Das-Kit CT5-I5 PAS Sensor D12L

Summary

This is a model for the magnetic ring part of the pedal assist sensor used with the Das-Kit CT5-I5 controller.

Needed Materials:

• Durable filament such as Nylon (or use a print service)
• 24 2x1mm neodymium magnets (or try 12 2x2mm magnets)

Optional Materials:

• Epoxy (to protect magnets)
• Thin plastic sheet (cut from plastic packaging)
• Grease

Link to model on Onshape where you can modify the design. For instance, to change tolerances, the clip angle, or magnet size.

Background

Pedal Assist Sensor (PAS)

If you are not familiar with electric bikes, most use pedal-assist power meaning they provide power to the motor only when the rider is pedaling. Therefore, there is a need to sense pedaling in order for the controller to know when to provide power. The sensor used to do this is called a pedal assist sensor or PAS. 

PAS often use magnets and Hall Effect sensors to sense the rotation of the crank spindle. As you pedal, small magnets embedded in a ring spin past the Hall Effect sensor which senses the change in magnetic field and sends that signal to the controller. 

How PAS Can Fail

Since the PAS is located at the bottom bracket where water and debris can be kicked up by the front tire, it is vulnerable to corrosion. This problem is particularly acute in regions where riding surfaces are treated with salt to melt snow and ice. If the sensor is not regularly cleaned or the magnets not protected, the magnets will eventually corrode enough to no longer produce a sufficiently strong magnetic field for the Hall Effect sensor to detect. In that case, the controller no longer provides power when pedaling. 

Symptoms of a Failing PAS

• Motor cuts out at low pedal cadence
• Motor cuts in and out constantly

When a PAS is failing, you may notice the power from the motor cut out more often when you are pedaling slowly, for instance when you begin to ride up a steep hill without changing gears. You may notice that changing down gears stops the problem. 

This is because some magnets will fall below the detectable threshold first, meaning there there will be gaps where there are effectively no magnets. The gap will cause the time between sensed magnets to be twice as long than where there is no gap. Since the controller turns off the motor after a certain period of time with no pedaling sensed, the increased time between magnets may now be long enough for the controller to think you have stopped pedaling.

Imagine a scenario in which every other magnet is missing from your PAS. The minimum pedaling speed needed to maintain motor power would be twice as fast as when all magnets are present. This is why when your PAS is failing, changing to a lower gear, i.e. faster pedaling, can help.

You may also experience the motor cutting in and out constantly. Imagine a scenario where magnets on one half of the disk are missing. For have the pedal stroke the controller will sense pedaling and provide power and for the other half it will sense nothing and cut power. The result will be a constant oscillation of power and no power.

As part of a differential diagnosis, try using your throttle. If the power is consistent then the problem is likely your PAS. If the power is still inconsistent, the problem is not your PAS and could be something else like your motor, battery, or controller.

Solutions

• Replace the controller (~$150)
• Replace just the sensor (requires modification) (~$15)
• Replace just the magnet ring portion of the PAS (<$10)

Replace the Controller

This is the way a bike shop will likely suggest you fix your bike if they are knowledgeable enough to diagnose the issue. For the Das-Kit CT5-I5 controller the PAS is wired in with no connector, so to replace the PAS you must replace the whole controller.

I want to say this bad design because it is not very serviceable for a part that is likely to fail with time, but mine lasted seven years, with four or five years of winter commuting in a region with salt. I can see a world in which the designers chose to not use a connector in order to improve reliability, but also that decision is easy to make when it saves production costs and increases the revenue for replacement parts. 

NCM did, at one time, produce controllers with PAS that used connectors. The P12L-10 old style matches the model I need, but I was unable to find any in stock. This shows that at some point the decision was made to move away from a replaceable PAS, and I'm not sure if that was a good decision.

Replace the Sensor

Since the Das-Kit CT5-I5 controller has a PAS that is hard wired in, you will need to find a replacement 12 magnet PAS and connectors that you can wire in. It seems like the KT-F12L WP ($10) and some 4-pin waterproof connectors ($4) on alliexpress could do the job. 

The downside is you need to do a good job of sealing up all connections and the connectors add bulk and length to the wire that may not fit in the original space.

Replace the Magnet Ring Portion of the PAS

This is the cheapest and least destructive option if we can pull it off. The model listed here is meant to replace the plastic part of the PAS that houses the magnets. In the picture below you can see the original black part on the left with the corroded magnets, and the printed replacement in white on the right.

The magnets used in the replacement are round 2x1mm neodymium magnets. Two magnets of this size are used in each slot. 

Magnet Ring Assembly Instructions

The orientation of the magnets in the ring is significant. The north poles of the magnets must face the clockwise rotation when looking down at the part with the flat side down; see the diagram below.

I used tweezers to hold the two magnets above the hole then used the flat back side of a non-ferrous screwdriver handle to push the magnets in. After all the magnets were in, I protected the magnets by covering them with epoxy, being careful to not create high spots that would wear the mating face.

When I installed the new magnet ring, I decided to cut a thin plastic sheet to act as a bearing surface between the magnet ring and the sensor ring. I added grease on each side of the thin plastic sheet in a hope to reduce wear. Time will tell if this is a good strategy. Your mileage may vary.

Materials Discussion

In my prototype, I printed the part using FDM without supports using PolyMaker PLA+. As of writing, it is winter in my region with temperatures below freezing and the part has worked for at least 40 miles and I suspect it will continue to hold up until outside temperatures warm up in summer or UV degradation finally weakens the part enough to fail.

Going forward, I plan to use PA12-HP Nylon printed using MJF. I ordered from JLC3DP at a cost of $1.91 per part and $1.50 shipping. I suspect the original part, although injection molded, is likely nylon and therefore nylon should be a good fit. Nylon is UV stable and is unlikely to deform in summer temperatures. 

Bike Models with Das-Kit CT5-I5 Controller

The Das-Kit CT5-I5 controller is likely a white-label product made a Chinese manufacturer and rebranded Das-Kit for the German bike manufacturer NCM. My bike is a Magnum Peak 29 from 2017 and I suspect that it is either a rebranded NCM Moscow or they are both rebranded white label bikes from a Chinese manufacturer. Either way here are some bike models that I have found that share this controller:

• Magnum Cruiser 1.0 
• Magnum Low Rider 1.0 
• Magnum Metro 
• Magnum Metro+ 
• Magnum Mi6 
• Magnum Payload 
• Magnum Peak 27.5" 
• Magnum Peak 29" 
• Magnum Ranger 1.0 
• Magnum Ui6 
• Magnum Ui6+ 
• NCM Milano
• NCM Milano Plus
• NCM Venice
• NCM Venice Plus
• NCM Moscow
• NCM Moscow Plus
• NCM Aspen
• NCM Aspen Plus
• NCM Miami
• ET Cycle ET T720
• ET Cycle ET T1000

Comment with additional models if you know of any so that folks can find this part.