Here is my quick youtube video for this project.
In my prior Instructable, I built an RC tank ( see it here ) using the guts of an RC toy car and an Arduino to handle the control signals for a Tamiya tracked chassis. Then, I thought about going one step higher: RC a ride-on electric car. One of my main directives is: KEEP EXPENSES TO THE ABSOLUTE MINIMUM. So, look and you shall find: a neighbor across the street discarded a Peg Perego Gaucho Grande Electric Jeep (these things were around $500 USD when they came out). I brought it home and, no surprise, both motors worked fine.
First, I wanted to just RC the Jeep; I figured I could use my Tank's Arduino, shield, sketch and Tx/Rx parts as long as I provided enough power and power control for the higher current motors. I would only need to reproduce the toy car signals. BUT, a powerful ($230 USD) 2000 oz-in torque servo was required to drive the Jeep rack-and-pinion steer. I tried with a "smaller" 200 oz-in servo but it was not powerful enough. Cleaning and lubing did not help. Other options were to use a cordless drill to rotate the steering wheel, but I did not have one nor was willing to spend money in buying not even a used one.
Soooo, plan B: I could use two more motors and use tank steering instead of rack-and-pinion steering. Then, another neighbor around the block discarded a Kid Trax Fire Truck. It was so big that I just removed the motor parts right there on the sidewalk. Brought them home and, again, everything worked just fine.
Step 1: The Control Boards
So, I took my control boards (Arduino, Arduino shield, and Tx/Rx parts) from the Tank project and the Arduino sketch. I disabled the extra functions (cannon and machine gun lasers, turret control) in the sketch by commenting them out and was set to go.
I did have to add relays to handle the bigger motors. I decided to use 12 V automotive relays which are inexpensive and can handle up to 20 A. These can be driven directly from my Arduino shield L293 driver chip. They are SPDT so 8 of them were required ( 4 per H bridge ).
Step 2: H Bridges and Crate Power Control Box Assembly
I mounted the relays on a piece of particle board; I also mounted terminal blocks for signal connections. I wired everything using quick-connect terminals.
Then, again, keeping costs to a minimum, used a milk crate as a container/control box. It has many openings to mount stuff, and as a gift from the robotics gods, it turned out to have the exact width needed to mount the Jeep motors.
I also assembled and mounted a base for the Arduino control boards from inexpensive Mechanix-type metal parts. A DB-9 connector plugs to the Arduino Control Boards.
I then mounted the crate onto a length of particle board as chassis
Step 3: Installing Back Motors
Installing the back motors was relatively easy. Just slid the shaft through the crate. The gears and motors slid right in. The body of the motor fit nicely into the handle opening of the crate, so the whole thing was held in place by the crate openings. I added tie wraps to hold shaft even tighter.
Step 4: Front Motors
Ideally, I would have preferred to have all motors and wheels the same. But finding on the curb two ride-on electric cars that are the same or close to the same is hard. Buying two identical motors, gears and wheels was out of the question. So, I happily made use of the ones I got from the fire truck. I cut and installed particle board brackets to hold these motors and smaller wheels. I used tie wraps to hold the motor bodies tight in place. A center screw was used in the original fire truck so I also added it to keep the shaft and motors from sliding left or right. Then, all wheels were in place !
Step 5: Installing the Arduino Control Boards
I took a round base that I had salvaged from a broken nightstand lamp to mount the Arduino control boards ( Arduino at the bottom, Arduino L293 shield in the middle, and Rx board on top). I wired a small connector harness between the Arduino shield and a DB-9 connector which I mounted on the base with hot melt. The 9-volt battery has a small 4-pin connector (only two pins are used; the others are just for mechanical support) which plugs into Vin and ground of the Arduino shield. A 12 volt lead for the L293 was plugged directly to the chip supply, bypassing the board edge connector.
Then the base was mounted onto the metal braces and was held in place with what else, electrical tape, of course.
Step 6: Finishing Touches and Field Test Run
i wanted to have an Emergency stop button. The only thing I had on-hand was the Jeep accelerator pedal. So I mounted it on the back of the crate with a tie wrap to keep it in the on position. I was to have my cutters handy when testing the Jeep and cut the tie wrap to cut the motor voltage off if something went wrong. I has not been necessary up to now.
I also added a 30 amp fuse in series with the Emergency Stop, right in the motor voltage line.
I used two 6 volt 4.5 Ah sealed lead acid batteries so that the motor voltage can be manually selected to be 6 or 12 v. With 12 volt, and without the additional Jeep and fire truck weight, my vehicle moved too fast. I settled with 6 volt only. It has the disadvantage of draining one of the batteries faster than the other. I can live with that.
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As always, researching, remembering or re-studying technical topics,assembling, wiring, mounting, testing, etc provides hours of learning, indignities, frustration and general fun.
For a future project, I would like to make this rover some form of autonomous, or to add vision cameras and the capability of being driven and steered by a driver in a remote location (ok, maybe next door at the beginning).
I will show this project to my students this upcoming year, when I will be my school Robotics coach, to hopefully motivate them into making stuff from an early age. See ya next project !