Inductive position sensing in linear actuators

#The invention relates generally to electromagnetic actuators, and more specifically to permanent magnet actuators, e.g. linear actuators.

#An earlier filed patent application, U.S. Pat. Pub. 2014/0312716, the contents of which are incorporated herein by reference, describes a linear actuator. In its simplest form, the linear actuator includes a linear array of coils wound around a core of magnetically permeable material and an actuator assembly encircling the array of coils. The actuator assembly, which is made up of a linear array of ring magnets made of a permanent magnetic material, is arranged to move back and forth along the length of the array of coils under the control of signals that are applied to the coils. By applying appropriate signals to coils within the array, the position and movement of the actuator assembly can be controlled.

#For such a linear actuator, it is useful to be able to accurately determine the location of the actuator assembly relative to the array of coils. The controller can use this positional information to determine which coils in the array to excite so as to optimally drive the movement of the actuator assembly and to accurately reposition the actuator assembly where desired. Typically, the information is determined by using an array of sensors that are mounted on the actuator. Two examples of such sensors that are mentioned in the earlier filed application are a linear potentiometer and a linear encoder.

#FIG. 1 illustrates a linear actuator.

#FIG. 2 illustrates in greater detail by way of a cross-section the arrangement of coils and magnets in the linear actuator of FIG. 1.

#FIG. 3 is an example of a key functional relationship between the inductance of a single coil and its position along the linear actuator shown in FIG. 1.

#Note that like components and features in the drawings may be identified by the same reference numbers.

#The embodiments described herein relate to methods of and systems for determining the position of a magnet or magnet array using a set of electrical coils and a volume of magnetically permeable material (e.g. iron or steel or Mu-metal).

#The specific example used to illustrate the techniques described herein is a linear actuator that includes a set of electrical coils arranged linearly along a cylindrical iron core with a magnet or magnet array constrained to move parallel to and along the core and coils, as illustrated in FIG. 1 (Further examples are described more fully in U.S. Pat. Pub. 2014/0312716 that has been incorporated herein by reference.)