The development of a controller for differential pressure calibrations

Pressure transducers are used to provide flight performance and tunnel conditions information during wind tunnel testing. All pressure transducers used during testing must have calibrations traceable to the National Institute of Standards and Technology (NIST); therefore, calibration is required on a scheduled basis for every pressure transducer used in the process of testing flight vehicles at Arnold Engineering Development Center (AEDC). To meet stringent data accuracy requirements, most transducers are calibrated on a daily basis: a time consuming process for the three thousand pressure transducers in use at the facility. These transducers are usually differential devices because they are easier to calibrate than absolute pressure transducers.

Initially, pressure calibration devices were very accurate portable air dead weight testers that had to be transported to the transducers under test. To eliminate this time consuming requirement, transducers were connected pneumatically to a differential pressure controller over lengthy pressure lines. Unfortunately, the pressure controller required over twenty minutes to set a stable output pressure due to the large volume of the pressure lines. The development of a differential pressure controller which substantially reduces the time needed to set calibration pressures, but maintains high accuracy and stability, is presented in this thesis.

The pressure controller accepts pressure commands in the range of +/- 15 pounds per square inch differential (PSID). A flapper-nozzle pressure servo valve provides stability to approximately +/-0.0015 PSID and can charge a 20 cubic inch volume to 12 +/- 0.01 PSID in approximately five seconds. The output pressure is controlled to better than +/- 0.002 PSID by using a differential pressure working standard to provide the calibration manifold feedback pressure. The pressure controller uses a proportional plus integral control algorithm that is satisfactory for volumes from five cubic inches to six thousand cubic inches.

The device has features and display options that are activated by a single switch making operation relatively simple. Routines to display the leak rate, zero the feedback transducer and calibrate the feedback transducer are included in the embedded program residing in memory on a STD BUS CPU card. All calculated control and display constants are stored in battery-backed memory. The controller is composed of two chassis connected by a multi-conductor cable. Separation of the electrical control chassis and the pneumatic control chassis allow the pressure source to be located near the pressure calibration manifold while the commands are issued by an operator in the wind tunnel control room. The pressure controller was evaluated in a laboratory setting, but it has been used on a daily basis in a transonic wind tunnel for over a year without failure.