If I understand correctly, the Sempu unit measures the flex in the BB when you pedal, but what happens when the motor starts pulling on the chainring?
NO.
When the motor pulls on the chainline, the Tq sensor output goes down ( cause you are no longer inputting power above what the motor is) ... and subsequently the sensor output drops...
Doesnt measure flex, to speak, but measures the tq produced ( through the flexing)... Measures the Tq created from input and makes this into a 0-5v signal. Yes, it utilizes flex, but not in the BB... the BB is only the case for the sensor... It measures the chainlint force (( lever arm length) x (force ON end of LEVER)) ( hint: when you are changing gears, does the lever arm length ( length of crank arm to BB center) grow or shorten? No. The output ratio changes ( through the gears to make certain desired speed) ( aka measured chain force) ... not the input ( force x rpm)...
( will not "flex" without "pedal cranking"...). It triggers the motor throttle input, and it is up to you ( the tuner) to make sure the power from the motor doesnt overwhelm the pedaling......
So if the motor is triggered, you are no longer inputting ( as much ) tq, and the power ( assist and human) goes down...or up, depending on desired speed... Motor can easily overwhelm human input.. and without careful tuning, this lurchy ON/OFF of the motor output ( on a graph) is a "Hunting Oscillation" of power input vs desired speed.
Fact of the matter, it is designed to "assist"..
and not to say a 1000w motor wont overwhelm a rider in an instant...
It will.... so you set the motor to not go over say 250-500w ( pas assist power) .. and the primary tension will always be on the chainline ( at human power levels) and contributing to the PAS output, and the CA input, then to the CA output, and finally the throttle input to the controller... where it gently pulses in ratio to the human power input, of 2-200w give or take.
I mean, hit the throttle on a 8Kw bike and there is no way you can pedal at all with any significant input, so it is designed to use the ( range of) HUMAN input force to "throttle " the power out to the wheel.
Im looking at putting together a DIY MY1016 kit with a CA and some kind of torque-sensing BB for my FS bike if the tsdz2-unit does not fit, but not sure if it will work.