Pneumatic Control

The RMC family of motion controllers excels at pneumatic motion control applications that demand precise, smooth motion.

Pneumatic Control has been serving industry for a long time and provides several advantages, not the least of which is less costly devices, clean operation and lower maintenance costs. However, precision and smooth motion has been elusive because of inherent drawbacks to pneumatics. The major drawback is the compressibility of air. This drawback has been minimized with new motion control techniques.

The RMC gives the user an option to replace start/stop or ”bang-bang” control that cause shock and wear allowing machines and equipment to last longer. This is done with motion controllers and by using feedback from position and pressure sensors. The RMC then controls the servo proportional valve to eliminate banging and bottoming out at the ends of cylinders. Use of proportional servo valves also allows a better option for accurate control in place of using a pressure relief valve. Delta recommends pressure relief valves be used for relieving pressure and not for controlling motion or pressure.

Precise midpoint positioning is also easily done with an RMC motion controller using servo valves by moving the spool to meter air in and out of a cylinder. Pneumatic motion control that is fast and smooth can make a system stiffer and act much like an elecro-mechanical system at a much less expensive cost.

High-performance motion control requires a servo valve and a high-resolution position transducer. For optimal control, a low-friction cylinder should be used. In addition, for the best control, either pressure transducers or accelerometers should be used.

Once the pneumatic system has been set up and tuned, it can be controlled much like any typical hydraulic or electric servo system.

Pneumatic Control Options

The RMC has several options for pneumatic control:

PID or I-PD with Active Damping with force feedback (load cell or pressure transducers) or accelerometers

Easy to tune without overshooting; smooth.

Force Feedback can be a load cell or differential force, which requires two pressure transducers; one on each end of the cylinder.

PID with Acceleration Control with accelerometers

This gives the tightest control. However, it really works the valve and will always overshoot slightly. Tuning hot tends to cause small high-speed vibrations and pushes stability limits.

This will generally require the use of a filter on the acceleration. Either the Low Pass filter or the Model based filter can be used, but Delta recommends trying the Low Pass filter first because it is easier to set up and does not require that an accurate model be generated by the Tuning Wizard. See the Acceleration Filter Type and Actual Acceleration Filter topics for more details.

PID with Acceleration Control without accelerometers

Much lower performance, may be good for some applications, especially for low-friction cylinders with small load. Does not require extra feedback devices or inputs on the controller. Least expensive option.

Required Modules

Active Damping and Acceleration Control require an AP2 module for the secondary acceleration or force feedback. Using Acceleration Control without accelerometers does not require any additional modules other than the axis module.

See the Active Damping or Acceleration Control topics for details on required hardware and how to define the axes.

Suggested Components

Pressure Transducers

Choose pressure transducers with a power supply range that includes 24V, and a voltage output range of 0 to 5 volts or more. The pressure transducers should be mounted on the ends of the cylinder so that they measure the pressure inside the cylinder, not the pressure in the A or B port lines.

Accelerometers

For pneumatic control, choose 1-axis accelerometers that cover approximately -2g to +2g, depending on the accelerations in your application. To simplify the wiring, choose one with an internal voltage regulator, a power supply range that includes 24V, and a voltage output range of 0 to 5 volts or more.

Delta recommends using two accelerometers, one on the stationary frame and one on the moving load. This is because the stationary frame is not usually truly stationary – its vibrations will impact the motion of the load.

Delta has successfully used accelerometers from Crossbow Technology Inc. (www.xbow.com). The model used was the LP series - General Purpose, part number CXL04LP1-R, which is a ± 4g, 1-axis accelerometer with an internal voltage regulator. It can be powered with a 24V power supply.

Pneumatic Servo Valves

Choose a zero-lap servo valve.

Pneumatic Cylinder

For best results, choose a low-friction cylinder.