GB1561080A - Dual output force motor - Google Patents

Description

(54) DUAL OUTPUT FORCE MOTOR (71), We, THE FOXBORO COMPANY, a Corporation organised and existing under the laws of the Commonwealth of Massachusetts, United States of America, of Neponset Avenue, Foxboro, Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to electric force motors. More particularly, this invention relates to a compact force motor of the moving coil type capable of providing two independently controllable output forces.

Force motors which convert an electrical input signal into an output force have been known in the art for many years. Motors of many varied designs have been used, but one type that has been used in a wide variety of applications is the so-called voice coil or moving coil motor. It is well established that when current is applied to a coil positioned in a magnetic field with a segment of the coil perpendicular to the field, a force is exerted on the coil segment that is perpendicular both to the magnetic field and to the flow of current. The direction of this force is dependent on the direction of current flow and the direction of magnetic field. For a magnetic field of given density, the force produced is directly proportional to the applied current.

Because of this linear relationship, moving coil force motors have been widely used as electro-mechanical transducers in instruments that measure force or pressure, for example differential pressure transmitters. Also such motors have been used as actuators to precisely control the position of an output member.

Typically, such motors employ a single coil as the force actuator, and thus applications which require processing of two or more independent output forces necessitate the use of multiple force motor assemblies, with an accompanying increase in cost and bulkiness. Consequently, prior art force motors having single actuators have not been wholly satisfactory in all applications.

According to the present invention, an electrical force motor comprises an outer structure of magnetic material having a recess opening out laterally through opposite ends of said structure; an inner structure of magnetic material positioned within said recess of said outer structure and spaced therefrom; pole piece means attached to opposite ends of at least one of said structures establishing air gaps at the respective ends; permanent magnet means forming at least part of one of said structures for producing flux which serially passes through said outer and inner structures through each of said air gaps; and first and second coils positioned within said air gaps respectively and independently movable therein, said coils being actuated by independent electrical signals to develop corresponding noninteractive output forces.

Thus, the invention provides an electrical force motor having two movable and independently actuable coils energized by a single magnet circuit. Such a device is of simple, compact structure and may be advantageously used with any system requiring dual actuation or force measurement where limitations of space and economy of manufacture are of paramount importance. The air gaps may be formed by mounting a cylindrical core of magnetically permeable material within the magnet and by attaching pole pieces to opposite ends of the magnet. This configuration provides a magnetic circuit serially connecting the magnet and the core through each of the air gaps and thus through the operative segments of the coils.

Each coil receives an independent input voltage which generates a current in the coil.

This current develops a corresponding force that independently moves each coil along the axis of the motor assembly in a direction determined by the polarity of the input signal.

The forces developed by the moving coils may be transmitted externally by any suitable means, for example, by an output member transversely coupled to the coils.

The overall configuration of the present invention provides reliable, accurate measurement/actuation of two independent forces in one simple, compact, co-axially symmetric assembly.

Other aspects and advantages of this invention will be pointed out in, or apparent from the following written description considered with the accompanying drawings, in which: Figure 1 is a perspective view, partially cut away, of a force motor embodying one form of the present invention; Figure 2 is a partial schematic representation of the motor of Figure 1 exaggerated to show details of the magnetic field distribution and the independent actuation of both coils; Figure 3 is a graph showing the demagnetization characteristics of the magnetic circuit used in the preferred embodiment.

Referring now to the drawings, an electrical force motor 10 is arranged as a dual actuator capable of receiving independent electrical input signals and converting these to corresponding forces in respective coils 11, 12. The motor shown in this embodiment has been arranged as a transducer for electromechanically controlling the position of multiple pen assemblies in an industrial process control recorder. This transducer actuates a clutching mechanism in the motor driven recording instrument so that the position of two pens may be independently and precisely controlled by one assembly.

The motor 10 includes a tubular permanent magnet 13, forming an outer structure, axiallymagnetized such that its north and south poles are located at opposite ends of the magnet.

Within the tubular structure of the magnet, a cylindrical core 15, forming an inner structure, of magnetically permeable material is concentrically attached such that the core is spaced from the magnet and extends outwardly beyond the ends of the magnet. The core is formed with a central bore 18 whose function will be discussed subsequently. The magnet assembly is completed by mounting cylindrical pole pieces 16A, 1 6B on the ends of the magnet such that circular, co-axial air gaps 17A, 17B are formed at opposite ends of the magnet. Thus a magnetic circuit is is formed whose lines of flux serially connect the permanent magnet and the inner core through each of the air gaps (see Figure 2).

The coils 11, 12 are wound on circular bobbins 21, 22 which are axially fastened to elongate tubular sleeves 23, 33 that are inserted within the bore 18 so as to centrally locate the coils within each air gap. These sleeves provide a low-friction sliding fit allowing bi-directional, axial movement of the coils when energized.

In addition, the sleeves serve as guides preventing the bobbins from cocking and possibly contacting either the pole pieces or the central core. Mounted within the sleeves and extending axially oeyona tne ooDom/coll assemblies are rods 24, 34 which externally transmit the forces generated by the current in the respective coils.

