Apparatus for measuring the position of a movable member

[0002] Not Applicable.

[0003] Not Applicable.

[0004] Not Applicable

[0005] The present invention relates to magnetic position encoders, and more particularly to magnetic position encoders utilizing mechanically shaped, magnetic field generating disks to optimize the resulting magnetic field pattern.

[0006] Many applications require instantaneous information regarding the linear or angular position of one element relative to another. For example, it is often desirable to measure the position of a rotating shaft with respect to a fixed reference, such as a stator holding the shaft bearings. Devices that measure such an angular position are well known in the art and include resolvers, optical encoders, magnetic encoders, rotary potentiometers, and others. Such devices, known for example as position encoders or position sensing devices, all have various economic and performance trade-offs. In particular, the least expensive high resolution position sensing devices are typically incremental as opposed to absolute. An incremental position sensing device is characterized by the fact that it only produces positional information regarding a movable member in the form of a quantized change from the previous position. The resolution of the device is defined by the smallest quantum of positional change that the sensing device can indicate (i.e., an angular increment for rotational sensing device or lineal increment for a linear sensing device). Such devices typically incorporate associated electronics that require external power. Once power is applied to the associated electronics of an incremental position sensing device, the device can begin producing incremental position information. By utilizing appropriate interface electronics (e.g., an accumulator), such sensors can keep track of how far the movable member has traveled since external power has been applied, but the absolute position is unknown, because the device has no information regarding the position of the movable member at the time the power was applied.

[0007] One type of prior art absolute position sensing device operates by producing a magnetic field that is fixed relative to the movable member, and sensing the magnetic field at a fixed reference position (relative to the movable member) as the movable member travels through its range of motion. In one such absolute position sensing device for sensing the angular position of a rotatable shaft, the magnetic field is generated by a permanently magnetized disk, fixedly mounted to the shaft. The fixed sensor produces a discrete output at the maxima of the detected magnetic field. Ideally, the field variation detected by the fixed sensor is sinusoidal, with the maxima occurring at the poles of the magnetized disk. One disadvantage to such a sensing device is that the sinusoidal field detected at the fixed sensor tends to be distorted, i.e., the detected waveform includes higher order harmonics, in addition to the desired fumdamental sinusoid. This distortion introduces an error in the estimate of position.

[0008] It is an object of this invention to provide a position encoder that substantially overcomes or reduces the aforementioned disadvantages while providing other advantages which will be evident hereinafter.

[0009] The present invention is an apparatus for indicating the instantaneous position of a movable member, movable along a range of motion with respect to a reference position. The apparatus includes a magnetic body that is fixedly attached to the movable member. The body has a contoured surface and produces a magnetic field. The apparatus further includes at least two magnetic field sensors. Each of the sensors is disposed adjacent to the contoured surface of the magnetic body, fixed with respect to the reference position, and is positioned apart from the other sensors, so as to form a non-uniform gap between the contoured surface and each of the magnetic field sensors. Each of the sensors produces a position dependent signal corresponding to the value of the magnetic field. The apparatus further includes a decoding circuit electrically coupled to each of the magnetic field sensors so as to receive each of the position dependent signals. The decoding circuit produces a position dependent signal that is a predetermined function of the magnetic field sensor position dependent signals.

[0010] In one preferred embodiment, the magnetic field sensors include at least two sets of two magnetic field sensors, so as to provide electrically redundant position dependent signals.

[0011] In yet another embodiment, the gap is tailored to vary as a periodic non-uniform function between g_maxand g_minand having an inverse sinusoidal component.

[0012] In another embodiment, the non-uniform gap is tailored so as to produce a sinusoidal variation with the position of the movable member with respect to the reference point.

[0013] In one embodiment, the magnetic body is selected from the group consisting of a permanent magnet, an electromagnet, a high mu material, a combination thereof, or a combination thereof with non-magnetic materials.

[0014] In another embodiment, the magnetic body is rotatable and includes N magnetic poles, where N is preferably an integer evenly divisible by four so that redundant magnetic field sensors can be incorporated.

[0015] In another embodiment, the movable member and the magnetic body are combined so as to include a single magnetic member.

