Abstract:
Inductive position sensors and circuit configurations are disclosed for the measurement of linear, rotary, or curved position along a motion axis. The simplified sensor structures combine one or two parts of a movable core element with a simple planar substrate having first and second inductances connected in series. Movement of the core element in parallel to the planar substrate causing the impedance of at least one of the inductances to change. Simple circuit configurations are taught by which the impedance change is converted into a useful output signal that indicates position along the motion axis.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to sensors and transducers for the measurement of position. The measured position is typically that of a linear or rotary motion; alternatively, the motion may take the form of a curved path. The position measuring element, also called the primary sensor or the sensing element, is herein called the sensor. The sensor translates the measurand into a usable electrical signal called the sensor output. The present invention teaches a non-contact sensor comprising stationary and movable component parts, the measurement being the position of the movable part, called the movable element, with respect to the stationary part, called the stationary element. A driver circuit drives the sensor, and a signal conditioning circuit converts the sensor output into a standard electrical signal transducer output, such as zero to ten volts DC. An electronics module comprises the driver and signal conditioning circuits. The combination of the sensor, electronics module, housing and other components together form a functional position sensing device, called the transducer.  
         [0003]     The present invention relates to a novel configuration of an inductive type of position sensor and associated electronic circuits. The present invention teaches an absolute, non-contact sensor and transducer, but alternatively, relative measurements can be derived from the absolute position data by storing and subtracting data, as is well known in the art.  
         [0004]     More particularly, the present invention relates to an inductive position transducer having a sensor that measures the relative position of a conductive or ferromagnetic movable element, comprising one or more parts, with respect to a stationary element that includes at least two inductors. The path over which the position is measured is called the motion axis.  
         [0005]     A position change of the movable element results in associated changes in the inductances of the inductors. The inductance changes are indicated through the use of an electronic circuit that responds to the resulting changes in impedance of the inductors.  
         [0006]     2. Description of the Prior Art  
         [0007]     Prior art inductive linear and rotary position sensors and transducers have disclosed a plurality of planar circuit boards onto which are disposed spiral conductive structures. Brosh, in U.S. Pat. No. 4,253,079, teaches a displacement sensor in which an array of such planar circuit boards are stacked, approximating the configuration of the well known LVDT (linear variable differential transformer). The spiral conductive structures each manifest a central hole, through which a magnetic core is disposed, whereby motion of the magnetic core results in variation of the coupling between primary and secondary windings of the spiral conductive structures.  
         [0008]     Brosh, in U.S. Pat. No. 4,507,638, teaches a rotary sensor formed of first and second planar circuit boards onto which four flat coils are disposed and arranged at ninety degree intervals. A rotatable plate capable of blocking a magnetic field is positioned between the first and second planar circuit boards and rotates about an eccentric axis. The assembly generates sine and cosine electrical outputs as the plate is rotated.  
         [0009]     Fiori, in U.S. Pat. No. 4,777,436, teaches a rotary sensor having first and second planar metal coils, not connected in series, attached to a first insulating board, and third and fourth planar coils, not connected in series, attached to a second insulating board, and a rotating planar metal member disposed between the first and second insulating boards. A first inductance is formed by series connection of the first and third coils, and a second inductance is formed by series connection of the second and fourth coils.  
         [0010]     Each of these prior art methods utilizes a sensing element having coils formed on more than one planar circuit board, and requires complex circuitry in order to develop a useful position output electrical signal. Using multiple planar circuit boards, and the required interconnections, also leads to higher cost and labor than designs according to the present invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     Both the prior art inductive position sensors and transducers, and the present invention, provide for the non-contact measurement of position. It is an object of the present invention, in contrast to the prior art, to provide a novel method for measuring position that combines a simple flat structure of conductors to form the inductive sensing element, together with a simple and inexpensive electronic circuit to drive the sensor and provide a useful electrical output.  
         [0012]     The novel method avoids problems associated with interconnecting more than one set of planar coils, as well as the cost of the complicated circuitry normally used to produce a transducer electrical output. In addition, the sensor and circuits of the present invention are easily configurable to support linear, rotary, or curved path position measurement.  
         [0013]     A preferred embodiment of the present invention teaches first and second conductor coils disposed on a stationary planar insulating member, essentially parallel to which is disposed a movable member that is formed of an electrically conductive or magnetically permeable material that moves in the motion axis while maintaining essentially parallel alignment with the stationary member. The first and second conductor coils are connected in series and driven by an alternating electrical circuit. The first and second conductor coils form first and second impedances. As the movable member moves along the motion axis, the magnitudes of the first and second impedances are affected. The series connection of the impedances acts as a voltage divider, providing a variable AC voltage at the interconnection point. A demodulator section of the electronic circuit converts this variable AC voltage into a DC signal voltage that varies with the measurand.  
         [0014]     To reduce errors that may occur due to mechanical misalignment, forming of the movable member as two interconnected parts is also taught. In this case, one part is disposed on each of the two sides of the planar insulating member. This same two-part configuration of the movable member also results in less sensitivity to changes in ambient temperature, due to the tendency of the two coils to undergo similar changes and cancel out temperature-induced errors. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  shows a circuit diagram where resistor  1  and capacitor  3  work together with inverting schmitt trigger  2  to form an oscillator circuit, well know in the art, that oscillates at a frequency dependent on the values of resistor  1  and capacitor  3 . The oscillating output voltages of inverting schmitt triggers  2  and  4  drive the series connection of first and second impedances  5  and  6 , that are formed by the two coils of the sensor. The operation of bilateral switch  7 , is controlled by the output of inverter  2  to demodulate the AC voltage appearing at the junction between impedances  5  and  6 . A DC signal voltage appears across filter capacitor  8  that varies with the ratio between impedances  5  and  6 . The signal output appears across output pin  9  and circuit common  10 .  
         [0016]      FIG. 2  shows first and second rotary sensor conductor coils  11  and  12  connected in series and attached to planar insulating rotary stationary member  13 . Pin  15  is the interconnection point of coils  11  and  12 . Pins  14  and  16  are the remaining connection points of coils  11  and  12 , respectively. Coils  11  and  12  form the respective impedances  5  and  6  that are shown in  FIG. 1 .  
         [0017]      FIG. 3  shows the mechanical arrangement of a rotary sensor according to the present invention, with shaft  18  extending through hole  20  in rotary stationary planar insulating member  13  and then attached to rotary movable member  17  at attachment point  19 . Rotary movable member  17  moves along an arc forming rotary motion axis  21 .  
         [0018]      FIG. 4  shows the same sensor configuration as  FIG. 3 , except that member  22  is shown as an additional part of rotary movable member  17 .  
         [0019]      FIG. 5  shows a configuration of first and second linear sensor conductor coils  23  and  24  attached to planar insulating linear stationary member  28 , with coil interconnection pin  27 , first coil pin  25  and second coil pin  26 .  
         [0020]      FIG. 6  shows the mechanical arrangement of a linear sensor according to the present invention, with linear movable member  29  positioned above planar insulating linear stationary member  28 , with linear movable member  29  moving along linear motion axis  30 .  
         [0021]      FIG. 7  shows the same sensor configuration as  FIG. 6 , except that member  31  is shown as an additional part of linear movable member  29 .  
         [0022]      FIG. 8  shows the same circuit as in  FIG. 1 , with the addition of an oscillator comprising schmitt trigger  34 , capacitor  32 , and resistor  33 . Resistor  35  connects to the oscillator of schmitt trigger  2  in a way to add variation to the schmitt trigger  2  oscillation frequency.  
