Design of a captive trajectory system

As part of a complete wind tunnel design project a control system for a Captive Trajectory System (CTS) was designed to meet certain specifications. The control system provides controlled movement of the model support in six independent axes. Since the wind tunnel facility was to be in a foreign country, off-the-shelf equipment, rather than custom design circuits, was selected to minimize problems with fabrication, installation and system check-out.

During the design process both analog and digital techniques were investigated. The final design uses a stepping motor controller driving an amplifier/motor combination that emulates the characteristics of a stepping motor. A true stepping motor with the required speed vs. torque characteristics for all axes could not be found. The stepping motor controller receives information from a computer regarding the distance to be moved, velocity and acceleration rate. An uncertainty analysis was performed on the position measurement to determine if it met the required specifications. Even though the uncertainty for the position measurement using a potentiometer failed to meet all specifications it was selected for economic reasons.

The control software for the computer was designed, coded and checked out on a mockup of the CTS that mimics system characteristics. As of this writing, the CTS is still under construction so no tests of the final configuration have been run. However, slightly modified versions of the CTS hardware and software have been used in successful tests of other model supports in the wind tunnel.