Pedelec sensor rings

[Beeping-Sleauty]

Can any-one please explain to me the operation of the magnetic sensor ring attached to the chainwheel bracket on pedelec bikes,

mine has 10 holes drilled in the ring, five of them have magnets in them.

thanks for any comments,
beeps

 

[Flying Kiwi]

Crank Speed/Position sensor

What you have differs to whats on my Giant Twist so it's not a standard electric bike thing. It sounds to me like some sort of crank rotational speed or position sensor. This information will be used by the motor controller circuitry to decide when to apply power and possibly in conjunction with a strain gauge (if it's a sophisticated proportional assistance system), how much power to apply.

Why do you want to know? Is it playing up?

 

[flecc]

As the pedals turn the system's momentary timer is switched on but will shut down almost instantly if the pulse isn't renewed by continuing rotation. In other words, it ensures that pedalling continues. The sensor is a Hall sensor, a transistor form that can detect a magnetic field and give a signal pulse.

In a non-pedelec mode, the timer is bypassed, permitting continuous feed from a throttle.
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[Beeping-Sleauty]

Thanks flying kiwi, i don't think there is a strain device, the ring looks fairly simplistic.

As the pedals turn the system's momentary timer is switched on but will shut down almost instantly if the pulse isn't renewed by continuing rotation. In other words, it ensures that pedalling continues. The sensor is a Hall sensor, a transistor form that can detect a magnetic field and give a signal pulse.

In a non-pedelec mode, the timer is bypassed, permitting continuous feed from a throttle.
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yes Flecc, the power comes off when i stop pedalling, i do not have a non-pedelec mode, but i have throttle feed all the time that over-rides the pedelec, what i am trying to ascertain, is whether adding or subtracting from the five magnets i have will affect the level of assistance delivered by the pedelec, it seems a little 'eager' in the assistance, especially in the top gears.

i removed the restrictor pot. this eliminated the slighty jerky cut-in/cut-out and improved the speed marginally, but sometimes the pedals seem to run away, a little too much pedelec assistance perhaps.

what would your thought be ?
thanks
beeps

 

[Flying Kiwi]

Thanks flying kiwi, i don't think there is a strain device, the ring looks fairly simplistic.

OK so if there's no form of strain gauge (as used in some torque sensors) then adding magnets would have the same effect of making the controller think you were pedalling faster and taking them away would have the effect of fooling it into thinking you were pedalling slower. If the magnets are equally spaced removing or adding magnets could throw a spanner in the works in terms of pulse smoothness though as there'd be a gap - a bit like having a tooth missing from a drive gear. Additionally it may also change the speed at which the motor assistance cuts out and anything above 15 mph wont be in compliance with the law.

 

[flecc]

I'm sure you'll find the only effect of removing magnets is that it might cut out intermittently and become jerky. This would be damaging to the motor. Adding magnets might smooth things out a bit but very unlikely. There is no speed control function from the rotor normally, it's just a detector of pedalling. Speed control of hub motor bikes is by design an approximation. There is no limiter, the motor and it's orbital gears are designed to reach approximately the legal speed on the design nominal battery voltage. Typically a hub motor bike is faster when a battery is just off the charger, but slows as the charge is used up.

The Torq is the one exception in having a limiter because it uses a motor designed for a 20" wheel (in the Quando) in a 28" wheel, so is capable of going 40% faster. Therefore to comply with the law a limiter is necessary. This limiting again doesn't operate from the pedelec rotor, it's electronic.

Most hub motor bikes use this kind of rotating pedelec sensor, mainly external just behind the chainwheel, but just a few are incorporated in the bottom bracket.
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[Carl]

The magnet sensor does measure the speed of the pedalling, but only in 4 or 5 increments, and it reaches top power output pretty quickly, so unless you are pedalling about 5 Mph or less, it will mostly be telling the controller to put out max power.
You need to keep the magnets evenly spaced, so if you could find a way of taking one or two away without messing the spacing up, it should add or subtract from the motor assistance at any given pedalling speed.

 

[flecc]

The magnet sensor does measure the speed of the pedalling, but only in 4 or 5 increments, and it reaches top power output pretty quickly, so unless you are pedalling about 5 Mph or less, it will mostly be telling the controller to put out max power.
You need to keep the magnets evenly spaced, so if you could find a way of taking one or two away without messing the spacing up, it should add or subtract from the motor assistance at any given pedalling speed.

But I think you'll agree Carl, not the legal 15 mph speed which I was referring to. Also, the pedelec sensor in the Giant Lafree Twist series is the opposite of the method you describe, giving most power at lower pedal speeds. Of course on that Panasonic motor system, the legal speed control is by another sensor system completely separate from it's pedelec sensor.

Vive la difference!

 

[Carl]

Ah, no, it wouldn't measure the top speed cut out speed, if that's how you mean
Just that it does measure the pedal revolutions and outputs the speed accordingly, whether slow revs/max assistance or fast revs/max assistance

 

[flecc]

Thanks Carl, understood. It sounds a bit contrary, but with thought it makes sense but in a different way to that Giant system.

 

[Beeping-Sleauty]

magnets, less is more

Thank you Gents,

...Just that it does measure the pedal revolutions and outputs the speed accordingly, whether slow revs/max assistance or fast revs/max assistance

as you all correctly surmised, sub-tracting magnets does increase the assistance given.

i increased the gap between the ring & the sensor with a spacer, blanked two of the five magnets with some copper shield using double sided tape, this kept them equally spaced, the increase in power under pedelec seemed about a third, i'm guessing as i don't have a way to measure.

This level of assistance is too much for comfort, the bike was almost pedalling me.... a bit like starting to pedal too early at the end of descending a steep hill, i need the resistance of the pedals to feel in control of the machine, i was seeking the opposite effect - less power under pedelec, i don't have magnets i can add to the ring so cannot test this.

however i did solve the problem by replacing the 38t chainwheel for a 52t and this has smoothed out the pedelec across the upper gears, it also means i don't look like a small furry pair of wind-mills coming down the street when i'm pedalling at 20 mph, considerably less tiring too.

there was no jerki-ness or other discernable effect on the pulse.

beeps

 

[flecc]

Good idea Beeps, as well as slowing the sensor rate, a high gear does smooth things out very effectively anyway, needs lots more power to jerk a bike about in a higher gear.
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