[0016] In another embodiment, each of the magnetic field sensors further includes at least one adjustment screw disposed substantially adjacent to the magnetic field sensor so as to interact with the magnetic field. The position dependent signal amplitude varies as a predetermined funtion of a relative distance from the magnetic field sensor to the adjustment screw. The adjustment screw includes a high magnetic permeability material.

[0017] In another embodiment, the magnetic field sensors include linear Hall effect sensors.

[0018] In another embodiment, the movable member includes a shaft rotatable about a rotation axis, and a disk having an outer periphery defined by a non-uniform radius. The disc is fixedly attached to the shaft at the disc center such that the disk rotates within a plane substantially normal to the rotation axis. The magnetic body is fixedly attached to the disk, and the magnetic field sensors are disposed about the outer periphery at substantially equal radii with respect to the rotational axis.

[0019] In another embodiment, the apparatus further includes two sets of three magnetic field sensors disposed at angular intervals substantially equal to 120 degrees.

[0020] In another embodiment, the movable member includes an elongated object movable along a linear axis. The elongated object has at least one exterior surface substantially parallel to the linear axis, and the magnetic field sensors are disposed along the exterior surface at substantially equal radial distances with respect to the linear axis.

[0021] The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which:

[0022] FIG. 1A illustrates an end schematic view of one preferred embodiment of an improved angular position encoder;

[0023] FIG. 1B illustrates a side schematic view of the FIG. 1A embodiment;

[0024] FIG. 2 illustrates a schematic view of one preferred embodiment of an improved linear position encoder;

[0025] FIG. 3A illustrates an end schematic view of one preferred embodiment of an improved angular position encoder; and

[0026] FIG. 3B illustrates a side schematic view of the FIG. 3A embodiment.

[0027] The invention is directed to an improved position encoder, also referred to by the Applicant as the “G-coder”. FIGS. 1A and 1B illustrate orthogonal schematic views of one preferred embodiment of an improved position encoder 100 that may be used to measure angular position of an object with respect to a fixed reference position (e.g., rotational angle θ_M=0), including a shaft 102, a shaped disc 104 fixedly attached to the shaft 102, a magnetic field sensor ring 106 concentric with the disc 104 and the shaft 102, and a decoder circuit 107 electrically coupled to the sensor ring 106. The sensor ring 106 is fixedly mounted to the stator housing 108 via ring mounting supports 110. The stator housing 108 supports the bearings 112 that rotatably secure the shaft 102 to the stator housing 108.

[0028] The disc 104 mounted on the shaft 102 includes a magnetic material disc having a predetermined magnetic field characteristic. This field characteristic may include a substantially constant magnetic field, produced for example by permanent magnet or a conductor carrying a direct current. Alternately, the predetermined field characteristic may include a magnetic field having a time varying amplitude that varies in a predetermined manner, e.g., sinusoidal amplitude modulation, as the shaft rotates. In one embodiment, the magnetic material disc 104 is fabricated entirely from one of several permanent magnet materials known to those in the art, or a combination thereof. In alternate embodiments, the magnetic material disc 104 includes a permanent magnet, an electromagnet, a high mu material (i.e., a material having a relatively high magnetic permeability), or non-magnetic materials, or a combination thereof (e.g., for a hub to mount the disc onto the shaft). In another embodiment, the magnetic field originates from an electromagnet, driven synchronously by a sinusoidal source, to produce an amplitude modulated magnetic field. In all of the preferred embodiments, the disk 104 produces a magnetic field that varies spatially with respect to shaft rotation angle θ. In FIG. 1, the magnetic field produced by the disk 104 is radially oriented with respect to the shaft axis of rotation 114. In an alternate embodiment, the shaft 102 and the disc 104 may be combined or integrated to form a single component that produces a magnetic field.

[0029] The magnetic field sensor ring 106 includes a plurality of magnetic field sensors 116 distributed about the shaft 102. The magnetic field sensors 116 linearly measure the value of the magnetic field produced by the magnetic material disc 104. In the illustrative embodiment of FIG. 1, each of the six independent magnetic field sensors 116 measures the radial component of the magnetic field produced by the disc 104, and substantially rejects components of the magnetic field orthogonal to the radial component. In a preferred embodiment, the magnetic field sensors 116 include linear Hall effect type sensors, although alternate embodiments may include other magnetic sensors known to those in the art.

