Basics
Pedal Assist Sensors (PAS) come in two different varieties: Cadence and Torque.
Cadence type sensors send a magnetic pattern signal from its magnets disc which is attached to your crank to your controller. The controller then knows the rotational speed you are inputting through the pedals and supplies the power assistance to the motor at the correct speed for your pedalling. All of the individual PAS's we sell are of this type as they are the most DIY friendly. Bafang mid drive motors use this type of sensor system internally, so no external magnet disc.
Torque type sensors similarly send a magnetic signal to your controller from your pedalling input, however the sensor reads how hard you are pushing the pedal. By measuring the twist in the crank it the controller then knows how much power to supply to the motor to assist you. This is a more refined and typically only found on ebikes built by big brands. However Tongsheng mid drive ebike kits do come with torque sensors if you are after a more factory feel for your DIY ebike.
Operation & Installation
A cadence PAS sensor consists of a sensor head and a disc with a number of magnets embedded within. Sometimes these two parts are contained within a single assembly, but more often than not, they are two separate pieces.
The magnet disc is installed on the bottom bracket spindle beneath one of the cranks so that it spins along with the crankset as the rider pedals. The sensor head is mounted in a fixed position adjacent to the magnet disc, usually secured beneath the bottom bracket securing nut. When the magnet disc rotates, the sensor detects each magnet as they pass.
In order for a cadence sensor to function correctly, both the sensor head and magnet disc have to be oriented correctly. Both parts are directional, so there are four possible combinations of sensor head and disc position. Only one combination will work, the other three will not. In addition to this, the magnet disc needs to pass the sensor head within a distance of ~4mm or less. It is recommended that you confirm the correct orientation of the sensor head and magnet disc by hand before fitting the parts to the bike.
Wiring
The majority of cadence sensors feature 3 wires - power (5V), ground and signal. 4 wire PAS sensors feature an additional ‘direction’ wire. Although I don't explicitly discuss 4 wire sensors here, testing and troubleshooting them is similar to dealing with a 3 wire sensor.
There is little in the way of standardisation when it comes to electric bike wiring, but some of the more common wire colour combinations for PAS sensors are as follows:
Please note that the exact wire sequence order might not match that listed above. The wire colours of the sensor and controller might also not match each other, but their functions must match in order for the sensor to operate correctly. An incorrect match could lead to damage of the sensor and/or controller, so care should be taken.
Testing, Troubleshooting & Fault Finding
With the aid of an electrical multimeter, there are a couple of basic tests that can be carried out to ascertain if the sensor is installed and functioning correctly. All testing should be carried out with the sensor connected to the controller and the controller powered on.
With your multimeter set to 20VDC, insert your multimeter probes into the rear of the PAS connector (sensor side) on the POWER and GROUND wires. You should see a voltage of ~5V. If you do not see 5V, there are a number of possibilities:
Next, insert the multimeter probes into the rear of the PAS connector (controller side) on the SIGNAL and GROUND wires and rotate the PAS sensor ring. You should see the output signal alter between high and low as the ring is rotated and the magnets pass the sensor. If you do not see the output signal alter between high and low, there are a number of possibilities:
Additional Troubleshooting Tips
If pedalling backwards causes the motor to engage, simply flip the magnet disc over to reverse the required direction of movement.
Cadence sensors sometimes feature an LED that flashes each time a magnet passes the sensor head. This is handy for verifying that the PAS sensor is functioning correctly “in the field”. Illumination however, does not necessarily mean that the sensor is wired correctly. If the 5V power feed is incorrectly backfed through the signal wire, the LED will illuminate, but nothing will happen. If this is the case with a newly installed sensor, it might be worth checking if the 5V and signal wires haven't been accidentally swapped.