OC Robotics has developed a range of intelligent CAN-enabled servomotor amplifier controllers to meet their requirements for distributed control of the large numbers of motors used within each of their snake-arm robots.
Named DC2, DC3 and DC4, the controllers have been designed for use in size-constrained applications. DC2 has a circular footprint and is designed to fit neatly in line with a 42mm diameter motor, enabling the closest possible packing of large numbers of motors into the smallest possible space. DC3 and DC4 have larger rectangular footprints but are designed with very low-profile skylines to minimise the volume they occupy.
The designs are based on DSPs for more demanding applications with servo loop closure rates of up to 2kHz. Microcontrollers are used for lower cost, less demanding applications with servo loop closure rates of 500Hz. All the controllers use similar communications protocols, allowing different types of controllers to be used in the same application. With on-board encoder decoding, the controllers perform position and velocity motion profiling, current-based or voltage-based loop closure, current-monitoring, I2t monitoring, error handling, homing and limit switch monitoring and all the necessary low-level housekeeping without needing to involve a host computer. Commands, demands and status information are all communicated over a CAN bus operating at up to 1 megabit per second. The amplifiers operate using sign and magnitude PWM for cool operation even with small motors and are rated at up to 50V and 10A making them suited to use with a very wide range of motors.
Brake control is also integrated into the firmware so that when the motor stops the brake (if used) is automatically applied. The brake control output is controlled using PWM to enable different pull-off and hold-off currents to be set to maximise efficiency and minimise brake heating as well as facilitating brake choice for different applications.
Applications include synchronous control of an OC Robotics snake-arm robot that has 32 motors. The snake-arm sits on top of a remote vehicle so the size and weight of the system is critical. One of the most significant advantages of the controllers is the ability to minimise wiring by engineering a totally integrated package. The firmware can be configured to optimise closed loop performance for different motor and gearbox combinations.
Monday, February 7, 2005