         [0023]      FIG. 9  shows examples of other shapes for the movable member of a position sensor according to the present invention. The arc shape of shaped rotary movable member  36  is suited to use in a rotary sensor for changing the rotation angle vs. output function, likewise, the sloped shape of shaped linear movable member  37  is suited to similar use in a linear sensor.  
         [0024]      FIG. 10  shows an example of a sensor configuration, according to the present invention, that measures motion along a nonlinear motion axis  44 . First conductor coil  38  is connected in series with second conductor coil  39 , with the interconnection point being brought out on pin  42 . The remaining coil connections are brought out on pins  40  and  41 . Movable member  43  follows the nonlinear motion axis  44 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     A first preferred embodiment of the present invention is shown in  FIG. 3 , and enables the measurement of rotary motion with the minimum of complexity in both the sensor and the associated electronic circuit. Together with the circuit of  FIG. 1 , a complete transducer can be constructed with a bare minimum of components, thus providing a simple and inexpensive transducer. As rotary movable member  17  moves along rotary motion axis  21 , the impedance of coils  11  and  12  change in response to the measured position along motion axis  21 . The movable member is fabricated from a ferromagnetic material, such as a nickel-iron alloy. Alternatively, it can be fabricated from an electrically conductive non-magnetic material, such as aluminum. With a ferromagnetic movable member, the coil impedance changes with position of the movable member due to the permeability of the ferromagnetic material, since the inductance and therefore the impedance of a coil varies approximately in direct proportion with magnetic permeability. With an electrically conductive movable member, the coil impedance changes due to eddy currents that flow in the movable member from electromagnetic induction. The production of eddy currents is more effective at higher frequencies, so operating frequencies of 100 kHz or more are preferred when operating in this mode. When using a ferromagnetic material movable member at such high frequencies, it is preferred to use a laminated material or a composite such as a ferrite. This reduces eddy currents when the effect of permeability is the preferred mode, rather than that of inducing eddy currents.  
         [0026]     When a ferromagnetic movable member is fully aligned over coil  11 , as shown in  FIG. 3 , a first impedance  5 , of coil  11 , is at a maximum, while a second impedance  6 , of coil  12 , is at a minimum. In this condition, the output at pin  9  is at its minimum voltage. When a ferromagnetic movable member is directly over coil  12 , the output at pin  9  is at its maximum voltage. For each angle of rotation in between these two outputs, there is an output corresponding to the angle, yielding a total rotational position measuring range of 180 degrees for the configuration shown in  FIG. 3 . It is not required that a rotary position sensor according to the present invention must use a coil shape covering 180 degrees of rotation. Coils of other angular dimension can be used. For example, a coil size in which each coil covers a rotation angle of 90 degrees, instead of the 180 degrees of  FIG. 3 , can be used.  
         [0027]     In a second preferred embodiment, according to  FIG. 4 , a rotary sensor is formed as in  FIG. 3 , with the addition to rotary movable member  17  of a second part,  22 . Disposing the two parts of the movable member so that the first part,  17 , is above stationary member  13 , and the second part,  22 , is below rotary stationary member  13 , yield an improved performance over the single part movable member shown in  FIG. 3 . The improvement is that the output at pin  9  of the circuit of  FIG. 1  remains relatively unchanged when the shaft  18  is allowed to move up and down or to wobble. In this case, any movement of movable member first part,  17 , toward or away from rotary stationary member  13  is compensated by a corresponding movement of movable member second part  22  away or toward, respectively, rotary stationary member  13 . Likewise, temperature-induced changes in the impedance of coils in rotary stationary member  13  are compensated due to a similar effect being introduced into each coil.  
         [0028]     A third preferred embodiment is shown in  FIG. 7 , in which a linear position sensor is formed by the movement of linear movable member  29 , and its second part, member  31 , along motion axis  30 . When a ferromagnetic linear movable member  29  and movable member  31  are aligned with coil  24 , the inductance of that coil and its impedance are maximum, while those of coil  23  are minimum. As the movable element parts move into alignment with coil  23 , the impedance of coil  24  reduces, as that of coil  23  increases. This causes the output voltage on pin  9  of  FIG. 1  to increase as the movable element moves along motion axis  30  in the direction shown in  FIG. 7 .  
