than a fixed rate. As the motor runs via gears to the wheels, I need something linked to RPM of the wheel.

For the sake of wiring I am thinking about a magnet on the wheel spokes and a sensor on the frame - and a hall switch linked to resetting some kind of countdown timer - while the timer is at zero, the switch stays open for a fixed period (enough time for an other revolution and the timer to hit zero again), if the timer doesn't get to zero in a fixed period then the wheel must be doing a revolution in less than the timer value, so the sensor switches off the power until the time does start getting back to zero (the wheel is now moving more slowly).

Is there a better way? If not, what sort of circuit should I be looking at for my timers?


  • \$\begingroup\$ Apparently, a version of a steam engine regulator is used on some vehicles nowadays. \$\endgroup\$ – Andrew Morton Mar 28 '17 at 13:03
  • \$\begingroup\$ As the wheel is run through gears, you can put the sensor anywhere convenient on the drive train (on the wheel-side of the clutch if there is one, of course). \$\endgroup\$ – Andrew Morton Mar 28 '17 at 13:05
  • \$\begingroup\$ Does this system need to be fail-safe? It sounds like it might be. \$\endgroup\$ – Jack B Mar 28 '17 at 13:08
  • \$\begingroup\$ Andrew, in principle the steam engine regulator is exactly what I am trying to recreate :) \$\endgroup\$ – user137049 Mar 28 '17 at 13:31
  • 1
    \$\begingroup\$ Hi Jack, the overall control system will have safety features, this is just a speed cut out (awkward legal requirement - electric assist bikes motor must not help propel it over a certain speed - go too fast and you are on pedal only.) \$\endgroup\$ – user137049 Mar 28 '17 at 13:33

Your idea is sound, however doing once per rev sensing the rider is going to feel like he is driving along on a train track bed.

You need much more frequent sensing, the more the better. You can still use a hall effect sensor and magnet, but instead of having the magnet on the wheel, arrange it behind the sensor and have the sensor close to a cog on a gear.

enter image description here

The on-off control will work but it will still feel "rough" to the rider. It is better to use a pulse width modulation system (PWM).

A (PWM) is basically the same thing as your idea, but instead of turning on and off every sensor pass, you turn on an off all the time at a much higher frequency. Perhaps 50KHz.

How much on time vs off time, the "duty-cycle" you "calculated" and adjust based on the sensor data.

enter image description here

As someone mentioned the system needs to be failsafe too. That is, it can NOT be allowed to get STUCK IN ON MODE.

When starting out the ON part could be 100%. However, you really do not want to go from full acceleration to cruise in a single move either. Especially, if the acceleration your vehicle can provide is large. The rider will be thrown forward..

The same goes for sudden deceleration.

Of course, if this is for a non passenger/"breakable cargo" application, the transitions do not matter so much, though wear and tear on the vehicle itself is still an issue.

| improve this answer | |
  • \$\begingroup\$ Hi, the plan for the timers was to delay the turn off. So the motor is always on, unless \$\endgroup\$ – user137049 Mar 28 '17 at 13:14
  • \$\begingroup\$ @pperrin, that is right, when under-speed the ON part could be 100%. However, you really do not want to go from full acceleration to cruise in a single move either. Especially, if the acceleration your vehicle can provide is large. The rider will be thrown forward... \$\endgroup\$ – Trevor_G Mar 28 '17 at 13:20
  • \$\begingroup\$ (soz prev comment went before ready, and too slow to edit!) Hi thanks, the plan for the timer(s) was to delay the turn off. So the switch is always open (motor is always on), unless the switch is is triggered too frequently (if a pulse is received too quickly after the previous one) - I am hoping for a stand alone gadget, that I can wire into the motor controllers 'brake' switch (which is designed to turn off the motor if the brake is hit). Other 'safety' features will be on the throttle side of the controller. Would a bicycles spokes act as the teeth of the gear in your diagram? \$\endgroup\$ – user137049 Mar 28 '17 at 13:27
  • \$\begingroup\$ @pperrin. The spokes would probably work, if they are steel, issue would be ensuring there is no wobble in the wheel as they rotate. You would not want them hitting the sensor. I know of no stand alone gadget to do this though. \$\endgroup\$ – Trevor_G Mar 28 '17 at 13:32
  • 1
    \$\begingroup\$ spokes should not be wobbling, so I could look at that - it would make fitting easier as it would all be in the one box, nothing needed on the wheel, just many more pulses. As electric assist, when the motor is off it freewheels, leaving the cyclist to do all the work - so sudden stop isn't a problem. The 'gadget' is what I am trying to build :) \$\endgroup\$ – user137049 Mar 28 '17 at 13:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy