eNotes: Mechatronics and Controls

42.1 TRAJECTORY CONTROL

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Given a set of points, we must move at given speeds.

42.1.1 Resolved Rate Motion Control

When the robot motion is important (such as welding applications), the resolved rate method may be useful

This method requires a good model of the manipulator.

The figure below shows an incremental interpolator, based on resolved rate motion control.

42.1.2 Cartesian Motion System

The figure below illustrates a system for making motions in cartesian space (straight line).

42.1.3 Model Reference Adaptive Control (MRAC)

To compensate for overly complex, or changing systems we can use an adaptive controller

42.1.4 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