How does the cadence sensor work?

[BigNerd]

Since @roshan is one of the few bike vendors here who actually interacts with us, I thought I would ask him this question as there is some confusion about how a cadence sensor works on a bike.

There are some that say it's just an on/off switch, once it detects pedaling, it tells the motor to run and that 100% power available at that PAS level will go to the motor.

Others, like myself, and the Evelo site, say that it's actually cadence based. The speed of the pedaling inversely tells the system how much power to give to the motor. The slower you pedal, the more power, the faster you pedal, less power.

Now, this could be brand/motor dependent so both are correct but what does Biktrix do on their bikes?

Thanks in advance.

 

[Taylor57]

Since @roshan is one of the few bike vendors here who actually interacts with us, I thought I would ask him this question as there is some confusion about how a cadence sensor works on a bike.

There are some that say it's just an on/off switch, once it detects pedaling, it tells the motor to run and that 100% power available at that PAS level will go to the motor.

Others, like myself, and the Evelo site, say that it's actually cadence based. The speed of the pedaling inversely tells the system how much power to give to the motor. The slower you pedal, the more power, the faster you pedal, less power.

Now, this could be brand/motor dependent so both are correct but what does Biktrix do on their bikes?

Thanks in advance.

 

[BigNerd]

When I see that dead horse... I can't help but hit it a few more times. But this time I want professional assistance.

 

[roshan]

Great question. Here's how a cadence sensor actually works:

On regular bikes, there is a magnet (that is usually attached to a moving part like spoke/pedal) and a sensor (that is attached to the frame of the bike). In this case the sensor indeed works like a switch which turns on or off (depending on the model) when the magnet passes close to the sensor. This is typical of how hall-effect sensors work. This on/off signal can be used with respect to time, to figure out how fast something is moving. Ie. if the sensor was on the frame and the magnet on the pedal, this could measure how fast the person is pedaling by looking at x rotations/min. If this sensor was on the rear wheel, it could measure the RPM of the wheel. This RPM can be used with the circumference of the wheel to calculate wheel/bike speed.

On most ebikes with PAS (Pedal assist systems), there is a magnetic disc attached on the crank arm with multiple magnets and a sensor on the frame. Say for example, there is only 1 magnet. In this case, the crank arm needs to move an entire rotation (360 degrees) for the magnet to send a signal to the motor controller. This would result in a very bad user experience as there would be a huge lag in response. Due to this, ebikes have a PAS magnet ring that has usually 3-12 magnets. As you can imagine, if the ring has 3 magnets, then each magnet would pass the sensor at 360/3 = 120 degrees per rotation while if it had 12 magnets, it would pass at 360/12 = 30 deg rotation. Now you can see how, if the ebike controller was getting feedback every 30 degrees of crank movement, then the controller would have a lot more information to process, thereby potentially resulting in a much sooner engagement of assist and much smoother assist. On mid motors like BBS02, there are about 52 points of engagement per rotation - resulting in a much, much smoother pedal assist and almost immediate engagement.

Okay, now that the primer is out of the way, let's look at your questions:

There are some that say it's just an on/off switch, once it detects pedaling, it tells the motor to run and that 100% power available at that PAS level will go to the motor.

This really depends on 2 things:
a. the number of magnets in the PAS ring
b. the way the controller processes the data from the PAS
As explained above, obviously more the magnets, the better. However, if the data from the magents isn't being used properly by the controller, then even with a lot of magnets, you could have a pretty shitty riding experience as the controller could just look at if a magnet passed the sensor and push out full power to the motor. From my experience, most chinese controllers have settings like:
- number of poles on PAS (essentially the number of magnets)
- start current: what's the starting amps put out by the controller when PAS is engaged
- current ramp: how fast the controller puts out full current
- max current output: this is often limited by hardware. More mosfets in the controller, higher max current
- max speed (rpm of the motor): this is often limited by voltage of the battery. Motors have RPM that is a function of input voltage (called kv rating of a motor) and motor gearing (if internally geared).