Each coil is electrically connected to an input signal, typically either a positive or negative voltage. Depending on the polarity of the voltage impressed across the coil, a d-c current flows through the windings of the coil which, in the presence of the magnetic field, develops a force in the coil causing it to move in a prescribed direction along the axis of the motor assembly (i.e., inwards or outwards). A flexible dust cover 28 mounted on opposite ends of the motor prevents contaminants from entering the motor without limiting the free motion of the bobbins.

The unique compact configuration of the present invention wherein two coils may be independently energized to develop corresponding output forces by a single magnetic circuit affords overall volume reduction. However in order to keep the size of the motor as small as possible and yet retain the high output accuracy required in most industrial applications, the interaction between each coil must be effectivelv miniminp.d Such inaccllr. acies typically result either from the steadystate magnetic tleld set up by tne current passing through the coil which could, dependent on polarity, oppose or enhance the per mnnp.nt m:}netie field nr from n hnr.k emf caused by the movement of one coil.

In the present force motor, the magnet 13 is formed from Alnico 8, which exhibits a relatively high coercive force (i.e., approximately 1600 oersteds maximum). The operating point for the magnetic circuit of the preferred embodiment (determined by the intersection of the load line 50 with the demagnetization curve 52 for Alnico 8) is about 1000 oersteds (see Figure 3). Under normal operating conditions the maximum input current available to each coil is 50 milliamperes. Assuming as a worst case condition that maximum steady-state current is applied to one coil, that coil is capable of generating only about 20 oersteds. This approximate 3% change in coercive force produces a parallel shift in the load line that, dependent on polarity, may be positive or negative as indicated generally by reference numerals 55A, 55B. Such a small change in the operating point has a negligible effect on the proper functioning of the other coil, thereby maintaining high overall output accuracy of the motor.

Although a sPecific embodiment of the Dre sent invent sent invention has been set forth above, it is to be empnaslzea tnat tills Is tor tne purpose or illustration and should not be considered limiting.

It will be obvious to those skilled in the art that modifications may be made without departing - - from the scope of the invention as set forth in the accompanying claims.

WHAT WE CLAIM IS: 1. An electrical force motor comprising: an outer structure of magnetic material having a recess opening out laterally through opposite ends of said structure; an inner structure of magnetic material positioned within said recess of said outer structure and spaced therefrom; pole piece means forming at least part of one of said structures for producing flux which serially passes through said outer and inner structures through each of said air gaps; and first and second coils positioned within said air gaps respectively and independently movable therein, said coils being actuated by independent electrical signals to develop corresponding noninteractive output forces.

2. Apparatus as claimed in Claim 1, wherein said outer structure comprises an axially-magnetized, elongate tubular permanent magnet exhibiting relatively high coercive force characteristics; said inner structure comprises an elongate cylinder of magnetically permeable material concentrically positioned within said tubular magnet; said pole piece means defining circular, coaxial air gaps.

3. Apparatus as claimed in Claim 2, wherein said permanent magnet is formed from Alnico 8.

4. Apparatus as claimed in Claim 2 or Claim 3, further comprising first and second circular bobbins on which respective coils are wound; said bobbins including respective tubular sleeves axially fastened to one end thereof; said cylinder having a bore along its longitudinal axis in which said sleeves are inserted to centrally locate said coils within said air gaps; said sleeves providing a low-friction sliding fit during actuation of said coils guiding the movement thereof and preventing said bobbins from contacting either said pole piece means or said cylinder.

5. An electrical force motor substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. the accompanying claims. WHAT WE CLAIM IS:

1. An electrical force motor comprising: an outer structure of magnetic material having a recess opening out laterally through opposite ends of said structure; an inner structure of magnetic material positioned within said recess of said outer structure and spaced therefrom; pole piece means forming at least part of one of said structures for producing flux which serially passes through said outer and inner structures through each of said air gaps; and first and second coils positioned within said air gaps respectively and independently movable therein, said coils being actuated by independent electrical signals to develop corresponding noninteractive output forces.

2. Apparatus as claimed in Claim 1, wherein said outer structure comprises an axially-magnetized, elongate tubular permanent magnet exhibiting relatively high coercive force characteristics; said inner structure comprises an elongate cylinder of magnetically permeable material concentrically positioned within said tubular magnet; said pole piece means defining circular, coaxial air gaps.

3. Apparatus as claimed in Claim 2, wherein said permanent magnet is formed from Alnico 8.

4. Apparatus as claimed in Claim 2 or Claim 3, further comprising first and second circular bobbins on which respective coils are wound; said bobbins including respective tubular sleeves axially fastened to one end thereof; said cylinder having a bore along its longitudinal axis in which said sleeves are inserted to centrally locate said coils within said air gaps; said sleeves providing a low-friction sliding fit during actuation of said coils guiding the movement thereof and preventing said bobbins from contacting either said pole piece means or said cylinder.

5. An electrical force motor substantially as hereinbefore described with reference to the accompanying drawings.