Null seeking position sensors

Abstract: A position sensor is claimed including a pair of primary sense windings (10, 12) for establishing a forward or drive field, a pair of secondary sense windings (14, 16) in which a voltage may be induced in the presence of the drive field, and at least one conductive screen (18) capable of blocking the magnetic coupling between the primary and secondary sense windings. When the screen (18) blocks the magnetic coupling between one pair of sense windings to the same extent as it does between the other pair of sense windings, the sensor circuitry generates a marker signal indicative of the position of screen (18).

Description: This invention relates to sensors, in particular to position sensors. Position sensors provide signals indicative of the position of a moving member with which they are associated relative to some fixed location. The invention also relates to sensors for use with electrical machines, such as, inter alia, brushless DC drives and synchronous motors. Machines and drives of these kinds are used in machine tool positioning and robots. In addition to these applications, angle sensing in general and motor commutation represent further favored fields of use. There are available position and speed sensors that include absolute and incremental digital encoders, resolvers based on the varying mutual inductance of windings moved relative to one another, variable-capacitance devices, hall devices sensing permanent magnet fields, and inductosyn type transducers. For some applications, however, these solutions for position sensing may be too costly or unreliable in hostile environments, particularly where the application requires detection of only one specific angular or linear position. A more recent invention in position sensors employs the screened magnetic coupling technique. According to this technique, a properly dimensioned conductive screen is attached to a rotating or linearly moving object such that the screen can pass through an air gap between at least two pairs of magnetically coupled coils. When the screen moves into the air gap, it blocks the magnetic coupling between the coils thereby reducing the induced voltage in at least one of the coils. From this varying voltage, the position or the screen can be determined. A position sensor utilizing this technique that provides a continuous stream of analog or digitally coded position signals is described in US-A-4,737,698. A screening micromotor for measuring displacements has been described in Electro-Technology 66, 11/60, page 11 . The screening micromotor comprises a screen member arranged in the air gap between two pairs of coils, an oscillator connected to one pair of coils and an amplifier connected to the other, a phase-sensitive detector connected to the oscillator and the amplifier and a center-zero meter to indicate the displacement. The displacement measuring device disclosed in JP-A-57/197418 comprises a deplacement detector connected to the moving core of a differential transformer. The primary coil of the differential transformer is connected to an AC power supply. Two secondary coils are connected to an operation circuit providing a difference signal and a sum signal. The difference signal is amplified, fed into a synchronizing detector, and synchronously detected using the sum signal. It is an object of the invention to provide a position sensor that can detect a specific angular position or a rotating object. It is a further object of the invention to provide a position sensor that can detect a specific linear position of a linearly moving object. It is another object of the invention to provide a low cost position sensor that is extremely accurate with substantially no thermal drift or hysteresis. According to the invention, there is provided a position sensor as claimed in Claim 1 and a method for sensing the position of a moving member as claimed in Claim 2. The following drawings, together with the detailed description, serve to illustrate the invention. Fig. 1 is a block diagram of a position sensor built in accordance with the invention. Fig. 2A is a view of the screen at one position. Fig. 2B is a view of the screen at another position. Fig. 2C is a view of the screen at a third position. The invention will be described by first explaining the physical layout of a position sensor built according to the invention and then explaining how the circuit operates. Fig. 1 shows a block diagram in which two primary sense windings 10, 12 and two secondary sense windings 14, 16 are separated by an air gap. Through this air gap, a screen 18 which is connected to a rotating or linearly moving object, can pass. An oscillator 20 drives both of the primary sense windings 10 and 12. Also, since the signal generated in the secondary sense windings 14 and 16 has the same frequency as that of the oscillator 20, the oscillator 20 controls a rectifying circuit that synchronously rectifies the signal which represents the difference between the signal from secondary sense winding 16 and that from secondary winding 14. To control the synchronous rectifying circuit, the output of oscillator 20 connects to a sign comparator 22. This sign comparator 22 controls the synchronous rectifier 40. Each secondary sense winding 14 and 16 is connected to both a difference amplifier 28 and a summing amplifier 24. The output of difference amplifier 28 feeds into the synchronous rectifier 40 both directly and through an inverting amplifier 26. The output of synchronous inverter 40 feeds into a null comparator 62. A rectifier circuit 30 rectifies the output of summing amplifier 24. This rectified signal feeds into a minimum comparator 60. To supply the reference voltage to the minimum comparator 60, it is also connected to a reference source 50. The reference source 50 voltage must be chosen such that the output of the minimum comparator 60 is at V(1) when the screen 18 is at a field interrupting position, and the output of minimum comparator 60 is at V(0) when the screen 18 is outside a field interrupting position. The output of both the null comparator 62 and the minimum comparator 60 feed into an AND circuit 70, the output of which indicates the position of the rotating or linearly moving object. Against this background of the physical layout of the position sensor shown in Fig. 1, its operation can be explained. The circuit provides different output signals based upon the location of the screen 18 relative to sense windings 10, 12, 14, 16. Therefore, the circuit behavior is illustratively explained for three different positions of the screen 18, these three screen positions being shown in Figs. 2A through 2C. Regardless of screen position, the oscillator 20 drives the primary sense windings 10 and 12. When the screen 18 is at the position shown in Fig. 2A, primary sense winding 10 induces substantially the same voltage in secondary sense winding 14 as the voltage primary sense winding 12 induces in secondary sense winding 16. In short, at point 21 the voltage substantially equals that at point 23. Since those voltages are substantially equal, the output of the difference amplifier 28 essentially equals zero. Similarly, the output of the synchronous rectifier 40 essentially equals zero causing the output of the null comparator 62 to go to V(1). Summing amplifier 24 adds the voltage at point 21 to the voltage at point 23. The resulting signal passes through a rectifier and charges a capacitor 52. With screen 18 at the position shown in Fig. 2A, the voltage across capacitor 52 exceeds the reference voltage 50; this causes the minimum comparator 60 to go to V(0). With the minimum comparator 60 at V(0) and the null comparator 62 at V(1), the output of the AND circuit 70, and thus the output of the position sensor, remains V(0). As the screen 18 travels towards the sense windings 10, 12, 14 and 16, it eventually reaches the position shown in Fig. 2B. At this point, screen 18 substantially prevents a voltage signal from being induced in secondary sense winding 14. Therefore, the voltage signal at point 21 does not equal the voltage signal at point 23. Since the voltages are not equal, difference amplifier 28 generates a non-zero output signal, which, after being synchronously rectified, causes the output of null comparator 62 to go to V(0) The minimum comparator 60, however, goes to V(1). Since almost no voltage is induced in the secondary sense winding 14, the rectified output of summing amplifier 24 charges the capacitor 52 to a voltage below the reference voltage 50. When this occurs, the output of the AND circuit 70 goes to V(0). With screen 18 at the position shown in Fig. 2B, the minimum comparator 60 goes to V(1) and the null comparator 62 foes to V(0). Therefore, the output of AND circuit 70 goes to V(0). Screen 18 eventually reaches the point where it blocks the magnetic coupling between primary sense winding 10 and secondary sense winding 14 to the same extent as it does between primary sense winding 12 and secondary sense winding 16. At this point, shown in Fig. 2C, the voltage signal at point 21 substantially equals the voltage signal at point 23. When the two signals are substantially equal, the output of the difference amplifier 28 goes approximately to zero, causing the null comparator 62 to go to V(1). The voltage across capacitor 52 remains below the reference voltage 50. Therefore, the minimum comparator 60 remains at V(1). With both the null comparator 62 and the minimum comparator 60 at V(1), the output of AND circuit 70, and therefore position sensor, goes to V(1). This indicates that screen 18 is at the position shown in Fig. 2C. The position sensor described above provides a signal when the screen member interrupts a field at a selected position among corresponding drive and sense windings. It would be apparent to those skilled in the art that there are many variations possible within the scope of the invention.

