Abstract:
Position sensors and methods having a coil, a magnetic target movable into greater and lesser proximity to the coil responsive to the position to be sensed, and circuitry responsive to an inductance of the coil to provided a measure of the proximity of the magnetic target to the coil. Various embodiments are disclosed, including embodiments having improved sensitivity and shielding from stray fields and for shielding from magnetic fields generated by the coil excitation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of position sensors and methods, and more particularly to position sensors and methods suitable for sensing and measuring substantial movement. 
     2. Prior Art 
     Position sensors and methods are well known in the prior art. Many of these sensors and methods are particularly applicable to instrumentation where the amount of motion or deflection is measured in thousands of an inch or less. There are, however, a number of applications wherein it is desired to measure position of a mechanical element throughout a range of motion of a substantial fraction of an inch or even a number of inches where the environment is less than ideal for a physically sensitive sensor and the mechanical requirements of the assembly cannot be substantially compromised or altered to accommodate the inclusion of the sensor. Such applications include the measurement of lift in internal combustion engine valves wherein the lift is electronically controlled, whether electro-hydraulically, electromagnetically or otherwise. Another application is in free piston engines such as that disclosed in U.S. Patent Application Publication No. US-2011-0083643-A1 entitled “Hydraulic Internal Combustion Engines”. In these applications, position as well as velocity are preferably measured, though velocity may be determined by the time and position difference between two successive position sensings. 
     U.S. Pat. No. 7,032,549 discloses a valve lift sensor for sensing lift in an internal combustion engine. That sensor uses a conductive target fastened to the engine valve stem that surrounds a coil fixed to the cylinder head. The coil is in series with a resistor with a signal generator energizing the coil with an oscillating voltage to create an oscillating magnetic field concentric to the coil. The oscillating magnetic field induces Eddy currents in the target, which in turn creates reactive magnetic fields. It is stated that “These increase in strength as the valve is opened and the target engages more of the coil magnetic field so that the eddy currents increase and a greater phase shift results between voltage of the signal generator and the current in the coil and the resistor.” “The reactive fields of the Eddy currents affect current flow through the coil by creating a lag in the current phase of the coil 36 and the resistor 38 relative to the phase of the voltage supplied by the signal generator 34”. This phase lag is sensed by a comparator which provides an exclusive OR output signal. “The Exclusive OR output signal is determined according to the following logic. When the state of the signal generator voltage and the resistor voltage are the same, the output signal is “Low”, as illustrated by portions 56 of line 50. However, when the state of the signal generator voltage and the resistor voltage are different, a “High” output signal is generated, as illustrated by portions 52 and 54 of line 50, until the states become the same. Thus, the duty cycle (or width) of the output signals 52, 54 is proportional to the response delay (phase lag) of the resistor voltage, which is proportional to valve lift.” 
     In the foregoing patent the signal generator voltage is a square wave and the phase lag is sensed at the leading edge and trailing edge of the square wave pulses with a voltage equal to one-half the amplitude of the square wave being taken as the dividing line between the states. 
     It is not clear how well the above described sensor would operate, or even how it operates. In particular, as the conductive target is lowered around the coil as the engine valve opens, the Eddy currents in the conductive target tend to confine the magnetic field caused by current in the coil, thereby reducing its inductance. At the same time, the Eddy currents increase the apparent resistance of the coil so that the overall effect is to reduce the time constant of the coil, yet it is stated in the patent that the duty cycle or width of the output signals is proportional to the response delay of the resistor voltage which is proportional to valve lift. That duty cycle, as shown in  FIG. 3 , is a measure of the time constant of the circuit, which would seem to decrease with increasing lift. 
     In any event, a thin conductive target as shown could be easily damaged during assembly of the engine or by inadvertent contact with hand tools during a repair. Also, while measuring lift on the leading and trailing edge of the square wave excitation has certain advantages, it also has a strong disadvantage. In particular, if the sensing point is not exactly midway between the upper and lower bounds of the square wave excitation, such as by drift of either or both voltages, there will be difference in lift measurement between leading edge measurements and trailing edge measurements. While this difference could be averaged out, two successive readings would need to be taken and then the midpoint between the two readings determined, not easily done and delaying the position determination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a  and  1   b  illustrate two exemplary configurations of the present invention. 
         FIG. 2  illustrates an example of the present invention being used for sensing engine valve lift. 
         FIG. 3  is a schematic illustration of a sensing circuit for the sensor of the present invention. 
         FIG. 4  presents exemplary waveforms for the operation of the present invention. 
         FIGS. 5   a  through  5   c  are cross sections of alternate embodiments of the present invention. 