         [0029]     Alternatively, in any of the preferred embodiments, the circuit of  FIG. 8  can be used instead of that in  FIG. 1 . The output of oscillator  32 ,  33 ,  34  (called the control oscillator) is connected to oscillator  1 ,  2 ,  3  (called the drive oscillator) through resistor  35 . The drive oscillator drives the sensing element at a frequency, for example 1 MHz. The control oscillator causes a relatively small variation in the frequency of the drive oscillator, typically on the order of two percent, or less, of the drive frequency; for example, a variation of 10 KHz. This percentage is adjusted through the resistance value of resistor  35 . The frequency of the control oscillator is typically on the order of less than one percent of the frequency of the drive oscillator, for example 1 KHz. The reciprocal of this frequency is the scan rate, and is adjusted through the value of resistor  33  or capacitor  32 . So, in the example, the frequency of the 1 MHz drive oscillator will vary by 20 KHz at a scan rate of 1 KHz. The advantage of this is to reduce the concentration of energy into any one frequency, thus making it easier to remain within the requirements of standards for limiting electromagnetic radiation. Instead of using the control oscillator of  FIG. 8 , a control signal from another source can be used to vary the frequency of the drive oscillator.  
         [0030]     Alternative shapes can also be used for a movable member. Examples are shown in  FIG. 9 . Shapes such as movable members  36  and  37  serve to modify the output function vs. measurand in two ways. The first way is to extend the measurement range for a given coil size. For example, by substituting a movable member shape as in  37  to replace  29  in  FIG. 6 , the length of motion axis  30  can be increased. In some cases, the inductance of both coils may be increasing at the same time, but one increases faster than the other. So, this function still results in a monotonic output on pin  9  in the circuit of  FIG. 1 . The same applies to increasing the range of a rotary sensor to more than the 180 degrees shown in  FIG. 3 , by using a shape such as that of  36  in  FIG. 9 .  
         [0031]     The second way in which the sensor output function can be modified by shaping the movable member, is to change the shape of the transfer function of output vs. measurand. For example, the movable member  36  can be used instead of the movable member  17  of  FIG. 3 , with a given set of coils  11  and  12 . In this case, the transfer function of sensor output vs. movement along the motion axis will change and this can be used to tailor the sensor transfer function as desired, for example: in the reduction of nonlinearity error.  
         [0032]     In addition to linear and rotary sensors, the present invention also teaches the use of similar movable and stationary member configurations to implement sensors with a nonlinear motion axis shape. An example is shown in  FIG. 10 . Coils  38  and  39  are shaped to conform to the desired nonlinear motion axis  44 . The coils are connected to a circuit such as that of  FIG. 1 . As movable element  43  is moved along the nonlinear motion axis  44 , the output at pin  9  of the circuit of  FIG. 1 , changes in response to the movement.  
         [0033]     For simplicity,  FIGS. 2 through 7  and  FIG. 10  show the inductor coils each as a single conductor pattern layer on the top of a flat dielectric material substrate, such as a printed circuit board. As it is well known in the art, a printed circuit board can be fabricated as having one or more layers of a conductor pattern in addition to the initial layer. In some cases, such additional layers are used in accordance with the present invention for the purpose of increasing the inductance of a sensing coil for a given surface area, and thus the impedance of that coil at a given frequency of operation. For example, coils  11  and  12  can be fabricated in a four layer printed circuit board, with each additional conductor layer positioned below the previous layer of the respective coil. Coil  11  would have four layers and coil  12  would have four layers, all included within one simple printed circuit board according to popular manufacturing methods well known in the art. According to the present invention, the two resulting coils would be connected together in series and form impedances as in  5  and  6  of  FIG. 1 .