CAN in personal transportation devices
Source: CAN Newsletter March 2004
The Segway HT (Human Transporter) has become a popular means of personal transportation in police work, mail delivery and other commercial uses. One of the most intriguing aspects of the transporter is its ability to balance a person on just two wheels while at rest or while traveling at up to 30 km/h. To achieve this balance, it utilizes two micro-controller boards, a BSA (Balance Sensor Assembly), a pair of electric gearmotor drives, a user interface and a pair of 12-Volt NiMH batteries. The link between the handlebar-mounted user interfaces and the micro-controllers uses two redundant Controller Area Networks. Each controller board uses a Texas Instruments DSP (TMS 320LF2406) and Philips CAN transceivers (82C250) that provide the intelligence and regulate the power. Each controller monitors the entire system and checks 100 times per second for any faults or conditions that might require immediate response. The micro-controller reads the information from the Balance Sensor Assembly (BSA) to determine if the rider is leaning forward or backward, and instantly uses this information to deliver power from the batteries to the motors through a set of twelve FETs (field-effect transistors).
The motors are updated at a rate of up to 1 000 times per second. Although each board is capable of operating the transporter alone, under normal conditions they share the load. Each handlebar houses a user interface that employs a Microchip micro-controller (PIC18C658) and a Philips CAN transceiver (82C250). Each user interface is a redundant two-node communication system with the controller board designed specifically for safe operation. Segway decided to develop their own higher-layer protocol and support tools to eliminate any issues with determinancy and to implement messages at 500 kbits/s in a highly scheduled manner. Internal memory constraints and the lack of a requirement to comply with an open higher layer protocol influenced the decision. The transporter features a user-detachable control shaft that is adjustable from 11 cm to 171 cm, which potentially created severe electrical and mechanical environments. Accordingly, the engineers implemented two electrically isolated CAN busses that each operate two nodes between the user interface and the controller boards but do not share data. The need for a robust network with rugged connectors and a cable harness that could accommodate these requirements led them to CAN in the first place and to adopt a redundant user interface.