         FIGS. 6 through 8  illustrate various implementations of the sensor of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First referring to  FIGS. 1   a  and  1   b , two exemplary configurations of the present invention may be seen. In  FIG. 1   a  a magnetic (ferrous) pin  20  is coupled to an element for which the motion is to be measured, and is positioned to be moved back and forth within coil  22  responsive to that motion. The magnetic pin  20  typically is a ferrous pin, which may be a reasonably soft magnetic material such as 1020 steel or a harder steel such as 52100. Thus movement of the magnetic pin  20  within coil  22  changes the inductance of the coil, which as shall subsequently be seen, is the key variable to be sensed. In  FIG. 1   b  a similar configuration is shown, though the magnetic pin  20 ′ is on the end of another pin  24  (not necessarily smaller than the magnetic pin  20 ′) which is non-magnetic. 
     A specific configuration of the present invention is shown in a valve lift sensing application in  FIG. 2 . Here the magnetic pin  24  and the sensor coil  28  are positioned at the top of the engine valve assembly with a hydraulic valve actuator assembly  30  therebelow which acts on the end of the valve stem  32  guided within valve guide  34 , with a valve seal  36  and a valve spring retainer  38  retaining valve spring  40 . Thus the magnetic pin  24  and sensor coil  28 , as well as the hydraulic valve actuator  30 , may be in a separate housing which may be a bolt-on assembly, as the engine valve assembly itself may be of conventional design and in fact might be a part of a preexisting engine or at least a preexisting design being converted to camless operation. In a free piston engine, a magnetic pin would either be fastened to or configured to follow the motion of the free piston and slide within an elongated coil to provide the sensor structure. In this case, the stroke of the free piston may be on the order of four inches, and accordingly, the coil itself as well as the pin would be of similar, perhaps slightly greater, length. 
     Now referring to  FIGS. 3 and 4 , an exemplary circuit for driving the sensor coil  28  and the associated signal waveforms may be seen. When the Fast Reset Signal is low, transistor Q 1  is off, so the current in the series connection of the sensor coil  28  and the resistor R is zero. When the Fast Reset goes high, transistor Q 1  is turned on, though the current in the series connection of the sensor coil  28  and the resistor R remains zero because the Excitation Signal and the output of the FET Driver is low. When the Excitation Signal and output of the FET Driver go high, a fixed voltage is coupled to the series connection of the sensor coil  28  and resistor R. Now the current in the series connection of the sensor coil  28  and resistor R starts increasing from zero, with the Excitation Signal voltage first appearing across the sensor coil  28 , but decreasing as the current increases and thus the voltage drop across the resistor increases. As the voltage across the sensor coil  28  decays, at some point it will cross the Comparator Threshold. The Comparator is triggered by the rise in the voltage across the sensor coil  28 , and again by the voltage across the coil decaying to below the Comparator Threshold, providing an output pulse T R  long. There are, of course, some Eddy current losses in the pin  24  ( FIG. 2 ), though these are relatively small. In a preferred embodiment, the excitation signal is maintained for a time period T e , after which the excitation signal goes low, followed by the Fast Reset Signal going low, turning off transistor Q 1 . This causes a very rapid collapse of the magnetic field in the sensor coil  28  because of the resulting high back EMF caused by the sudden open circuit, the back EMF being limited only by the voltage across the Zener diode commonly included in such field effect transistors (not shown). Because of this, the Fast Reset Signal and the Excitation Signal may soon thereafter be driven high again to repeat the sensing cycle. Thus in comparison to the prior art, the present invention depends on the magnetic field coupled to the pin as opposed to Eddy currents in the pin caused by magnetic fields therethrough. Also in comparison to the prior art, for a fixed position of the pin  24  in the sensor coil  28 , the present invention provides a stable output for each Excitation Signal, in spite of a possible error in the reference voltage Vref, typically, but not necessarily, set at approximately one half of the Excitation Signal voltage. The prior art, on the other hand, will provide alternating higher and lower signals with a separation of twice the error in the reference voltage, which will definitely cause noise in the system and could lead to instabilities. Another advantage of the present invention is that such sensors may be used to balance or equalize the motion in multiple elements. By way of example, in the engine valve example of  FIG. 2  it is desirable in multi-cylinder engines that valve lift for each cylinder be equal. This can easily be done with the present invention by merely providing the same reference voltage Vref for all such sensors. Finally, if desired, the Fast Reset Signal and the Excitation Signal could be triggered low at or just after the trailing edge of the Resultant Signal for substantially immediate repeat of the operating cycle to obtain an even higher frequency of sensing, particularly when the inductance of the sensor coil  28  is low (such as when pin  24  has withdrawn most of the way through sensor coil  28 ). Since the inductance is essentially proportional to the position of the pin within the coil, the pulse width T r  of the resultant signal is substantially a linear measure of the pin position within the coil. In a typical application the pulse width T r  of the resultant signal would be converted to a digital signal and if better linearization is required, any output signal could be corrected by an appropriate lookup table and interpolation between points. In that regard, a slight delay in the effective triggering of the Comparator on the Excitation Signal going high has been found to improve the linearity of the sensor, which delay may be imposed in the analog domain or in the digital domain. Also any zero offset can be cancelled, either initially, or repeatedly when the position sensor is in a known position, such as in hydraulic or other engine valve actuation systems, when the valve is known to be closed. In an engine valve actuation system, this has the advantage of being able to cancel any changes in zero offset faster than changes in zero offset can accumulate because of such effects as differential expansion, thermal expansion and wear. Also one can measure the temperature of the sensor coil  28  by momentarily interrupting the position measurement to pass a known current through the sensor coil  28  and measure the voltage across the sensor coil  28  to determine its resistance and hence its temperature. Note that by definition, a fixed current through the sensor coil  28  eliminates the effect of its inductance, so the voltage drop across the sensor coil  28  is entirely due to its resistance. If possible, the temperature measurement can be done while the sensor pin  24  is in a fixed position, such as in a hydraulic engine valve actuation system, sometime while the engine valve is closed, as the pin position need only be measured once (or a few times) when in the known position for correcting the zero offset. 