Others, like myself, and the Evelo site, say that it's actually cadence based. The speed of the pedaling inversely tells the system how much power to give to the motor. The slower you pedal, the more power, the faster you pedal, less power.

The slower you pedal, more power, the faster you pedal, less power: you can see that this statement is more of a "feeling from the rider" because technically when you start pedaling from a standstill, the controller pushes power to the motor (based on the current ramp), and say you already reach full power in about 2 pedal strokes. After this point, no matter how much you pedal, you won't get additional assist as the motor is already at peak power. Thereby making you feel like the motor isn't doing any work or the motor is putting out less power.

what does Biktrix do on their bikes?

The answer is, it depends on the bike.
Juggernaut Classic, Stunner X, Stunner: BBS02 motor (highest resolution cadence sensor on the market). Our motors are optimized for response and natural feel. Max current 25A.
All Ultra bikes: Torque sensor - most natural assist with max current 30A.
Swift, Stunner LT: Bottom bracket torque sensor assist
Swift lite, Kutty: 8 magnet PAS

Hope that helps!

 

[Afhill]

Great question. Here's how a cadence sensor actually works:

On regular bikes, there is a magnet (that is usually attached to a moving part like spoke/pedal) and a sensor (that is attached to the frame of the bike). In this case the sensor indeed works like a switch which turns on or off (depending on the model) when the magnet passes close to the sensor. This is typical of how hall-effect sensors work. This on/off signal can be used with respect to time, to figure out how fast something is moving. Ie. if the sensor was on the frame and the magnet on the pedal, this could measure how fast the person is pedaling by looking at x rotations/min. If this sensor was on the rear wheel, it could measure the RPM of the wheel. This RPM can be used with the circumference of the wheel to calculate wheel/bike speed.

On most ebikes with PAS (Pedal assist systems), there is a magnetic disc attached on the crank arm with multiple magnets and a sensor on the frame. Say for example, there is only 1 magnet. In this case, the crank arm needs to move an entire rotation (360 degrees) for the magnet to send a signal to the motor controller. This would result in a very bad user experience as there would be a huge lag in response. Due to this, ebikes have a PAS magnet ring that has usually 3-12 magnets. As you can imagine, if the ring has 3 magnets, then each magnet would pass the sensor at 360/3 = 120 degrees per rotation while if it had 12 magnets, it would pass at 360/12 = 30 deg rotation. Now you can see how, if the ebike controller was getting feedback every 30 degrees of crank movement, then the controller would have a lot more information to process, thereby potentially resulting in a much sooner engagement of assist and much smoother assist. On mid motors like BBS02, there are about 52 points of engagement per rotation - resulting in a much, much smoother pedal assist and almost immediate engagement.

Okay, now that the primer is out of the way, let's look at your questions:

This really depends on 2 things:
a. the number of magnets in the PAS ring
b. the way the controller processes the data from the PAS
As explained above, obviously more the magnets, the better. However, if the data from the magents isn't being used properly by the controller, then even with a lot of magnets, you could have a pretty shitty riding experience as the controller could just look at if a magnet passed the sensor and push out full power to the motor. From my experience, most chinese controllers have settings like:
- number of poles on PAS (essentially the number of magnets)
- start current: what's the starting amps put out by the controller when PAS is engaged
- current ramp: how fast the controller puts out full current
- max current output: this is often limited by hardware. More mosfets in the controller, higher max current
- max speed (rpm of the motor): this is often limited by voltage of the battery. Motors have RPM that is a function of input voltage (called kv rating of a motor) and motor gearing (if internally geared).


The slower you pedal, more power, the faster you pedal, less power: you can see that this statement is more of a "feeling from the rider" because technically when you start pedaling from a standstill, the controller pushes power to the motor (based on the current ramp), and say you already reach full power in about 2 pedal strokes. After this point, no matter how much you pedal, you won't get additional assist as the motor is already at peak power. Thereby making you feel like the motor isn't doing any work or the motor is putting out less power.

