Lawn Mover – Motor Algorithm

Running a BLDC (3-phase motor) you need to insert and tune some parameters with regards to size of motor, number of winding, how much current do you output etc. But, more important is the algorithm and technique you chose matching the job at hand. You have three different algorithms (FOC, Sinusoidal, Trapezoidal) and a variety of input sensors techiques (BEMF, Hall, Current and position encoders).

FOC (Field Oriented Vector) is excellent for running fast motors, but the algorithm requires heavy math and current sensors that don’t work that well on slow speeds giving me a problem on the lawn mower wheels. Classic Trapezoidal or Sinusoidal combined with Hall sensors do actually have an advantage at very low speeds. Both are also table driven, meaning we can act more or less as a stepper motor. The accuracy of Hall sensors are not even close to that of a stepper, but we have a belt that introduce a gear ratio that in effect will increase the accuracy.

Sinusoidal can be calculated, but a neat trick is to pre-calculate x number of vectors in a table.

Vector driven means we use current, hall, bemf or a position encoder to detect rotator position and calculate an output vector that is 90 degrees – this is where FOC is good as it is more efficient than the other algorithms assuming it has accurate current sensors. All methods can be used without any sensor input – running a BLDC with Trapezoidal with no sensors is very easy as you just rotate the field based on timing outputting one PWM combination at the time. It work decently well assuming the motor follow your output, but as you are blind for the actual position you get a glitch at start or if the motor is stuck. It is also very difficult to get up in higher speeds, but this work excellent at low speeds.

I have current sensors on the drivers, but as I will be driving slow I expect there to be more noise than input on the wheel drivers. Hall Sensors are far more reliable as they work even at stand-still.

In my case I will just use Trapezoidal on the wheels for now since it is dead easy to code up and fits well with what I need to do. Trapezoidal combined with Hall sensors should work just fine. Having three Hall sensors I will get an encoder that givers me 0 to 7 as input – 8 positions. These will give me the ca position of the rotor and should be sufficient to index the next step in a Trapezoidal algorithm that easily can be adjusted to this.

Changing subject to the grass cutter I basically need a different approach as I in this case is interested in running at 80% possible speed (80% speed is ca 50% efficiency) on a 1,5KW motor. In this case I don’t have hall sensors, but as I run faster I should have working current sensors.

One challenge is however that as I start the cutter I don’t know the position, so I do a trick and start running the motor Trapezoidal until I get readings and from there I run FOC (or sinusoidal). By doing this I take advantage of the fact that slow Trapezoidal will more or less force the motor from stand-still up in a minimum speed where I can start accelerating based on phase current readings.

At this point I am only interested in spinning the motors – I will implement more optimized algorithm’s later. My decition is basically not to use time on this at precent because motor algorithms and optimization can be very time consuming.

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