Claims: A position sensor comprising a pair of primary sense windings; a pair of secondary sense windings, said pair of primary sense windings being separated by an air gap from said pair of secondary sense windings; at least one screen member (18) of a material capable of interrupting a magnetic drive field and adapted to be positioned in said air gap in correlation with the position of a moving member being sensed; oscillator drive means (20) applying current to said pair of primary sense windings, said current in said pair of primary sense windings electrically inducing corresponding current in said pair of secondary sense windings, characterized in that said primary sense windings are connected in parallel to said oscillator drive means; and that it further comprises circuit means including sign comparator (22), summing amplifier (24), inverter amplifier (26), difference amplifier (28), rectifier (30), synchronous rectifier inverter (40), reference source (50), minimum comparator (60), null comparator (62) and AND circuit (70), said circuit means providing a first signal corresponding to the sum of signals from said pair of secondary sense windings, said circuit means providing a second signal corresponding to the difference of signals from said pair of secondary sense windings, said first signal corresponding to a maximum value and said second signal corresponding to a minimum value when said screen member is outside the air gap, and said second signal corresponding to a minimum value again when said screen member is located at a certain position within the air gap, said circuit means performing a minimum signal test on said first signal by comparing said first signal to a reference signal and combining the results in the AND circuit (70), said AND circuit (70) generating said indication when said screen member (18) is located at a certain position within the air gap. A method for sensing the position of a moving member comprising the steps of positioning a pair of primary sense windings (10,12) near a pair of secondary sense windings (14,16), said primary sense windings and said secondary sense windings being separated by an air gap; coupling a screen member (18) with said moving member, said screen member being of a material capable of interrupting a magnetic drive field and adapted to be positioned in said air gap in correlation with the position of said moving member being sensed; applying oscillating current to said pair of primary sense windings, and inducing current in said pair of secondary sense windings; and detecting the presence of said screen member within said air gap; and providing an indication when said screen member is located at a certain position within said air gap, characterized in that said oscillating current is supplied to said primary sense windings in parallel; and that it further comprises providing a first signal corresponding to the sum of signals of signals from said pair of secondary sense windings; and providing a second signal corresponding to the difference of signals from said pair of secondary sense windings, wherein said first signal corresponds to a maximum value and said second signal corresponds to a minimum value when said screen member is outside the air gap, and said second signal corresponds to a minimum value again when said screen member is located at a certain position with the air gap; and performing a minimum signal test on said first signal by comparing it to a reference signal; and performing a null test of said second signal such that a minimum value thereof is determined; and combining the results in an AND circuit (70), said AND circuit (70) generating said indication when said screen member is located at a certain position within the air gap.

Inventors: ELLIS GEORGE H, KIRBY JAMES CHRISTOPHER, ELLIS, GEORGE H., KIRBY, JAMES CHRISTOPHER
Applicants: KOLLMORGEN CORP, KOLLMORGEN CORPORATION
Application year: 1990
Main classification: G01D5
IPC codes: G01D5, G01B7