Constant misreads and frequent downtime on the programmable-ride chairs prompted Paramount's Great America to switch from potentiometers to Micropulse linear-position transducers from Balluff Inc., Florence, Ky.

These noncontact, industrially hardened transducers provide continuous, absolute position and velocity feedback to three hydraulic cylinders, each with an 18-in. "throw" range. This allows for simultaneous up-and-down, side-to-side, and front-to-back motions that precisely match movie-film action.

Potentiometers have a high failure rate, says Jose Rios, the theme park's maintenance technician. As they get dirty or wear down, they send intermittent, faulty signals causing the chairs to get out of sync, slow down, or not move at all.

Now, each Micropulse-equipped chair hooks to the theater's programming source, an Allen-Bradley PLC. A motion-based interface card connected to the main CPU controls chair motions, and opens and closes the theater doors through a closed-loop, self-regulating feedback circuit.

A projectionist controls the action from a pulpit located above and behind the 40-seat theater. The system signals the chairs precisely in sync with the actions on the screen and sends the data to a separate computer screen specifically designed for reviewing chair performance and overseeing maintenance.

An auto-tuning feature lets the transducers compensate for temperature fluctuations. This allows Balluff's transducers to operate consistently over temperatures ranging from -40 to 185°F. Improved waveguide technology results in a temperature coefficient of just 20 ppm/C°, about one-half that of other magnetostrictive devices, say company officials.

In the Micropulse, the magnetostrictive element is an extremely small diameter Ni-Fe alloy tube that sits inside a protective outer tube. This so-called "waveguide" runs the length of the transducer. To measure position, a circuit in the housing pulses a current on a conductor wire which has been threaded coaxially through the waveguide. When energized, a rotating electromagnetic field surrounds the waveguide. Simultaneously, lines of force from a magnetic field in the marker ring or block, which has been attached externally to the moving member of the user's equipment, come to focus on the waveguide. These two fields generate a magnetostrictive strain wave just below the magnets, which ripples back down the waveguide to a receiver in the transducer head. This mechanical pulse is then converted into an electrical signal. A high-speed clock or an integrator measures the time between the current pulse's beginning and the torsional wave's arrival. Because torsion pulse velocity is known from material properties, and time has been determined by the clock or integrator, distance is decided.

Basically, a sonic (or ultrasonic) delay line has been created which shows the magnet's position to 2.5 mm resolution, depending on the output signal type and stroke length.

The unit is available in stroke lengths to 142 in., and provides hysteresis of just 0.00003 in. and linearity of +/-0.02%. ---

Magnetostrictive transducers operate by sending a pulse down a wave guide then measuring the elapsed time until the reflected pulse returns. No mechanical contact is required, only the field from the position magnet.