What is in this article?:
The compliance of compressed air has kept it from gaining widespread use in motion bases for ride simulators and virtual reality (VR) attractions. But at least one company has turned a liability into an asset by developing a pneumatically powered motion base that offers impressive performance without an exorbitant price.
The secret to accomplishing all of this lies in the software, which incorporates fuzzy logic. The fuzzy logic tackles another universal challenge in designing motion control systems — finding the optimum gain. If gain is too low, actuators will operate slowly, and precious time will be spent waiting for them to move the load to where it should be. On the other hand, if gain is too high, the actuators shoot right past the endpoint and probably begin oscillating to either side of the endpoint without ever stopping there. The software we developed uses a fuzzy-logic algorithm that essentially changes the gain of the system as needed.
A truly unique aspect of the motion control signal is that it is embedded within the video signal. It works the same way closed captioning is embedded into a video signal for broadcast or cable television. This allows conducting an interactive virtual reality experience where the participant determines what he or she will see, hear, and feel. Or you can record an experience on a standard VCR through a pair of RCA jacks provided on the side of the motion base. When you play back the tape and connect it to a sound system and video monitor, you’ll not only see and hear what was recorded, you’ll also recreate the six-axis motion program through the motion base.
A 1½-hp compressor provides all the mechanical energy needed by the Cyber Air Base. Motion is introduced through three pairs of pneumatic cylinders using non-lubricated, dry air. Each 2½-in. bore cylinder is attached to a stationary base platform at the bottom and articulated motion platform at the top through stud-mounted ball joints. The configuration of the motion base is patented, but uses standard off-the-shelf components wherever possible. The angle at which the ball joints are mounted, which is also patented, offers full freedom of motion for all six axes through standard 90° ball joints. This arrangement handles load to 300 lb. We were unable to find cylinder manufacturers that offered a tapped hole in the cap end to accommodate the ball joint. Consequently, the cylinders are custom made. Another special feature of the cylinders is that the end caps are secured by retaining rings instead of tie rods or other methods. This allows quick disassembly to replace seals.
Sizing the cylinder bore turned out to be a real compromise, because you normally specify a large bore to move a heavy load. On the other hand, a smaller bore keeps the volume of air displaced manageable. Cylinders with a 3-in. bore have also been used successfully to handle loads to 600 lb. Plans for the future include using 5-in. bore cylinders with 1-in. rod end bearings to handle loads up to a ton.
Instrumentation for the motion base itself is rather simple: a linear potentiometer mounted alongside each cylinder and pressure transducers to monitor supply pressure. Electronic operator-input devices for the Cyber Air Base are more extensive and include an absolute rotary encoder for steering, linear potentiometers for the accelerator and brake pedals, and a switch for forward or reverse. The joystick is a key component, because its electronic signal is fed simultaneously to the visual program and to the motion program. Another input device is a kind of three-dimensional electronic compass. As the user turns his or her head left, right, up, down, or tilts his or her head left or right, the video image reacts accordingly.
A potential limitation is that this electronic compass cannot distinguish between when the user turns his or her head up or down, or if this motion was generated by the motion base. For example, if the operator looks up, he or she should see the ceiling of the vehicle. However, if the motion base tilts upward, the user should see a view out the front windshield with the front hood of the vehicle pointing upward. To allow the software for video to distinguish between motion generated by the user and that generated by the Cyber Air Base, a similar electronic compass is provided for the motion base. The software then compares the two inputs and generates a video image that accounts for both motions.
Potential industrial applications
With the right marriage of valves and cylinders, ¼-in. resolution can be obtained with varying loads from –30 to 300 lb. This enables pneumatic equipment to work in many different orientations relative to gravity or friction points, not just pick-and-place applications. A quarter-inch is too sloppy for most industrial applications, but is well beyond the realm of what humans can perceive in a dynamic environment. On the other hand, many industrial applications could benefit from a motion system that can react to dynamic load changes while maintaining ¼-in. repeatability — especially when the system is relatively simple, economical, and uses clean, reliable pneumatics. Repeatability should improve, but because 1⁄4in. is more than adequate for VR applications, we have not made much effort to improve repeatability beyond ¼ in.
Embedding the motion-control signal in a video signal could have direct application in the industrial world. For example, say someone uses a manipulator to position a part in an assembly operation. The software then assumes a teach function: you record the moves, edit them, and play them back so the manipulator duplicates all the moves. Signals from the joysticks could be recorded, so the manipulator could duplicate all the movements by playing back the tape. Editing errors out of the tape, optimizing movements, and playing back the tape at a higher speed would cause the manipulator to perform a task over and over, limited only by the speed of the machine.
Signals for teaching can originate from separate joysticks of from positioning the machinery by hand. Imagine using a VCR to record motions visually while you record mechanical motions. Later, you can play back the tape to watch the machine simultaneously performing the same operation. This is a simple and easy-to-understand teaching method for both people and robots.
Brad Engstrand was chairman of Virtogo Inc., Chicago, which manufactured the Cyber Air Base, when this article was opriginally published. He is now president of Motion Controls LLC, Hartford, Wis., a manufacturer of pneumatic cylinders and the InSight positioning system using QVLA sensors.