[0030] The rotational nature of the structure shown in FIG. 1 causes the output from a given magnetic field sensor to be periodic as a function of the rotational angle θ_M. Any periodic function can be decomposed into its harmonic terms via a Fourier Series, which is repeated below for reference: $Y_{\mathrm{A1}}\left({\theta }_M\right)=\sum _{n=-\infty }^{+\infty }\left[A_n\cos \left(n{\theta }_M\right)+B_n\sin \left(n{\theta }_M\right)\right]$

[0031] The output of the other magnetic field sensors would ideally be given by Y_A1(θ_M−θ_O) where θ_Ois determined by the angular distance between the magnetic field sensors.

[0032] To illustrate how the embodiment of the invention shown in FIG. 1 measures the shaft mechanical angle, assume there are only two magnetic field sensors on the sensor ring 106 separated by 45 mechanical degrees instead of six magnetic field sensors 116 being distributed about the sensor ring 106 at intervals of 60 mechanical degrees, as shown. Then, given appropriate magnetic material disc magnetization and air gap shape, the outputs of the two sensors are given by:

[0033] Y_A1(θ_M)=Acos(2θ_M)

[0034] Y_A2(θ_M)=Asin(2θ_M)

[0035] The angle θ is easily determined when both the cosine and sine of θ are known. Thus, for a system with two magnetic field sensors, is it possible to uniquely determine 2θ_M, i.e., determine the absolute angular position of the shaft within one half of a mechanical revolution. Circuitry to perform this function within the decoder circuit 107 is well known for interfacing to resolvers, which have the cosine sine output signal format given above, except for being amplitude modulated by a carrier. U.S. Pat. No. 5,162,798, entitled RESOLVER TO DIGITAL CONVERTER, invented by Yundt, discloses an example of such an angle conversion apparatus.

[0036] Note that the sensor output signals, Y_Ax(where x=1, 2, etc.), do not have to be purely sinusoidal. A decoding circuit 107 could be constructed for any a priori sensed magnetic wave shape, as long as the generalized function of the output of all the magnetic field sensors taken together is a single valued function of the position. The pure sinusoidal case is shown in the illustrative embodiment because it is one of the simpler magnetic wave shapes to decode. In another embodiment, the sensor output could be in the form of a triangle wave, e.g., vary the sensed magnetic field linearly from minus B_oto plus B_oover 180 electrical degrees and visa versa over the next 180 electrical degrees.

[0037] If the desired result is a pure sinusoid and the resulting output from the magnetic field sensor does have some distortion, then additional magnetic field sensors can be used to partially cancel that distortion. Consider the case shown in FIG. 1A which includes three magnetic field sensors 116 (H_A1, H_A2and H_A3) spaced at intervals of 120 electrical degrees. (Note that because the disk 104 includes a magnetic pole every 90 mechanical degrees, one mechanical degree corresponds to two electrical degrees for angular measurement about the rotation axis 114 with respect to a fixed reference point. Thus, placing the magnetic field sensors 116 about the disk at intervals of 120 electrical degrees is equivalent to placing the magnetic field sensors about the disk at intervals of 60 mechanical degrees.) If the Y_Axoutput signals are purely sinusoidal with third harmonic distortion, then by looking at the line-line output signal (i.e., the difference signal between any two sensors), the third harmonic distortion term may be canceled. For example, consider Y_A1-Y_A2for the disk shape and sensor placement of FIG. 1A: $\begin{array}{c}{Y}_{\mathrm{A1}}\left({\theta }_M\right)-Y_{\mathrm{A2}}\left({\theta }_M\right)=\left[A\cos \left(2{\theta }_M\right)+B\sin \left(6{\theta }_M\right)\right]-\left\{A\cos \left[2\left({\theta }_M+2\pi /3\right)\right]+B\sin \left[6\left({\theta }_M+2\pi /3\right)\right]\right\}\\ =\left[A\cos \left(2{\theta }_M\right)+B\sin \left(6{\theta }_M\right)\right]-\left\{A\cos \left[2\left({\theta }_M+2\pi /3\right)\right]+B\sin \left[6{\theta }_M+4\pi \right]\right\}\\ =A\cos \left(2{\theta }_M\right)-A\cos \left[2\left({\theta }_M+2\pi /3\right)\right]\end{array}$

[0038] Thus, the third harmonic distortion term for a line-line signal is canceled. In fact, any distortion term representing a harmonic number divisible by three would be canceled by this technique. More generally, for two magnetic field sensors placed 2π/N electrical degrees apart, N being an integer, the line-line output signal rejects any harmonic distortion term which is an integer multiple of N. Note that looking at sensors line-line also has the advantage of greatly helping to reject any common mode noise that might be injected into the signal over the cable between the magnetic field sensors 116 and the associated decoding electronics 107.

