Servopneumatics Go Analog

The ongoing effort to bring pneumatics beyond the so-called bang-bang world has taken a new twist. Engineers from Enfield Technologies (http://www.enfieldtech.com) have rolled out an analog-based system that can provide mid-stroke positioning as well as force and pressure control.

Known as the LS System, the new technology departs dramatically from previous efforts in servo-pneumatic positioning. Instead of using microprocessors to deal with the inherent non-linearities caused by the compressibility of air, the LS employs a high-speed servopneumatic valve and an analog control card. Together, the card and valve cost approximately $1,000 (without an actuator), making them less than half the cost of the electric servo drives used in motion control applications today. Enfield Technologies engineers say that the LS's lower costs make it ideal for process control applications in food, beverage, and pharmaceutical industries, as well as in traditional motion control applications.

To be sure, the technology's ability to provide mid-stroke positioning using air is not new. For the past 15 years, pneumatic systems manufacturers-such as Rexroth, Mosier Industries, Parker Hannifin, and Festo-have offered pneumatic systems capable of tightly-controlled positioning accuracy using air cylinders. Such systems were a departure from the traditional bang-bang nature of pneumatics, in which air cylinders stopped their motion by slamming into mechanical stops.

But Enfield's technology is different in its approach, and its engineers claim that the differences (especially the absence of a microcontroller) make it simpler and less costly than competing servopneumatic systems.

No more control loops

"One of the great advantages of our control system is that the designer doesn't have to develop a control loop in order to run our valve," says Edward Howe, chief executive officer of Enfield Technologies.

Instead of using a digital control loop, the LS System works by taking an analog command signal from a PLC (programmable logic controller) or industrial computer. The command signal is sent to the LS control card, which compares it to a set point command from a sensor. The analog control card processes the difference and determines the direction and magnitude of valve movement needed. It then passes that information to an amplifier that converts the voltage signal to an electrical current command, which actuates the company's servopneumatic valve.

Electric servo replacement

Enfield engineers say the key to this simple approach is the valve itself: Instead of a solenoid valve, the LS employs a high-speed servopneumatic valve coupled to a linear force motor. The force motor enables the valve to open and close in a mere 2.5 msec, compared to 10, 20 or 30 msec for a solenoid-based valve. This high-speed valve operation allows the system to achieve mid-stroke positioning accuracy of about half a percent of full stroke. Thus, an application with 12-inch stroke can be positioned within about 0.06 inches.



Enfield Technologies' LS System employs a high-speed servopneumatic valve coupled to a linear force motor.

Moreover, the LS can be used in applications requiring force and pressure control in the food, beverage, and pharmaceutical industries. Howe says one customer is using it to control the pressure of viscous fluids, such as yogurt, as the fluids are moved from a vat to an individual container. The servopneumatic controller, he says, adapts to natural pressure changes in the vat, and then adjusts the dispensing pressure so that the yogurt is always delivered in the prescribed amount of time.

Enfield engineers claim that the technology is well-suited for test and measurement-particularly in laboratory-type applications where pressure and force control are key-as well as for motion control.

"Our accuracy is good for approximately 80 percent of all motion control applications," Howe says. "If an engineer needs very, very precise positioning or extremely high speed, then maybe an electric servo drive is worth the extra cost. But if half-a-percent of full stroke is sufficient, then this system can provide benefits of lower cost and greater simplicity."

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