Michael Walsh
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Michael Walsh - Minneapolis, MN
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- Type: Radio or Stamp controlled robot arm
- Purpose: Acquire mechanical skill towards building future autonomous robots
- Controller: Mark1: Human operator via R/C; Mark2: Basic Stamp 2 program
- Software Development Environment/Methods: Basic (sort of)
- Sensors: None
- Actuators: Four servos: shoulder rotation, shoulder lift, elbow rotation, and gripper
- Height, width, length:approx. 16x6x6"; the arm segments are each 12" long
- Weight:A few pounds; requires two sledge-hammer heads at its base so it doesn't tip over
- Power source: Ni-Cad rechargeable batteries
- Speed: Natural arm-speed; but if you spin the shoulder too fast, the gears pop off
- Contruction history:
This was inspired by some leftover R/C model parts, including a radio
transmitter, servo controller/receiver, and several servos. I built the
wooden stand out of some surplus maple boards. The arm consists of aluminum
stock and coaster wheel bearings.
I admit I had visions of the arm hoisting beer steins, but ran into
the inconvenience that servos don't really have enough torque for this
application. 48-ounce-inches is fairly typical. That corresponds to
hoisting a 12-ounce beer - at a 4-inch lever arm. Just in order for
the arm to move its own weight around I had to invest in some high-torque
servos, an oversized 130 oz-in for the shoulder rotation, 125 oz-in for
the shoulder lift.
- Operational description:
Mark1 could pick up a piece of paper or a small object and move it around ...
if the operator was very skillful. Mark2 follows a preprogrammed series of
motions. The operator can cycle through five modes by hitting a button.
Modes 1 through 4 move individual servos through their (safe) range of
motion. Mode 5 is more interesting and moves the arm though a loop
which has been compared by observers to a karate chop. I thought it
was a handshake, whatever.
- Future Enhancements/Plans:
The arm could profit from a rebuild with tighter joints that have
less play in them. Also, the servos could be taken off the arm and mounted
on the shoulder. They would run the arm over bicycle brake lines (wires
inside rigid tubes). This would take weight off the arm and allow it to
lift heavier objects.
However, I think I'll move on to an autonomous rover instead. The last
modification to the Red Hand is likely to be a microswitch inside the
hand that triggers the gripper motion.
- Status: The Red Hand Mark1 and Mark2 are complete.
- More Info: I hope to do a detailed writeup with source code
listings and circuit diagrams. In the meanwhile, send me email.
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- Type: Mobile robot
- Purpose: Autonomous mobility
- Controller: Basic Stamp 2
- Software Development Environment/Methods: Basic (sort of)
- Sensors: Current sensing
- Actuators: Two gearmotors
- Height, width, length:approx. 12x12x8"
- Weight:several pounds
- Power source: 7 Ah lead-acid gel battery
- Speed: several feet per second, and that's running 24V motors on 12V at a 30%
duty cycle.
- Construction history:
Twelve-inch plywood rounds, two motors, lead-acid battery, and a Basic Stamp 2 ...
hmmm. Actually the toughest part for me was the mechanical construction, using bent and
drilled steel plates to screw everything down to the plywood base. Then I built a
hexagonal plywood shell to sit on the base and peg into the second plywood round that
forms the roof. In case it rains. And then I polyurethaned the whole thing, and added
an external power switch.
There is a single small electronics board, with the Basic Stamp, the motor driver
chip, and some related logic. The Basic Stamp sits in a socket which sits in another
(soldered) socket, so that I can easily remove it, including its first socket, for
reprogramming. The most recent addition is a current sensor. This consists of a 358 op
amp to amplify the voltage drop over a small resistor into the motor driver chip, and a
339 voltage comparator to compare the result to a potentiometer-set threshold. The
result is an input line to the Basic Stamp. This is intended as a bump sensor, but I
haven't used it this way yet.
- Operational description:
It can do a little dance: forward, back, rotate one way, forward, back, rotate the
other way. An alternate program runs straightaway ahead, for debugging the current
sensor. The next program will use current-sensor-based bump sensing to explore the
floor.
- Future Enhancements/Plans:
The following is a monster wish list: Speed control and odometry using PWM and the
(presently unconnected) mechanical encoders on the motors. Orientation using a compass
and/or gyroscope. More sensors, such as real bump sensing and infrared and ultrasonic
range finders, possibly mounted on a servo or stepper motor programmed to sweep back and
forth. More microprocessors to control the above. An interface to a laptop for real
navigation with mapping and landmarking. Actually, I will have to build a second mobile
bots based on some better motors I have acquired, with real 500-count-per-revolution
optical encoders rather than the 6-count mechanical ones I have now.
- Status: Work in progress.
- More Info: Send me email with any questions.