The answer is, it depends on the bike.
Juggernaut Classic, Stunner X, Stunner: BBS02 motor (highest resolution cadence sensor on the market). Our motors are optimized for response and natural feel. Max current 25A.
All Ultra bikes: Torque sensor - most natural assist with max current 30A.
Swift, Stunner LT: Bottom bracket torque sensor assist
Swift lite, Kutty: 8 magnet PAS

Hope that helps!

@roshan now I'd like to know why you do so many different things! Pros/cons and why you chose the different type of sensor for each bike.

 

[roshan]

@roshan now I'd like to know why you do so many different things! Pros/cons and why you chose the different type of sensor for each bike.

Each bike meets different price points and needs for different customers!

 

[voidedwarranty]

Great question. Here's how a cadence sensor actually works:

On regular bikes, there is a magnet (that is usually attached to a moving part like spoke/pedal) and a sensor (that is attached to the frame of the bike). In this case the sensor indeed works like a switch which turns on or off (depending on the model) when the magnet passes close to the sensor. This is typical of how hall-effect sensors work. This on/off signal can be used with respect to time, to figure out how fast something is moving. Ie. if the sensor was on the frame and the magnet on the pedal, this could measure how fast the person is pedaling by looking at x rotations/min. If this sensor was on the rear wheel, it could measure the RPM of the wheel. This RPM can be used with the circumference of the wheel to calculate wheel/bike speed.

On most ebikes with PAS (Pedal assist systems), there is a magnetic disc attached on the crank arm with multiple magnets and a sensor on the frame. Say for example, there is only 1 magnet. In this case, the crank arm needs to move an entire rotation (360 degrees) for the magnet to send a signal to the motor controller. This would result in a very bad user experience as there would be a huge lag in response. Due to this, ebikes have a PAS magnet ring that has usually 3-12 magnets. As you can imagine, if the ring has 3 magnets, then each magnet would pass the sensor at 360/3 = 120 degrees per rotation while if it had 12 magnets, it would pass at 360/12 = 30 deg rotation. Now you can see how, if the ebike controller was getting feedback every 30 degrees of crank movement, then the controller would have a lot more information to process, thereby potentially resulting in a much sooner engagement of assist and much smoother assist. On mid motors like BBS02, there are about 52 points of engagement per rotation - resulting in a much, much smoother pedal assist and almost immediate engagement.

Okay, now that the primer is out of the way, let's look at your questions:

This really depends on 2 things:
a. the number of magnets in the PAS ring
b. the way the controller processes the data from the PAS
As explained above, obviously more the magnets, the better. However, if the data from the magents isn't being used properly by the controller, then even with a lot of magnets, you could have a pretty shitty riding experience as the controller could just look at if a magnet passed the sensor and push out full power to the motor. From my experience, most chinese controllers have settings like:
- number of poles on PAS (essentially the number of magnets)
- start current: what's the starting amps put out by the controller when PAS is engaged
- current ramp: how fast the controller puts out full current
- max current output: this is often limited by hardware. More mosfets in the controller, higher max current
- max speed (rpm of the motor): this is often limited by voltage of the battery. Motors have RPM that is a function of input voltage (called kv rating of a motor) and motor gearing (if internally geared).


The slower you pedal, more power, the faster you pedal, less power: you can see that this statement is more of a "feeling from the rider" because technically when you start pedaling from a standstill, the controller pushes power to the motor (based on the current ramp), and say you already reach full power in about 2 pedal strokes. After this point, no matter how much you pedal, you won't get additional assist as the motor is already at peak power. Thereby making you feel like the motor isn't doing any work or the motor is putting out less power.

