Unmanned Vehicle Uses a Distributed-Motion Network

As a result, the vehicle had limited performance on anything but flat surfaces. The vehicle also lacked true velocity control for each of its independent drive wheels. This created a motion twitch whenever the vehicle came to a stop. The twitch was traced to the servodrives playing catchup to close any lingering position error.

The contractor designed the vehicle to suit both industrial and military applications such as hauling, transportation, disarming and disposing of explosives, and surveillance. The independent drive wheels let the craft move in any direction through harsh terrains and environments, yet could propel the vehicle at speeds over 25 mph when conditions were right. The vehicle’s manipulator arm with five degrees of freedom can lift and transport 100-lb payloads, even with the arm at full extension. A CANopen DSP402 network handles communication between the controllers, drives, and remote-control circuitry.

The nature of this vehicle demanded compact electronics with high-power densities, along with a minimum of cabling. To address the power and motion issues, engineers at Elmo Motion Control Inc. in Westford, Mass., analyzed the power demands of the battery-operated vehicle and recommended three different motion control drives to handle all aspects of the craft. As the vehicle was already designed to use CANopen communications, substituting the new Elmo drives became almost a drop-in replacement for the original ones.

Two Elmo Eagle digital drives feed the main drive wheels. The 25-oz devices can continuously deliver up to 60 A to the main drives, powering the vehicle across rough terrain and inclines. Encoder-only velocity control in the Eagles eliminates the need for tachometergenerators typically used for velocity feedback.

Falcon digital drives run the three high-power arm joints. Analog absolute encoders give absoluteposition feedback of the arm joints as well as velocity-control signals.

Finally, Harmonica digital drives handle steering and the arm’s lowpower joints. Steering feedback comes from Hall-effect sensors, while absolute encoders track the arm’s joints and provide encoderonly velocity.

The CANopen communication link lets the drives sit close to the axis they control to reduce cabling, save weight, and shrink the chance of cable failure. Smaller cables aren’t affected by vibration and shock as much as their longer cousins. CANopen in and out connectors on each drive make hook-up a simple daisy chain.