     Now referring to  FIGS. 5   a  through  5   c , cross sections of alternate embodiments of the present invention may be seen.  FIG. 5   a  shows the embodiment of  FIG. 1   a  with a further ferrous metal member  26  substantially encircling the coil  22  and target  20 . The purpose of the ferrous member  26  is primarily to confine the magnetic field generated by the coil  22 , which otherwise may cause noise in any adjacent circuitry, and to avoid interference from external fields. The presence of a full circular ferrous member  26  has been found to actually decrease the sensitivity of the sensor, though the placement of a small longitudinal slot in the otherwise circular ferrous member  26  actually increased the sensitivity of the sensor by eliminating or substantially reducing the Eddy currents generated in the ferrous member  26  and by providing a more complete magnetic circuit with the magnetic pin  20 . In the embodiments disclosed herein, the magnetic pins are generally circular rods, the coil and the ferrous metal member substantially encircling the coil and target are generally tubular in cross section having circular inner and outer diameters, though this is not a limitation of the invention. 
       FIG. 5   b  is similar to  FIG. 1   b  but also further incorporates the ferrous metal member  26 , again in a split tubular form.  FIG. 5   c , on the other hand, is similar to  FIG. 5   a  in that it has the ferrous metal split tubular member  26  but further incorporates a ferrous end plate  28 , which has been found to further increase the sensitivity of the sensor, again by providing a more complete magnetic circuit with the magnetic pin  20 . 
     Now referring to  FIGS. 6 through 8 , various implementations of the sensor of the present invention may be seen. In this embodiment the target (magnetic pin)  70  is connected to the hydraulic valve actuation actuator therebelow, with the sensor coil  72  surrounding the target  70  with a steel or ferrous sleeve  74 , again with a longitudinal slot therein, generally encircling the coil  72 . In this embodiment a plastic housing  76  is used, with the top of the housing being open to the atmosphere. Thus this embodiment is an example of the application of the embodiment of  FIG. 5   a.    
     Now referring to  FIG. 7 , an embodiment incorporated within a hydraulic actuator for a hydraulic engine valve actuation system may be seen. Here the ferrous target  41  also acts as a hydraulic piston to open an engine valve in response to actuation fluid pressure in the region  42  above the target  41 . The target  41  slides within a ceramic cylinder  44  with sensor coil  46  on a bobbin surrounding the ceramic member  44 . The steel sensor sleeve  48  may be seen at one side of the cross section of the sensor assembly but not on the other side, as the cross section is taken through the slit in the steel sleeve  48 . 
     In  FIG. 8 , the sensor is disposed between the top of an engine valve stem (not shown) and a hydraulic actuator for actuating the same (also not shown). The sensor coil  50  surrounds the ferrous metal target  52  with the split steel sensor sleeve  54  encircling the sensor coil  50 . Member  56  is a non-ferrous (non-magnetic) metal with the intersection between the ferrous metal target  52  and the non-ferrous metal  56  being labeled  58 . The entire assembly is retained within a plastic housing  60  with the steel sensor sleeve  54  being shown only on one side of the cross section of  FIG. 8 , as the cross section is taken through slit  62  in the steel sensor sleeve. 
     It should be noted that one can apply the principles of the present invention to other sensing circuitry as desired. By way of example, one could sense the voltage across the resistor R, and take the output as the time during which the voltage across the resistor does differs from the Excitation Signal by a predetermined voltage. One could also sense position using both a positive pulse and a negative pulse, though this is not preferred for reasons set forth herein, or using a negative pulse only, effectively an inversion of the embodiment disclosed. The key point is that one senses the inductance of the sense coil by sensing a time period responsive to the time period that the magnitude of the voltage across the sense coil exceeds to a predetermined value, whether sensed directly across the sense coil, or indirectly sensed by sensing the opposite voltage waveform across the resistor, given that the sum of the two voltages must equal the voltage step of the Excitation Signal. 
     Thus the present invention has a number of aspects, which aspects may be practiced alone or in various combinations or sub-combinations, as desired. While certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the full breadth of the following claims.