Spaceborne gimbal systems are typically bulky with large footprints. Such a gimbal system may consist of a forked elevation stage rotating on top of the azimuth motor, and occupy a large volume. Mounting flexibility of such a system may be limited.

Figure 1. New Gimbal consists of two DC brushless motors.

A low-mass, small-volume gimbal unit was developed that consists of two DC brushless motors, each rotating the payload about one of two orthogonal axes (see Figure 1). An elevation axis motor is located beside the azimuth axis motor, which reduces the height of the gimbal. An adaptor secured to the elevation axis supports the payload.

The bracket that holds the elevation stage is clamped at one end to the azimuth stage motor shaft. At the other end, this bracket is supported by a precision ball bearing, the outer race of which is pressed into the bracket, and the inner race of which rides on a short stationary shaft. The stationary shaft has an integral flange that is bolted to the azimuth stage housing. Thus, as shown in Figure 2(a), the bracket is supported at both ends by bearings: at the motor shaft end it is supported by the shaft end ball bearing in the azimuth motor, and at the other end it is supported by the external precision ball bearing.

The payload adaptor is similarly coupled to the elevation motor through an off-the-shelf flexure coupling to the motor shaft on one side and a precision ball bearing through a stationary shaft on the other [see Figure 2(b)]. As in the case of the elevation stage bracket, the outer race of the bearing is secured by a press-fit into the payload adaptor.

Figure 2. Design Details: (a) elevation stage support cutaway and (b) elevation stage cutaway detail.

Although the design is suitable for a variety of motors, brushless DC motors were used with integral encoders for their smoothness of operation. These motors lack the cogging torque of a stepper motor, and the friction between brushes and a commutator found in a brushed DC motor. Both of these mechanisms are sources of torque variation that can compromise pointing accuracy in a gimbal. For both azimuth and elevation stages, DC brushless motors were used with encased encoders. This gimbal is a low-mass unit suitable for a small payload such as a small lasercom terminal or a small imager. The prototype unit has outer dimensions of approximately 127 × 127 × 127 mm. The gimbal may, however, be scaled to accommodate heavier payloads if desired.

The gimbal units may be used to manipulate various payloads such as remote sensing instruments, laser communications, and radar for space applications.

This work was done by Vachik Garkanian, Joseph M. Kovalik, and Martin W. Regehr of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47824

Motion Control & Automation Technology Magazine

This article first appeared in the February, 2015 issue of Motion Control & Automation Technology Magazine.

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