It was obviously not really designed to be an outside toy, as was evident from the decay of all the decal stickers from just one summer outside. We don't have the space play with it inside though, so it shall remain an outside toy. However, while I had it in for the winter I decided to clean the whole thing. It was already dirty from the thrift shop in addition to a summers worth of daily play. I took the whole thing to pieces and washed everything carefully. This made it look almost band new, but I also wanted to update some functionality.
The scooter originally just ran current through a foot pedal switch directly from the battery to the motor. This is quite hard for the transmission as there is a huge torque spike when the pedal is pressed. It also requires connecting and disconnecting quite large DC currents through an inductive load. This has left quite clear marks on the contact surfaces inside the pedal. Without any speed regulation, the scooter runs slow uphill and fast downhill. Finally, there is no reverse implemented, which together with the rather large turning radius causes problems in tight corners.
I wanted to address all of the problems through an electric speed controller. The speed controller can either limit the current through the motor to an acceptable level, or it can start the motor up more slowly. In both cases limiting the amount of torque through the transmission. Also proper driving of the motor ensures that current is continuous and hence no arcing will occur. Finally, implementing velocity feedback control allows the scooter to run at constant velocity.
I didn't want to add any sensors to read the motor velocity, so I had to do some reading on sensorless motor velocity feedback control. The main idea is that when the motor is turning without current flowing, the motor acts as a generator producing a voltage proportional to the velocity. The only downside is that in order to measure the velocity, we have to cut power to the motor and wait some time for the magnetic energy in the motor to dissipate. The time it takes for the energy to dissipate is also strongly dependent upon the velocity of the motor (which could be used as a secondary velocity measurement as well).
After some bread boarding of the technique I designed and etched a PCB to put everything on. To keep things simple I implemented the drive using a single N-channel MOSFET, while the reverse function is achieved through a DPDT relay. I went with a silicon diode for the freewheel diode instead of a Schottky one. This is a trade-off between efficiency and the time the motor takes to demagnetize. A Schottky diode would be more efficient, but it would take some time longer for the magnetic energy in the motor to dissipate than with a silicon diode. Also, I had silicon diodes capable of handling the current already lying around. The layout of the motor controller is shown in the figure below, while the schematic is available here.
|My oldest son showing the position where he wants the ignition lock to be mounted.|
|The ignition lock in position.|
|Direction switch mounted. My son in the background supervising my work.|
|Connecting power wires. The black block at the back is the 5A circuit breaker. The reset button of the circuit breaker is accessible through the underside of the scooter.|
|A better view of the inside compartment, showing the speed controller board. The motor is just dangling from the wires for testing. The off-white part in the compartment is the gearbox to which the motor connects.|
|Flashing the software. The speed controller is seen dangling from the wires. The scooter is too large to comfortably fit in my hobby room, so all of this was done in the living room and on the dinner table.|
|Taking it for a test spin. Works great!|