This was a fairly successful, although never completed (of course), robot that used a pair of bicycle wheels driven by a friction roller.The frame was made of aluminum extrusions bolted together. An IBM PC served as the main controller, with the same old Z80 wirewrapped board to control the PWM amplifiers. Power was from 3 12V gelcell type batteries, with dc/dc converters used to generate +5V and +12V as required.

System Block Diagram - 2K

Block diagram (13K GIF) (50K visio)

Navigation was by a combination of odometry/dead reckoning and a scanning sonar sensor.

PWM controllers

I went through a lot of design iterations on the PWM controllers for Roger. I started with using some commercial PWM chips from Unitrode and so forth. In theory, these chips allow one to sense the back emf to measure the motor speed, however, in practice, this is pretty darn tricky, because you're also switching power to the motor, which has a LOT of inductance, etc.

The final design iteration was a fall back to using the Z80 and programmable timers, but this time, driving VMOSFET H-bridges.

On board computer

By this time, I had also realized that having some on-board intelligence would be useful, and since I had a spare motherboard from an IBM PC, I mounted that, along with some DC to DC converters on the chassis. A monochrome 12V monitor, scavenged from a Bondwell portable computer, served as the display. I booted off a single floppy drive (visible in the photo above on the right hand side).


One of the Polaroid Sonar Developers Kits was used to make a scanning sonar sensor. A 200 step/rev stepper motor scavenged from a floppy disk drive was used to scan the sensor in azimuth. A simple transistor buffer drove the motor from a PC parallel port, with other bits on the port used to trigger the sonar, and to look for the return (by counting loops in a BASIC program).

The original goal was to use the sonar to produce a map of the area in front of the robot, then match that to a stored map for position finding and obstacle avoidance. In practice, I learned that there are some serious shortcomings in using a broad beamed sonar sensor:

Walls provide a specular reflection, and hence, disappear. The problem is particularly severe in hallways, where the navigation problem is most difficult.

Sonar reflectivity is not, by any means, consistent for a given object. From different angles, or even at different times, the reflectivity of things like furniture can vary a lot. To a certain extent this is due to scintillation (the object dimensions (or components of the object) are comparable to a wavelength (at 40 kHz, a wavelength is around a cm).

Lessons Learned

Roger was originally designed when I worked in a place with a big lab area, wide doorways, etc. When I moved it home, I discovered that navigating a 20" wide robot through a 24" wide doorway is a real challenge. It requires great precision and slow speed. Try it yourself while walking holding a stick or a box.

Homegrown PWM controllers are a pain, no matter how you look at it. This is especially true if you want to try and close a velocity or torque loop.

Accurate feedback from the wheel isn't all that necessary. Sure, on a flat, level surface, with no slip, you could use it to do accurate navigation, but in the real world, factors like wheel slip and uneveness mean that you need some other way to figure out where you are and how to go where you want to go.

In fact, trying to put tight feedback loops around the drive is probably not worth it, since you have relatively little a priori information about the terrain, state of battery charge, traction, etc. A fairly simple strategy of setting the "throttle" works as well as anything other.

Some scheme for reasonably accurate navigation (or position finding, at least) is needed.

robot/roger.htm - 6 November 2001 - Jim Lux
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