The vehicle platform is designed to fit within a 2-foot circle so it can navigate doorways and turn within its width. Currently equipped main wheels are 11.5 inches high. The platform structure is made up of 1-inch square steel tubing in a box structure. Vertical hard clearance when level is 2 inches. The top of the box structure is 12 inches above the floor. In general, platform structure below the 12 inch height is intended to be dedicated to platform mechanicals, and passenger equipment will mount to the top surface. The upper surface consists of a perimeter structure of 1-inch square tube, 10 inches wide by 11 inches fore-aft. We will equip it with mounting holes or tabs, or studs, or tape things on top with duct tape.

Platform main wheels are driven from wheelchair motors via #35 roller chain. Based on preliminary observations, motive force of the system will be limited primarily by tire friction. Based on measured motor torque of ________, and a __:__ reduction ratio, the horizontal stall force will be ___________. Thus a heavily loaded platform will be able to push over or drive through certain static objects. Some motor current sensing or other safety mechanisms may be appropriate. A perimeter bumper system with contact switches will likely be installed eventually.

Control is currently passed to the EVB via a standard serial port for testing in conjunction with a data terminal. Eventually control will be migrated to the HC11 Serial Communications Interface (SCI) for interprocessor communication.

The motor drive power electronics reside on a circuit board with the same dimensions as the EVB and similarly placed 60-pin connector. It is capable of being mounted, with a suitable connector, as a daughter board on top of the EVB. Logic and motor drive circuitry are separate, with optical coupling of control and status signals in both directions.

Motors are controlled via enable and direction signals. The enable is pulse width modulated (PWM) to control motor speed. In addition to the individual motor enables, a global driver enable allows shutdown of the entire power output stage. An overcurrent sense signal is sent back from the motor driver circuit to the HC11.

Each motor driver consists of four MOSFET transistors in an H-bridge configuration, controlled by a __________ IC. The most recent circuit version is equipped with IRF540 MOSFETS, giving it current capability of 27 A continuous, 108 A peak. This appears to be a reasonable transistor size for efficient operation at anticipated motor power levels. MOSFETS with higher current ratings may be substituted as required. The motor driver circuit is currently programmed for overcurrent limit of approximately 20 A, which may be modified by changing the current sense shunt.

Optical encoders on the motor output shafts are used to feed back motor position to the HC11. Each encoder consists of a pair of optical interrupters and an encoder wheel. The encoder sensors use a Schmitt trigger buffer to control encoder hysteresis. Optical encoder outputs are fed to the HC11 via connectors on the motor driver board. Connectors are 4-pin .100 type.

The motor driver circuit board is designed to operate with input voltage in the 5V-30V range. Full turn-on saturation of the MOSFETS is not guaranteed below 10V, so operation is not recommended in this range with a large motor load. Power connectors on the power board are Molex .093 series 2-pin connectors, connecting to male connectors on batteries and female connectors on motors. The motor drive circuit is fused with a standard automotive type fuse. The fuse should protect against melted wires and boiling batteries in the event of an output short. Current limiting by the transistor driver IC should protect the MOSFETs to some degree. This will no doubt be determined experimentally at some time.

There is an undesirable and potentially exciting design defect on the current version of the power board, resulting in runaway full-speed drive of the motors when power is removed from the control logic. This will be corrected on future versions. For the moment it is necessary to ensure that logic power is applied prior to application of motor drive voltage, and motor drive voltage disconnected prior to disconnection of logic power.

Calibrate

- sets setpoint position equal to current position (overrides move in progress)

Translate

(direction,magnitude) - forward or reverse increment to current setpoint position

Rotate

(direction, magnitude - rotates right or left about platform centerline

Velocity

(direction, magnitude) - gives platform velocity in specified direction

Radius

(direction, length) - specifies curve radius in conjunction with velocity commands

Motor

(direction, magnitude) - allows independent increments topology motor setpoints

Control

(various parameters) - allows resetting of control equation constants and limit values

"Magnitude", a parameter in many commands above, is expressed in terms of encoder counts, and the relation to physical movement distance is a function of encoder resolution, gear ratio, and wheel size. Encoders should eventually be sized such that some whole number relation exists between counts and distance in standard units of measure. Radius is specified in increments of half the vehicle track.

Commands as currently implemented return prompt strings to the data terminal. Such information will be omitted for interprocessor control via the SPI.

Translation, rotation, and velocity commands may be overlaid, with predicatable though not always intuitive results. In general, translation and rotation commands are intended to be applied disjointly, and separately from velocity commands. Velocity and radius commands are intended to be used together in a smooth manner. All commands take effect immediately, but translations and rotations modify endpoint positions, while velocity and radius commands modify the incremental setting of new endpoints.

Eventual additions to the command set may include profiled moves, consisting of acceleration, maintained velocity, and decceleration. Other means of limiting acceleration and jerk forces via programmable limits are possible. Another feature, primarily applicable to translation and rotation commands, would be the ability to queue commands, i.e. move forward 5, turn left 2, move forward 8. Eventually the command set will probably need to be divided into two or more distinct modes of operation.