1.1.4 Digital Control System
• It is more common to use computers to control operations
• The computer must examine sensors, then decide how to change outputs to actuators. But, computers can only do one thing at once, so the approach is limited by CPU speed.
• Note the controlling computer still needs a control algorithm such as PID, resolved rate, MRAC, etc.
• e.g. to control a simple robotic arm with proportional control
• Some sensors, such as encoders can return binary numbers to the computer, instead of analog voltages, this leads to more accurate, noise resistant position measurements.
• The main functions of the control computer are,
1. plans and interpolates a path from the start, to the endpoint in world coordinates.
2. Transforms path points from world to joint coordinates
3. Receives a signal about a completed move, and signals next moves on a path.
4. Executes other motions, not directly driving motion.
• There are two servo loops for each joint,
- the inner loop may consist of an amplifier (voltage or current), a joint drive, and a tachometer for velocity feedback.
- the outer loop may consist of incremental encoder, counter or microprocessor,
- Microprocessor samples the contents of the counter (equal to the BRUs) and transfers this to the computer. The computer compares the counter contents with a program reference to produce the system error. The error signal is fed each ms (for example) to a D/A converter which supplies a voltage proportional to the required axis velocity.
• Advantages,
- complex calculations for control are possible
- software is easy to change for new control laws
- the controller can be adapted easily to new operating conditions
- the computer does not need retuning, reducing the amount of maintenance required
- faults can be easily detected
• Disadvantages,
- the computer may be unable to calculate fast enough
- the computer is generally less reliable
