Economical actuator provides simple, precise motion control

Hamel, MN -When it comes to moving an object from point A to point B, nothing beats pneumatics due to the wide availability of air, low maintenance, and low cost. However, because of its compressibility, air is notoriously difficult to manage. As a consequence, controlling the velocity, deceleration, and position of the carrier can be difficult and costly. When precise motion or mid-stroke positioning is required, design engineers in the past have employed proportional valves, which use internal spool position feedback to set the orifice size proportional to an applied control voltage.

Tol-O-Matic has developed what it says is a simpler, lower-cost alternative to traditional servopneumatics. Since the company considers the design unique, they call it a "precision air device," rather than a servopneumatic device, says Tony Braga, product manager.

Instead of using proportional valves, which regulate the air pressure at both ends of the cylinder, engineers directly control the position of the Tol-O-Matic carrier with a proportional brake. When valves are opened or closed at either end of the stroke, air enters the piston chamber, applying a pressure force that causes the piston to accelerate. A continuously controllable magnetic particle brake provides a smooth resistance torque that finely controls the movement of the carrier when a magnetic flux is applied.

A patented control system regulates the current supplied to the brake. The closed-loop system controls the brake current with four gain settings-proportional, velocity, integral, and deceleration torque-to govern the position and velocity of the carrier. To ensure the proper positional repeatability and velocity control, these four parameters can be adjusted for a specific motion profile and load.

Based on internal test results, Tol-O-Matic engineers say that their technology can control the position of the carrier with a repeatability of plus or minus 0.01 inch at a stroke length of up to 16 ft. Maximum stroke length for comparable repeatability with a proportional valve system is less than 6 ft. "One problem with traditional proportional servopneumatics is that it is difficult to predict the motion of the carrier. That's because of the numerous variables, including cylinder friction, changing loads, cylinder orientation, and cylinder leakage, which influence how the carrier/load will react to the amount of air entering the piston chamber," says Keith Hochhalter, director of product development. "With our design we simply switch the valves in the on or off position, allow the air to fill the pressurized piston chamber, and control the velocity and position of the carrier using the brake."

In the process of developing the new design, the goal of Tol-O-Matic engineers was to cost-effectively fill a performance gap between end-of-stroke pneumatics and electric linear motion systems. To achieve that goal, the engineering team investigated a number of alternatives to proportional-valve braking technology. "We played around with several concepts to control the device. The challenge was to keep it cost effective against the continuously dropping prices of electric systems," says Hochhalter.

At first, the design team tried using an electric, solenoid brake pad system, but they could not obtain enough braking force to stop the carrier effectively with heavy loads. Brake pads were also unappealing, because they wear out and need to be replaced. The idea of using a magnetically responsive brake came from an ad in a trade magazine. "With a magnetic particle brake, you get a lot of torque in a small package" recalls Hochhalter. At 3.5x3.5x1.74 inches in size, the brake achieves a maximum torque of 200 inch-lbs at 2.5A.

Initially, the design team incorporated the brake into a rodless cable cylinder system, which involved attaching steel cables to the carrier that ride on pulleys at each end of the stroke. However, the design was not reliable because at high decelerations and with heavy loads, the cable would stretch and at times slip on the pulleys, compromising position repeability.

By designing the brake into a rodless piston cylinder configuration, the team was able to optimize performance. "Belts are commonly used with electric motors for repeatable positioning to 0.005 inches, and they provide a reliable and cost-effective link between the carrier and rotary encoder," says Hochhalter. "The advantage the belt has in this design is that unlike in an electric system, it sees very little loading during the acceleration, slew, and the high-speed portion of the deceleration. The control system converts the rotational velocity data provided by the encoder to linear velocity, which in turn is integrated to give the position of the carrier."

Using a rotary encoder also allowed engineers to dramatically reduce cost. In traditional servopneumatic systems, a linear encoder is required to determine the position of the carrier along the entire length of the stroke. This technique can be costly, particularly at long strokes.

Tol-O-Matic's linear motion system can be used for horizontal and vertical positioning and may offer a unique and effective alternative to traditional servopneumatic systems. In fact, engineers at a major automaker have evaluated the technology and successfully integrated it into a welding application.

Additional details

PrecisionAire...Contact Tony Braga, Tol-O-Matic Inc., 3800 Country Rd. 116, Hamel, MN 55340-9360; Tel: (612) 478-8000; Fax: (612) 478-4309; E-mail: [email protected] .

Rheonetic(TM) brake...Contact Eddie Houchin, Lord Corp., Box 8012, Cary, NC 27512; Tel (919) 469-2500 ext. 2118; Fax: (919) 469-5991; E-mail: [email protected].