Machine Design

Servopneumatics offers reliable, economical motion

Advances in controls and networks combined with components that pack more punch have opened the door to more-precise, durable, and economical pneumatic systems.

The SPC-200 is a universal axis controller that supports linear and rotary pneumatic drives, as well as electrical stepper motors. It offers "self-tuning" control of a pneumatic axis, with travel speeds to 3 m/sec, accelerations to 30 m/sec2, and typical accuracy of ±0.2 to 0.8 mm.
When it comes to motion control, conventional thinking usually produces designs with electromechanical drives and computer-controlled electric motors. While such systems can accurately position within 0.0001 in. and, in some cases, to 1 µm, these solutions are often "overkill" and ignore the cost/benefit relationship in many applications.

According to engineers at Festo Corp., Hauppauge, N.Y., servopneumatics represents an innovative approach to midstroke positioning and motion profiling. To them, it's an inexpensive, reliable, and accurate alternative for applications that don't require extreme precision. For instance, the company's servopneumatic systems have a typical positioning accuracy between ±0.1% and ±0.04% of stroke, or within ±0.008 in. for an 8-in. stroke. This is precise enough for an estimated 80% of typical industrial positioning requirements. Servopneumatics also eliminate the complexity, expense, and maintenance of motors, large power supplies, and servoamplifiers.

Modular servopneumatic systems consist of only four main components: controller, proportional valve, actuator, and axis interface. The axis controller provides processing, comparing, and diagnostic functions. It sends positioning control signals to the valve, which supplies air that moves the actuator to a preprogrammed stop position. A transducer reads actual position, and returns a signal to the controller for comparison with the programmed position. If there is an error between the two, the controller sends a correction signal to the valve.

The servopneumatic valve regulates airflow to the actuator in direct proportion to the electrical signal from the axis controller. Festo offers linear actuators with both integrated Temposonics magnetostrictive feedback from MTS Sensors, Cary, N.C. (, and external feedback.

The axis interface provides electrical connections to the valve and feedback device. Each axis uses an interface module that conditions signals required for communication to the controller over the Festo CAN bus. This eliminates the need to have different controllers for different types of feedback devices.

The "Smart" Positioning Axis Controller (SPC-200), Festo's servopneumatic positioning system, is a two-axis unit (minimal configuration). A subcontroller card permits control of up to four axes. The SPC-200 position controller functions both as a stand-alone system and can be networked to higher-level systems.

The controller can be programmed to control the axes autonomously with its own set of independent programs or in a coordinated mode. Its multitasking capability allows for programming of complex positioning tasks.

The basic unit provides 10 inputs and eight outputs, including five programmable inputs and three programmable outputs. It is expandable via additional digital I/O modules and many functional I/O possibilities, such as operating grippers and auxiliary actuators.

A keypad lets users enter system parameters, operating modes, program changes, and modify position values. Positioning information can be stored in the controller and move commands can include additional parameters such as speed and acceleration. The SPC-200 can also link to a command control system via a fieldbus such as Profibus, DeviceNet, or Interbus.

Festo's WinPISA application-development software has been specially written for the controller. The Windows-based program provides a familiar operating platform and uses NC commands compliant with DIN 66025.

Graphing capability allows users to upload performance data from the axis during debugging or during the actual process run. The graphing feature plots desired or actual curves for position, speed, and acceleration, as well as valve-output and motion-complete signals on screen. Users can zoom-in on specific details, and several different graphs can be displayed simultaneously.

The SPC-200 provides a straightforward approach to sophisticated applications. The modular servopneumatic system can be assembled quickly, and programming tasks that represented significant effort in the past can now be handled with easy-to-use, Windows-based software.

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