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Note #20
Auxiliary sensors

John Nagle
Last revised April 7, 2003.

Lesser, but important devices

Short-range rangefinding devices

We need the usual "ring of dumb obstacle sensors" on the vehicle. Shown here are a few options.

Doppler radars

Short-range Doppler radar devices are widely used for sensing obstacles behind trucks and buses. Eaton VORAD makes one, as an adjunct to their forward-looking radar. Unfortunately, all it indicates is presence/absence of a target. (Eaton's engineer tells me that it has a DSP and has distance information internally, but doesn't bring it out to an external connection.) Competing products include the Rostra "RearSentry" and "School Bus Protection System".

Rostra "RearSentry" range sensor

A small Doppler radar for rear bumpers of large trucks.

Range about 4 meters. Senses only moving objects.

The standard product connects two ranging sensors to a 3-LED display; it's not intended for computer interfacing.

These devices have good immunity to dirt, but cannot detect stationary objects. So they're not that useful for side-looking sensors, where the range might be constant, or for looking ahead of the front wheels to detect drop-offs when moving slowly.

Ultrasonic sonars

There's been some progress since the days when the fragile Polaroid ranging sensors were the only available devices.

Senix "ToughSonic" range sensor

A small ultrasonic ranging device for industrial applications.

30mm diameter.
Ranges from 0.15m to 4.25m (or 0.08 to 1.12m) Submersible. A narrow beam device. Available in RS-232, analog, various other digital outputs. Multiple units can be synchronized for use in the same area without interference. About $395-$435 each. Made in Vermont.

"SonarVision" range sensor

A backup warning device for autos.

30mm diameter.
Ranges to 2m (Minimum range unknown). Output is a driver display with beeper and 3 colored LEDs. About $89 each. Made in Canada.

"Migatron" range sensor

Another industrial sonar sensor.

Teflon-enclosed sensor for dirty/corrosive environments.

These devices vary in beam width. The narrow-beam Senix unit seems ideal for looking for a surface ahead of each front wheel. But it would take too many of those units to provide full perimeter coverage. We still need to find a wide-angle unit that is very rugged.

Murata makes both types of sensors as OEM components, but their web site has problems.

Speed sensors

We'll need to measure shaft speeds at the engine and driveshaft, but just knowing what the wheels are doing doesn't mean that much off-road.

Dickey-John speed measurement unit

A speedometer for off-road vehicles. Used on tractors and earthmovers.

This is a tiny Doppler radar.

Speed range 0..44mph. Transmits at 24.125 GHz. Distance to ground must be 457 to 1219 mm (18 to 48 inches) 610mm (24 inches) nominal.