[0039] The six magnetic field sensors 116 shown in FIG. 1A are set at the same radial distance from the shaft and they are spaced at equal 60 mechanical degree intervals about the periphery of the disc 104. These six magnetic field sensors 116 are segregated into two groups of three; the three on the right side of FIG. 1A (H_A1, H_A2and H_A3) and the three on the left side of FIG. 1A (H_B1, H_B2and H_B3) form these two groups. Each of these groups forms a fully functional sensor output that may be used to determine the absolute angular position of the shaft within one half of a complete mechanical revolution. The dual electrical outputs from the two sets of sensors of this embodiment provides redundancy that may be used to form a high reliability system. In addition, using three magnetic field sensors 116 in each group instead of the minimum requirement of two per group allows redundancy within the set. For a fully functioning system, the three magnetic field sensor outputs from each group form a balanced three-phase waveform set, and the sum of the three signals Y_A1+Y_A2+Y_A3should equal 0. A non-zero sum may be interpreted by the decoder circuitry 107 to detect faulty hardware.

[0040] The goal of the magnetic design of the magnetic material disc 104 and the geometric placement of the magnetic field sensors 116 is to create the desired Y(θ_M) on the output of the magnetic field sensors 116. It is theoretically possible with a sufficiently complex magnetizing fixture and magnetizing process to completely tailor the magnetic field of a uniform radius permanent magnet magnetic material disc 104. However, such a magnetizing fixture design can be extremely complex, and the magnetizing process can be quite variable and hard to control, providing inconsistent results. In one embodiment of the invention, the magnetization fixture design is simplified, and the process variabilities are reduced, by incorporating a variable air gap between the magnetic material disc 104 and the magnetic field sensors 116 as another way to control the shape of the magnetic field.

[0041] For the simple case of a magnetic material disc 104 including a permanent magnet, and the desired Y(θ_M) being a sinusoid, it is useful to design the radial air gap between the edge of the magnetic material disc and the magnetic field sensor so that the inverse of the radial air gap distance includes a sinusoidally varying component. This inverse relationship follows from the desire to have a sinusoidally varying component in the air gap permeability, and the fact that magnetic permeability is inversely proportional to the air gap distance. Define the following terms as:

[0042] R_o→radial distance from shaft to magnetic field sensors

[0043] P→number of magnetic pole pairs

[0044] g(θ_M)→air gap distance from the magnetic material disc surface to the magnetic field sensor

[0045] R_M(θ_M)→radius of magnetic material disc

[0046] g_min→minimum air gap

[0047] g_max→maximum air gap

[0048] Then, the air gap distance is:

[0049] g(θ_M)=(2g_ming_max)/{(g_max−g_min) sin(Pθ_M)+(g_max+g_min)}

[0050] g(−π/2)=g_max

[0051] g(0)=(2g_ming_max)/{(g_max+g_min)

[0052] g(−π/2)=g_min

[0053] The radius of the magnetic material disc is just the magnetic field sensor radius minus the air gap distance, i.e.:

[0054] R_M(θ_M)=R_og(θ_M)

[0055] Holding the desired magnetic field sensor radius R_oconstant, the minimum and maximum gap values are then iterated until the resulting field exhibits the desired sinusoidal shape and amplitude. The magnetizing fixture for this magnetic material disc would be P crossed solenoidal fields. In another embodiment, the magnetic circuit may be closed by a return path concentric to the shaft and at a larger radius than the magnetic field sensors to increase the magnetic field strength and to make the relative effect of the air gap variation with position stronger. The return path may include high mu material, or other magnetically permeable material.

