Sensing Taut Cable Orientation

Instrumented fairleads sense tension, direction, or both on reeled cables.  Sensing instrumentation is core to active fairlead control.  Active control goes beyond passive sensing to control direction and tension on a reeled cable.  Sub-elements of tension-sensing, tension-setting, direction-sensing and direction-setting are applied in many applications from weaving, sailing, tethering and rappelling to gravity offload. A common approximation of reduced gravity for earth-testing of planetary condition is to uplift a fraction of an object’s weight using an actively controlled fairlead to impose a constant, upwardly vertical pull.  We are developing a gravity offloader to emulate lunar gravity for testing robot mobility.  One technical development within the project is determination of cable orientation, which is discussed here.

Figure 1: Active fairlead for robot rappelling.


Since a taut cable approximates a line, just two spatial angles determine its unit vector.  Figure 1 illustrates an active fairlead for robot rappelling that senses cable angle by measuring two angles.  The upside is simplicity.  Downsides are poor resolution of angular measurement and side-forces imparted to the cable.

Figure 2: Delta arm used to determine cable angle.


The device described here (Figure 2) measures three angles of a parallel-link delta mechanism for determining cable angle. This is akin to (and has advantages of) Stewart platforms or Delta  robots that are commonly used in automation.  The novelty of the development is that there is no actuation, only angular sensing, and that the mechanism is back-driven by side-motion of a taut cable that passes through it.    This minimizes side-force imparted to the cable so that the devices is not corrupting the angle that it is measuring.  The big advantage is that it improves estimation of the cable’s unit vector by measuring three magnified rotations within the parallel mechanism. Angles are read by 2500 count incremental encoders.  The forward kinematic equation of the mechanism utilizes these angles to to determine the unit vector of the taut cable.

When actuated by motors and used as a robot as in common application, the Delta mechanism displaces from base A to tip B to perform useful work.  As applied here, no motors or actuators are involved.  The taut cable origin is A, and motion of the cable backdrives the mechanism tip to location B. Two points, A and B, determine line… in this case line AB, which determines the line of the taut cable.

This works as intended.  First experimentation is illustrated in the appended video.

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