RobotBuilder

E20 Electronics

The Main Shunt Motor

The 36 V main shunt motor contains an armature and field. More power given to the armature causes the motor to speed up. The field controls the direction of rotation and also increases the motor speed as it goes into field weakening.

The electronics for the main motor contain a half-bridge with current sensing for the armature and full-bridge for the field. For half-bridge there are four paralleled IRF1407 mosfets per leg. The design was based on the Open Source Motor Controller which is a for a 100 A/50 V dc motor controller. Current sensing is done with Allegro's ACS752SCA-100.

The difference is that I use an IR2184 half-bridge mosfet controller that is internally protected from turning both the low and high side on simultaneously. This chip can't turn the mosfets on 100% so I needed to make a homemade mosfet power supply that would add voltage to the high voltage supply.

Improved Duel Lift Motor Controller

The electronics are capable of running two lifts, each of which is either an original lift motor or new 24 V wheelchair motor from 36 Volts. It can safely do this by limiting how much power is delivered to the desired lift motor through current limiting.

For electronics, there are 2 full bridges with with one IRF1407 mosfet per leg and two other mosfets to turn on the brakes for the new wheelchair motors.

Mower Deck Motor/Rototiller Motor

The mower deck runs from a half-bridge of MOSFETS and is able to supply and sense up to 100 Amps. It's powerful enough to run the five blade mower deck or a rototiller. The electronics are identical to the armature.

Analog and Digital Inputs

Analog inputs are used for Rotary Hall-Effect Position Sensor for desired speed, current sensing, brake switch (really digital but didn't have an extra digital input available) and temperature sensing (for board temperature and motor temperature). Sixteen extra analog inputs were added to the PIC with Microchip's serial ADC MCP3304.

Eight digital inputs are avaible for switches on the dash such as front lift up/down, rear lift up/down, mower, headlights, seat switch, and internal motor over-temp. These eight inputs use a 74LS151 which decrease the number of I/O pins required on the PIC from 8 to 4.

Headlights

A DC/DC converter runs 12 V lights from 36 V

Power Supplies

Running all the electronics takes about 100 mA at 36V. Incoming 36 V is dropped to about 14 volts with an adjustable switching regulator, part number LM2594HVN-ADJ. Then this voltage is dropped to 12 V for the mosfet drivers and 5V for the PIC, ADCs, etc. Five volts also is given to a 2.7 volt voltage regulator to power half a MB of external flash memory.

PIC Microcontroller

A PIC18F448 is used to gather all inputs and control all E20 motor controllers. The E20 Software for this PIC controls all lawnmower functions.

Charger

Design in process for a charger to observe how fast the voltage is rising and will shutoff charger when it slows down. Uses a small eight pin PIC12F675. I use a contactor to connect the charger to the batteries, turn the large transformer on, and turn the fuel guage on while the elec-trak is charging. The board is very large for now because of the large solid state relay and the 500 mA transformer but otherwise is very simple. I plan to leave the original timer in series with the coil to the contactor as a safety feature while I test in order to not overcharge the batteries.

Layout of the Electronics

The main drive motor (armature) and mower deck motor electronics are nearly identical, each capable of supplying up to 100 amps continuously. The field can be weakened electronically with the full-bridge controlled by the PIC so there are more speeds available instead of only varying the PWM (Pulse Width Modulation - basically the on time of the MOSFETS, between 0-100%) of the armature. Both the armature and mower have nice heavy duty ground terminal strips from Home Depot with brass screws holding them onto the board. They're non-removable, everything on this board carrying high power is soldered in. The board has eight mounting holes, 4 in the corners and one in between on each edge in order to secure the board to the tractor. Also, it's less likely having the board crack when tightening all the screws. Traces carrying lots of power were covered in solder in order to lower their resistance.

Small 20 amp connectors were used for lifts, field, headlights, fan, brakes, and the key wire. These connectors, from All Electronics, originally had wings and two more holes to mount the connector but I sanded them off in order to save space on the edges of the board.

The connectors for analog and digital inputs are nice because the board doesn't take any extra room since wires enter straight in. I didn't want to use connectors that just plug in because I figured screws would be more realiable in the long run. There's quite a few external wires for all these but digital inputs were simplified using ribbon cable. Each input is programmable, so it doesn't matter which digital input was which, each was configured in seconds. Digital inputs came from the dash in a ribbon cable.