Limit current of ESC by software

Question

I am designing a cheap electric skateboard.

I have played around with various parameters for quite a while until I finally converged towards a specific motor and a specific battery. Lithium batteries are too expensive so I'm going for an AGM motorcycle battery. The problem is that the 0.025Ohm resistance of the motor leads to a huge current (hundreds of amps) up to 10km/hr which is most of my operational range. Other motors do not provide me with sufficient torque, therefore I need to include a current limiter to protect the ESC and maximise the battery capacity. For information, the continuous rating of the ESC I'm using is 60A.

I'm planning on measuring the current with a Hall Effect current sensor such as the Allegro modules out there, and an Arduino. Then map the current to a % of the duty cycle to subtract to the setpoint, before sending it to the ESC. For example, up to 30A => 0 feedback, 60A and above => 100% feedback, varying linearly in between. My gut feeling says it's basically a bang-bang controller which should be converging.

Any better alternatives?

Answer 1

You are probably overthinking this.

First, as soon a DC motor is rotating, there is an internal voltage source which works against the outer voltage. Second, the rotor coil of a DC motor is fed an AC voltage because of the commutator, which means its inductivity adds a lot of additional resistance to the circuit, depending on speed.

So, overcurrent is only a problem at a standstill and very low speeds. The usual way to handle this is to have thermoswitches at all interesting points (battery too!) and thermofuses as a measure of last resort.

Answer 2

Although limiting the current by acceleration control as proposed by @Tony_Stewart should work up to a point, limiting or controlling the torque should provide better performance. With a DC motor, controlling current quite effectively controls torque. In engine powered vehicles, the throttle essentially controls torque by regulating fuel flow. That essentially provides torque controlled operation with an outer-loop acceleration and speed control implemented by observation and throttle control by the operator.

I would propose proportional control of current from 0 to 150% of the rated continuous safe operating motor current with an inverse time override for operation above rated motor current. The ESC should be selected to accommodate the maximum safe operating current of the motor.

Answer 3

The trick to solve this is to analyze the Motor DCR coil resistance to Battery ESR ratio and you will find LiPo is your best solution considering heat rise in the battery and RdsON switches is proportional to both ESR in battery and RdsON in MOSFETs.

Then limit the current by controlling the servo acceleration. F=ma and F is proportional to current so limit the acceleration , a and then current will be limited. This applies to both start and stop.

Servo systems need to be complex to handle a variety of inputs and desired response. Ultimately heat and temperature rise must be controlled and detected not just current or acceleration or velocity. But ultimately you want a smooth ride so the best control is power input and the limiting factor is temperature rise. But as in any engine without active cooling, if you do not anticipate the temperature rise you may have to wait for it to cool down when it overheats.

Therefore I still suggest acceleration control is the best for smooth operation and velocity limits will and thermal rise will occur according to demand and power dissipation.

Therefore you must have some thermal feedback to limit current*time product both which are proportional to temperature rise and not just current limit for brief instances.

As with any "underpowered" system you cannot get max power all the time nor would want it to spin wheels with constant current incase of loss of traction, so you need load sensing, thermal sensing, velocity sensing, and acceleration sensing for a smooth control.

You certainly wont get a complete design here either without requirements, which is your first obligation.