RobotBuilder

The E-20 elec-trak is a full size electric riding lawnmower. There are many improvements from the original Elec-trak incuding:

• Electronics controlled by a PIC microcontroller
• 5 blade mower deck made from scratch
• New belt that does not slip

Being programmable by the PIC is nice because new features can always be added. The PIC can control how the lawnmower accelerates so the driving much smoother. Regenerative braking actually slows the tractor down at a programmable speed too, so instead of coasting, returning to neutral will slow the tractor down rather than coasting. The kinetic energy from the lawnmower gets put back into the batteries. Mowing on hills works good now since the tractor doesn't roll back.

Electronics

Overview: All motors and sensors connect to a homemade PCB (Printed Circuit Board) I made during the winter of 2004/2005. It contains special hardware to control the main shunt motor, mower deck, front/rear lift motors, and a few other devices. The biggest improvement on the E20 PCB is simplified external wiring. It requires just two power wires of ground and 36 volts. It can run two lift motors and each can be either the original lift motor or a new 24 V wheelchair motor with a brake. The lift motors also run on 36 V but with current limiting so that they aren't being overpowered. Even the 12 V lights run from 36 V through an onboard DC/DC converter. More details of the E20 Electronics are available here.

Designing and Etching the Printed Circuit Board

I designed the circuit board using the Ultiboard 2001. I printed just the the 9" x 8" traces with the pads left open onto regular paper using an injet printer. Then I used a photocopier to copy it onto Press and Peel Blue paper (made by Teckniks) because the iron-on transfer requires a laser printer or photocopier. The next step was to iron this transfer onto the PCB, remove the transfer under running water, and touch up areas which don't transfer with an etch-resistant marker. Ferric chloride removes all the copper on the PCB that isn't covered in etch resistant, so all the copper traces stay. The heater in the etch tank reduces the etch time but it still took around an hour to etch because the board is covered in 2 oz copper and the solution is old. Last step before populating the board was to drill holes with a dremel tool so component wires could be soldered to the back of the board.

Populating

Most of the components came from Digi-key, Newark In-One, Jameco, and All Electronics. Some of the components are shown in a pile below. Each of the photos shows the order in which the board was built starting with the power supply and ZIF (Zero Insertion Force) socket for the PIC microcontroller. Then as things were tested little by little, MOSFETS (devices cable of switching high current) were added to the project.

Some Finished Photos

Schematic of Wiring to Board

For power, the board only requires ground and +36 volts. Wires from motors and most other devices are connnected directly to this board. Digital inputs are Lift Up/Down Switches, Seat switch, Brake switch, etc. They are all either connected to ground (closed switch) or not. Analog inputs read voltages between 0 and 5 volts. A rotary hall effect position sensor is used for determining the velocity of the main motor. Other analog inputs are for ambient temperature inside the case (fan comes on when inside case gets hot), main motor temperature, and current sensing.

5 Blade Mower Deck

Here is the beginning of the 5 blade mower deck that was created from scratch.

The mower deck in the wash position (it latches into this postion), the driving position, and the mowing position.

This is the first lawn the five blade mower deck ever mowed.

Note to elec-trak fans: Sorry it isn't yellow. Blue and silver are much sharper looking colors.

Belt Drive

The original belt designed by General Electric could slip and was probably necessary because it sorta clunks when switching between the seven original speeds. Many times either plowing snow or pushing dirt around the belt would slip. But with my electronics, and nearly infinite variable speed, a gear belt works better. The belt chosen is 1 1/2" wide and capable of transmitting up to 20 HP. Some modifications to the frame were made so that the belt could fit in the original frame and there is a way to tighten the belt with two threaded rods.

Some before and after photos of paiting

Putting it Together and Wiring

Here is a rototiller attachment for the Elec-trak tractor.