[0056] FIG. 2 shows another embodiment of the an improved position encoder 200. The embodiment of FIG. 2 may be used to measure the linear position of an object with respect to a fixed reference position (e.g., position offset distance d_M=0), and includes an elongated object 202 movable along a linear axis 204, a sensor assembly 206, and a decoder circuit 208 electrically coupled to the sensor assembly. The elongated object 202 includes a magnetic material component 210 having a predetermined magnetic field characteristic. This field characteristic may include a substantially constant magnetic field, produced for example by permanent magnet or a conductor carrying a direct current. Alternately, the predetermined field characteristic may include a magnetic field having a time varying amplitude that varies in a predetermined manner, e.g., sinusoidal amplitude modulation. In one embodiment, the magnetic material component 210 is fabricated entirely from one of several permanent magnet materials known to those in the art, or a combination thereof. In alternate embodiments, the magnetic material component 210 includes a permanent magnet, an electromagnet, a high mu material (i.e., a material having a relatively high magnetic permeability), or non-magnetic materials, or a combination thereof (e.g., for a hub to mount the disc onto the shaft). In another embodiment, the magnetic field originates from an electromagnet, driven synchronously by a sinusoidal source, to produce an amplitude modulated magnetic field. In yet another embodiment, the elongated object 202 and the magnetic material component may be combined or integrated to form a single movable component that produces a magnetic field. In all of the preferred embodiments, the magnetic material component 210 produces a magnetic field that varies spatially with respect to the linear offset position d_Mof the elongated object 202 along the linear axis 204. In FIG. 2, the magnetic field produced by the magnetic material component 210 is oriented in a direction substantially normal to the linear axis 204.

[0057] As with the rotary example of FIG. 1, the magnetic material is shaped to vary the air gap between the magnetic material and the magnetic field sensor as the elongated object 202 moves, so that the magnetic field shape sensed by the magnetic field sensor can be controlled with greater accuracy and predictability than via a conventional magnetizing fixture. Similar to the rotary example of FIG. 1, the linear form of the invention includes multiple magnetic field sensors 211 distributed along the sensor assembly 206 at intervals of less than 180 electrical degrees and positioned at a sensor offset position 214. The decoding circuit 208 receives the position dependent signals from the magnetic field sensor 211 outputs and processes the position dependent signals to extract the linear position information in a manner analogous to that described herein for the rotary example of FIG. 1.

[0058] FIG. 2 also shows a simple way to correct for variations in magnetic field sensor output amplitudes. Output amplitude variation between the magnetic field sensor outputs can be caused by imperfections in the magnetic field sensor scale factor or by variation in the distance between the magnetic field sensor and the magnetic body. Although shown herein only for the linear case, this correction method may also be used for the rotary position encoder application. High magnetic permeability adjustable slugs or screws 212 (hereinafter referred to as “H-adjust screws”), fabricated from a high permeability material known to those in the art such as iron, steel, cobalt, nickel, ferrites, mu metal, various alloys or combinations thereof, are mounted behind the magnetic fields sensors in line with the field being sensed. If a particular sensor output is lower than desired, then the screw 212 is adjusted to be positioned closer to the back of the sensor, so that the screw 212 increases the flux density at that sensor. If a particular sensor output is higher than desired, then the screw 212 is adjusted to be positioned further away from the back of the sensor, so that the screw 212 decreases the flux density at that sensor. The gain variation caused by the non-ideal magnetic field sensor may be mitigated via electronic calibration in the sensor assembly 206 or in the decoding circuit 208. However, correcting the gain on the magnetic field sensor assembly 206 via electronic calibration is typically more expensive than the H-adjust screws shown in this embodiment, and correcting the gain variation in the decoding circuit 208 requires that the sensor assembly 206 and the decoding circuitry 208 be calibrated together. Using the H-adjust screws or electronic adjustment on the magnetic field sensor assembly 206 allows sensors and decoders to be readily interchanged, and still have the interchanged sets be calibrated.

[0059] There can also be measurement errors caused by incorrect location of the magnetic field sensors in a direction parallel to the relative motion, i.e. perpendicular to the air gap. In FIG. 2 this error occurs if the distance between the two sensors does not match ¼ of the pitch between consecutive North poles. In FIG. 1 this error occurs if the angular distance between consecutive sensors is not exactly 120 electrical degrees.

[0060] Consider the case shown in FIG. 1 where the location of H_A2is assumed arbitrarily to be exactly π/3 in electrical degrees. H_A1, and H_A3are assumed to be misplaced from the ideal 2π/3 electrical degree spacing by θ₁and θ₃respectively. With H-adjust screws, or appropriate electrical adjustment, each of the three sensor output amplitudes can be adjusted individually. If the position decoding electronics uses two line-line outputs, then the three sensor gain adjustments can be used to set the two line-line amplitudes to the desired value and to correct for the non-ideal angular locations.

