Robotics

Project Examples

Industrial Motion Control

Motion Control of Multi-Axis Precision Stages


Linear and rotary electric motors are commonly used for operating stages with two, three, or more axes of motion.




Precision Control for Linear Motors


Linear motors are often preferred for high-precision stages with sub-micron positioning accuracies, and for cleanroom environments where minimal particle generation from moving parts is desired.
Heavy, massive granite frames can be used to isolate the motion system from external vibration.
See Figure 1.

Precision Control for Linear Motors Fig. 1


Zidecs Automation has designed and implemented advanced motion control software for clean and vibration-controlled linear motion systems.
Example: a 3-meter long, horizontal axis driven by a linear motor with a positioning jitter of better than 50 nm (standard deviation < 5 nm) using a 1.0 nm encoder resolution. The tracking error at a velocity of 0.6 meter/sec was 500 nm (1 sigma), an excellent performance considering the 70 kg load and the disturbance injected by the cable carrier (Figure 2).
The control system is used on manufacturing tools in the semiconductor and flat panel display industries.

Precision Control for Linear Motors Fig. 2




Dual-Loop Control for Rotary Motors

Rotary motors with lead screws, either belt-driven or directly coupled, offer proven and cost-effective linear actuators. However, dynamic performance can be impacted due to the reduced stiffness and backlash between the motor and the load. Single-loop position control is sufficient for repeatability and accuracy on the order of 0.1 mm. Zidecs Automation has designed and implemented advanced motion control software for vertical axes of motion with heavy loads that achieved a 0.001 mm positioning repeatability and accuracy. The axes were operated with common, off-the-shelf rotary motors (Figure 1).

Dual-Loop Control for Rotary Motors Fig. 1


Dual-loop control uses two feedbacks: load position and motor position (Figure 2). The load position feedback is used for the position control loop. The motor position feedback is used for the velocity loop and for motor commutation. Safety is provided by an integrated motor brake. Other safety measures, including pneumatic counter balances, are not needed.

The linear motion systems are implemented on manufacturing tools for ultra clean manufacturing environments.

Dual-Loop Control for Rotary Motors Fig. 2




Closed-Loop Control of a Pneumatic Counter Balance

The position control of a vertical axis of motion with a heavy load can be achieved with an well-sized electric linear motor. However, continuously supporting the heavy weight with only the motor is not an inefficient use of energy. Furthermore, it would also heat up the motor and the motor assembly.

The resulting thermal expansion can compromise the accurate positioning of cameras or laser systems and should be prevented. A better and commonly used method is to balance the load with a pneumatic cylinder and to perform the position control with a smaller and more energy-efficient electric motor. Zidecs Automation developed and implemented an advanced algorithm with nested control loops that achieved this two-fold control objective: achieve the commanded position while minimizing the motor load. The method actively controls the ‘large-signal’ transient behavior with a pneumatic cylinder and the ‘small-signal’ position control with a linear motor. Figure 1 shows the electronic pressure regulator that is used to command the pressure set point.

Closed-Loop Control of a Pneumatic Counter Balance