I tried making a body with the lego parts and motors that I already had, but this didn't work out too well. It looked like a very lumpy tricycle; it had two (powered) rear wheels, and one (un-powered) front wheel. It also had a four-speed gear-box (controlled by a servo), which was supposed to enable the relatively weak 4.5volt lego-motors to move the robot at a decent speed. This body failed on several counts:
- It was slow and noisy.
- The four-speed gearbox was overly complicated, and would probably have been unreliable.
- When the robot hit a door-sill, it didn't manage to climb it. The rear wheels just spun.
- It looked like an obese tricycle that had been assimilated by the Borg.
I also discovered that my two remaining servos were also starting to malfunction (they are 10+ years old)
Unfortunately, I forgot to take a picture of it before I dismantled it.
I obviously needed a new approach, so I used the old proverb "If in doubt, use brute force. If that doesn't work, apply more brute force.".
Time to go shopping for new parts:-)
This is what I bought:
- Four 9V lego technic motors (lego nr. 8287). These are several times more powerful than my old ones, and I planned to use four of them, as opposed to the two motors in the old body.
- Two new servos, which the guy in the shop claimed should be perhaps twice as fast and strong as my old ones. I haven't measured it, but he seems to be right.
- A 9.6volt 2500mAh NiMh battery-pack (designed for a RC boat, I think). This replaces my ancient 7.2volt 1200mAh NiCd battery-pack.
The new lego motors have a built-in gearbox, which greatly simplifies the gear-chain that I need to build. After some experiments, I found that I only needed a single extra reduction. The new body has four wheels, each with its own motor. Each of the two front wheels is steered with a servo. The robot is (for now) manually controlled by a pair of cobbled-together joysticks, so that I can easily test the mechanics.
Here is a picture of the robot (with a beer-can to show the scale).
Close-up of the front wheels:
Close-up of the rear wheels (the new-style heavy-duty gears are visible on the right side):
The battery-pack (which is usually hidden below the electronics-board):
The manual (RC-style) controller:
This is basically two potentiometers connected to a couple of levers so that I can control the robot the same way as a radio-controlled car. The potentiometers are wired to the microcontroller, which uses its build-in analog-to-digital converters to read the position of each of the control-levers. The microcontroller then uses this information to position the servos, and to apply power to the motors.So what are the results?
In a word: great!
The new body is far, far superior to my first attempt. The new motors are awesome. At full throttle, the robot moves at a fast walking pace, which is more than enough. It has enough torque to push small chairs around. The four-wheel drive enables it to easily climb over the door-sills.
I'm not quite sure what the next step should be. Most of the brawn of the robot is now in place, but quite a lot of the brains are still in the mail. I'm still waiting for the shipment of sensor-, bluetooth- and motor-control parts from www.sparkfun.com. Hm, maybe I should start on the beer-dispenser thingy that will sit in the fridge.