The answer is, it depends on the bike.
Juggernaut Classic, Stunner X, Stunner: BBS02 motor (highest resolution cadence sensor on the market). Our motors are optimized for response and natural feel. Max current 25A.
All Ultra bikes: Torque sensor - most natural assist with max current 30A.
Swift, Stunner LT: Bottom bracket torque sensor assist
Swift lite, Kutty: 8 magnet PAS

Hope that helps!

@roshan Where does the Juggernaut HD duo fit in to this? I just purchased one a few hours ago

 

[AHicks]

Where does the Juggernaut HD duo fit in to this? I just purchased one a few hours ago

You're going to need to identify your purchase as the hub drive version, or the mid drive. They are each goig to be WAY different PAS systems...

 

[voidedwarranty]

You're going to need to identify your purchase as the hub drive version, or the mid drive. They are each goig to be WAY different PAS systems...

My bad, I thought HD only referred to the BBSHD. I bought the BBSHD 1000watt version

 

[Jtdev]

Thanks for the helpful explanation of how torque and cadence sensors differ. However, I have a Stunner LT, which Biktrix advertises as having a TORQUE + CADENCE sensor. I assume this is some type of hybrid system, can anyone explain how it works and/or what I should expect from it?

 

[scrambler]

Torque + cadence sensing offers the best of both world if the Controller programming takes good advantage of it.
Torque sensing provides the most natural feel of power response based on the pressure you apply on the pedal, but can sometimes lack smoothness at sustained speed, where a cadence sensor gives better results.
If the controller is well programmed, it can blend the information from both sensors, to provide the best feel in all situations.

 

[AHicks]

Torque + cadence sensing offers the best of both world if the Controller programming takes good advantage of it.
Torque sensing provides the most natural feel of power response based on the pressure you apply on the pedal, but can sometimes lack smoothness at sustained speed, where a cadence sensor gives better results.
If the controller is well programmed, it can blend the information from both sensors, to provide the best feel in all situations.

'IF" being the key word here. Secondary to that would be regarding if it's user adjustable. Biktrix makes very little regarding it available on the website. Maybe a deeper dive into the user or owner's manuals would reveal more?

The Bafang Ultra is also described as torque + cadence, as it uses both type sensors to provide a torque sensing PAS system. Then there's the Juiced torque + cadence system on their bikes. They mention that you can turn their torque sensing off, leaving you with strictly cadence sensing. Point being, is that at the moment the manf's seem to be all over the ball park, with little uniformity.

 

[scrambler]

IF Indeed

 

[Jtdev]

Thanks for the responses. I assumed the idea was to get the best of both worlds. The risk with that strategy though, is that you can end up with the worst of both worlds instead. I guess that underscores the importance of good programming.
Not that I am saying the PAS on the Stunner LT is poorly programed. Overall, I am very pleased with the performance and responsiveness. I will make two observations though. First, there is a sweet spot, where the motor and my pedaling seem to be in perfect sync. It changes depending on the slope and wind, but for the most part it is when the PAS and the gear levels are aligned. Riding in both PAS 3 and 3rd gear, for instance. I am not sure if that is planned or if it just works out that way for me given my weight and driving conditions. If I move out of that sweet spot, I have to peddle harder to maintain my speed. Second, when I do move out of the sweet spot, there is also something a little counter-intuitive to the the way the motor responds. For instance, shifting into a higher gear, which involves more pressure on the pedals, often leads to less input from the motor, perhaps because my pedaling slows down a bit. I wonder if this is common with hybrid PAS systems?
Again, these are not complaints so much as observations. The LT is my first e-bike so I am still getting a feel for it, and I have no experience with other PAS systems with which to compare.

 

[Deafcat]

Every torque sensor bike also uses cadence. Pretty sure there's no such thing as "torque only" PAS sensor, regardless of how the mfg advertises it.

 

[Jtdev]

Every torque sensor bike also uses cadence. Pretty sure there's no such thing as "torque only" PAS sensor, regardless of how the mfg advertises it.