[0061] To prove that the line-line output amplitudes and the relative phase shift between the two line-line outputs can be calibrated first define the define the individual signals and the line-line quantities.

[0062] θ_E=P*θ_M+π/3

[0063] where P is the magnetic pole pair count

[0064] Y_A1(θ_E)=A₁cos(θ_E+π/3+θ₁)

[0065] Y_A2(θ_E)=A₂cos(θ_E−π/3)

[0066] Y_A3(θ_E)=A₃cos(θ_E−π+θ₃)

[0067] X_A1(θ_E)=Y_A1(θ_E)−Y_A2(θ_E)

[0068] X_A2(θ_E)=Y_A2(θ_E)−Y_A3(θ_E)

[0069] Now use the desired amplitude match and the desired 120 electrical degree phasing to relate the two line-line signals.

[0070] X_A2(θ_E+2π/3)=X_A1(θ_E)

[0071] A₂cos(θ_E−π/3+2π/3)−A₃cos(θ_E−π+θ₃+2π/3)=A₂cos(θ_E+π/3+θ₁)−A₂cos(θ_E−π/3)

[0072] A₂[cos(θ_E+π/3)+cos(θ_E−π/3)]=A₁cos(θ_E+π/3+θ₁)+A₃cos(θ_E−π/3+θ₃)

[0073] A₂[(2/2)cos(θ_E)]=A₁cos(θ_E) cos(π/3+θ₁)+A₃cos(θ_E)cos(−π/3+θ₃)+−A₁sin(θ_E)sin(π/3+θ₁)−A₃sin(θ_E)sin(−π/3+θ₃)

[0074] Since sine and cosine are orthogonal functions, then the above can be separated into two simultaneous equations.

[0075] A₂=A₁cos(π/3+θ₁)+A₃cos(−π/3+θ₃)

[0076] 0=−A₁sin(π/3+θ₁)−A₃sin(−π/3+θ₃)

[0077] It should be appreciated that the correction can be provided with the decoding circuitry.

[0078] Although the above analysis has been described in connection with the FIGS. 1A and 1B rotary embodiment, the same mathematical analysis applies to the FIG. 2 linear embodiment.

[0079] The principles of the present invention can be applied to other arrangements. For example, a second preferred embodiment of the rotary position encoder is shown in FIGS. 3A and 3B, the latter illustrating orthogonal schematic views of a second preferred embodiment of an improved position encoder 300 for measuring the angular position of an object with respect to a fixed reference position (e.g., rotational angle θ_M=0). This embodiment includes a shaft 302, a disc 303 fixedly attached to the shaft 302 and supporting a magnetic “annulus” 304, a magnetic field sensor ring 306 axially spaced from the annulus 304, and a decoder circuit 307 electrically coupled to the sensor ring 306. The sensor ring 306 is fixedly mounted to the stator housing 308 via ring mounting supports 310 and support a plurality of magnetic field sensors 316. The stator housing 308 supports the bearings 312 that rotatably secure the shaft 302 to the stator housing 308. In this embodiment the “annulus” is a ring having a mean diameter (the average diameter between the inner and outer diameters of the annulus) so that the magnetic field sensors 316 are axially spaced adjacent to the annulus as the disk 303 rotates about its axis. The confronting surface of the magnetic annulus is contoured so as to provide the non-uniform gap as prescribed in accordance with the present invention, so as to produce a sinusoidal variation with the rotational position of the disk. The thickness of the ring varies from a maximum thickness (providing the minimum air gap, g_min) to a minimum thickness (providing the maximum air gap, g_max). The ring would be magnetized so that the maximum thickness locations provide alternating north and south poles of a magnetic field, while the minimum thickness locations provide zero magnetic flux. In this regard it should be appreciated that the term “magnetic annulus” as used herein includes a substantially annular structure in which the minimum thickness parts of the ring include any minimum thickness including zero. Further, measurement errors caused by incorrect location of the magnetic field sensors can be corrected in the same manner as described above, with H-adjust screws as shown, or appropriate electrical adjustment.

[0080] The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.