Cable-driven gimbals boost pointing accuracy

In the world of directional pointing and tracking systems, many low-end systems are found for under $5000 with relatively low pointing accuracy for long-range applications.

Oct 1st, 2003
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In the world of directional pointing and tracking systems, many low-end systems are found for under $5000 with relatively low pointing accuracy for long-range applications. Various high-precision systems also exist for $50,000 and more, usually designed by systems integrators for specific camera or payload devices. But there is not much in between, according to engineers at Sagebrush Technology (Albuquerque, NM) who have developed a high-precision plug-and-play gimbal technology for the midprice range that is being customized to serve a broad range of payloads, such as cameras, lasers, mirrors, and biopsy needles. They have seen an increase in interest among users who have worked with an inexpensive system good to a half or fourth of a degree but who now require a higher accuracy—from a tenth of a degree to tens or hundreds of microradians.

Cable vs. worm drive

Long-range precision is obtained by replacing the traditional worm-gear drive with servo-controlled cable-driven gimbals. Worm-drive gearboxes for 30- to 50-lb payloads generally have error specifications on the order of 3 to 6 arcmin.

The precision gimbals system developed at Sagebrush Technology is based upon surface-wound cables that use a spring mechanism to produce a preloaded tension. Under load, the cable can't relax until the applied load exceeds the designed-in spring tension. Deflections of cable-based gimbals under load are on the order of 10-5 rad/in.-lb of applied torque for the smaller drives (several inches in diameter) to 10-8 rad/in.-lb for larger drives (1 ft and larger diameters), several orders of magnitude less than either the backlash of traditional gearboxes or the compliance of belt-drive systems.

The cable-driven gimbal (right) forms the inner workings of the minimally invasive stereotactic breast-biopsy system (left), which allows clinicians to precisely target a breast lesion from an optimal angle.
Click here to enlarge image

The degree of real-life precision also depends upon the control system. In the simplest and least expensive version, a stepper-motor controller counts the motor steps and provides reasonable accuracy for many applications due to the lack of backlash error. An intermediate solution derives feedback from an encoder on the motor shaft, while the highest precision comes from placing the measurement instrument directly on the output shaft.

In all cases, the feedback signal must be processed through a computer system that essentially tunes the gimbal to the payload. The gimbals system is tuned to the weight, size, and inertia of the payload it will be moving, using an iterative process in which a swept-sine excitation is applied to the payload so that filters can be applied and gains adjusted based on a review of the resulting bode plot.

"The first part of tuning and balancing the gimbals is to work with the customer to optimize the system in terms of the inertial field," said David McCreery, program manager. "The second part of it is to look at the platform. If we are on a helicopter, we know there are certain vibration modes. And by understanding what those are and working with how the gimbal's mechanical structure reacts, then we can put notch filters into the control loop that will filter out the resonance that is being generated that we simply don't have the ability to control."

A third part of the tuning process involves working with the gimbals to understand the response of internal gains and control loops to real-world conditions, which may involve taking the gimbals out on a boat for a shipboard application and capturing the actual conditions at sea, or flight testing on a helicopter for an airborne application.

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