Yes, that was my understanding. However Biktrix advertises some of its bikes as just torque sensors and others as cadence+torque system, suggesting there is a difference between their systems and regular torque sensors.

 

[AHicks]

Yes, that was my understanding. However Biktrix advertises some of its bikes as just torque sensors and others as cadence+torque system, suggesting there is a difference between their systems and regular torque sensors.

You need to consider as well the fact Biktrix (and some others) sell Bafang geared hubs as well as the Bafang mid drives, and even when just considering the mid drives, they sell the fairly sophisticated Ultra equipped bikes that come standard with torque sensing built right into the drive, AND the BBSHD equipped bike with NO torque sensing, just cadence.

Agreed too, that today's bikes use a cadence sensor signal to the controller to allow the controller to turn power to the motor on/off. -Al

 

[BigNerd]

It's a on off switch.
It does not measure the cadence. What's the point of measuring cadence if it doesn't know the gear ratio?

For example, let's say you're pedaling SLOW.
Does that mean you're pedaling SLOW and HARD (top gear, high speed) or SLOW and EASY (low gear, easy pedaling at 5mph on parking lot).
The context can differ so much. The cadence itself doesn't tell you much.

The cadence sensor might be able to tell you're pedaling SLOW, but what does it mean? It can be any situation.

That's not the case with at least my Espin Sport.

I have a power output setting on my display and the power output varies based on how fast/slow I am pedaling, when I pedal slower, power output is higher, when I pedal faster, power output is lower.

The way this works is similar to a torque sensor... when you pedal slower, the assumption is that you are having a harder time pedaling so it increases the power to the motor... like when you first start pedaling or when you start pedaling up an incline. As you pedal faster, it reduces the power which follows the logic that you are having an "easier" time so the motor needs less power.

 

[scrambler]

That's not the case with at least my Espin Sport.

I have a power output setting on my display and the power output varies based on how fast/slow I am pedaling, when I pedal slower, power output is higher, when I pedal faster, power output is lower.

The way this works is similar to a torque sensor... when you pedal slower, the assumption is that you are having a harder time pedaling so it increases the power to the motor... like when you first start pedaling or when you start pedaling up an incline. As you pedal faster, it reduces the power which follows the logic that you are having an "easier" time so the motor needs less power.

This matches my understanding of the best approximation a Cadence sensor can make.

Assuming you always wish to pedal at 80rpm for example, if you are pedaling slower, it assumes you are struggling to reach that cadence and raises the assist, as you are able to pedal faster it reduces the assist assuming you are having an easier time

That scenario is a valid one, but it is not the only one. There will be cases, where you are pedaling slowly with little effort because of leisurely pedaling, and in that case the fact that it will push the assist wont be a good thing.
But once you know this is how your assist is programmed, you can adjust your gear and pedaling habit to match its assumption.

It is definitely not as good and exact as a torque sensor, but can get closer for a majority of situation.

 

[scrambler]

Yeah as I noted on example I just gave, I'm not convinced that that's how cadence sensor works.

If you decide to pedal slowly on parking lot, now all the sudden bike will boost up the assist? It doesn't make sense.

There is no such thing as "how a cadence sensor works"
A cadence sensor just provides one information to the controller.
It is then then up to the bike manufacturer to decide what to do with that information. Controllers are programmable, and can use a signal to do all sort of things with that information. This is how a controller can blend a torque sensor signal and a cadence sensor signal, to provide the best possible response to specific scenarios.

With a cadence sensor only signal, some only switch on or off the assist on a set level causing a burst of assist as soon as you start pedaling and then keeping it constant (on/off switch behavior)
Some improve the start by ramping up the assist power progressively, but still keeping it constant. It is still an on/off behavior but with a smoothed ON.
And some as mentioned above could modulate the Assist power based on pedaling speed to try and provide a modulated assist, similar in a way to a torque sensor, but that would only matches one usage scenario, and would therefore be imperfect.