Abstract:
A proximity transducer system including an eddy current effect proximity probe, a proximity electronics module coupled to the probe, a monitoring system and a two-wire, current interface connecting the proximity electronics module to the monitoring system is disclosed. The proximity transducer system is operative for measuring position and vibration of a component to be monitored. The current interface is made up of a pair of wires, e.g., a twisted pair, and is adapted for providing power to the electronics module and an electrical signal from the proximity electronics module that is representative of (e.g., proportional to) the length of the gap between the probe and the component being monitored. That signal has a DC component that represents a steady state distance and an AC component that represents active movement of the component, such as vibration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     “Not Applicable” 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     “Not Applicable” 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK 
     “Not Applicable” 
     FIELD OF THE INVENTION 
     This invention relates to generally to proximity transducer systems and particularly to non-contact transducer based proximity systems including a proximity electronics module, a monitoring system, and a two-wire current interface coupling the proximity electronics module to the monitoring system. 
     BACKGROUND OF THE INVENTION 
     For the past fifty years proximity transducer systems based on the eddy current effect have dominated the measurement of vibration and shaft position in machines with fluid film bearings. These systems convert the physical distance from a probe to a metal target into a voltage proportional to that physical distance. A proximity transducer system has by its nature a frequency response from DC (static distance) to about 10 Khz of AC or (dynamic distance). This allows these sensors to be useful for measuring static (DC) distances such as the position of a shaft relative to a thrust bearing and the dynamic AC movement of a machine shaft (e.g., vibration). Proximity transducer systems are also used for measuring the dynamic position of rods, pistons or other and mechanically moving parts on reciprocating machines. When a proximity transducer system is mounted to a fixed part of a machine observing the rotating shaft the AC component of the output is proportional to vibration of the shaft relative to the machine case or bearing. This direct vibration measurement has become the industry standard. 
     Typical proximity transducer systems consist of a probe tip located within a probe body, a separate electronics module, a monitoring system, and an interface cable connecting the electronics module to the monitoring system. The probe tip typically contains a coil of wire that is located within the probe body and arranged to be placed in close proximity of the component to be observed (e.g., a machine shaft or thrust collar). The probe body not only supports the sensing tip, but also allows setting the static distance from the tip to the target. The material making up the target has to be metal for the eddy current effect to be realized. A coaxial cable may be provided for connecting the probe tip&#39;s wire coil to the electronics module when a separate electronics module is used. If the probe is an integral one, i.e., the probe contains the electronics, a separate electronics module is unnecessary, as is a coaxial cable. In any case the electronics module contains electronics for driving (powering) the probe tip and for converting the output signal from the probe, i.e., the measured distance, to a linear voltage signal which is represented in volts/distance units. The interface cable serves to connect the proximity electronics module to the monitoring system is typically a three-wire twisted shielded cable. The monitoring system can take various forms, e.g., it may be designed to protect machines, provide current values, alarms, diagnostic information, or many other uses. In all case, the monitoring system provides power to the proximity transducer system and accepts the signal from the proximity transducer system. This signal is then analyzed for various useful attributes such as overall vibration, vibration waveforms, vibration spectrums, vibration phase and amplitude, thrust position, compressor rod position, compressor piston position and so forth. The monitoring system may be custom made, built of existing systems such as PLCs, machine unit controllers, computer DAQ functions or any number of realizations. 
     As should be appreciated by those skilled in the art, the three-wire cables used in the foregoing proximity transducer systems have a number of drawbacks. For example, each channel requires a shielded three-wire twisted cable. Three-wire cables are not as common as shielded two wire cables and are more expensive. Moreover, three wire cables exhibit significant cable bulk, requiring a larger conduit. Further still, the voltage interface used in the current interface is typically terminated with a 10K ohm load resistor. This makes the internal signals, power, signal and common, susceptible to conducted EMI. Unwanted conducted currents entering the system will generate voltage across a relatively large load resistor. Because the interface is not differential, this voltage can create error signals that can cause significant performance issues with the monitoring system up to and including creation of false alarms. Furthermore, in potentially explosive or hazardous applications where proximity transducer systems are used, such systems commonly incorporate the use of a zener diode as a safety barrier between the monitoring system and the proximity electronics module. Such an arrangement with conventional three-wire voltage based systems results in a reduced linear range and a decreased scale factor. 
     Thus, a need exists for a cable connection between the probe electronics module (connected by coax or integral) and the monitoring system (or equivalent) which overcomes those disadvantages. 
     The subject invention addresses that need. To that end, this invention targets the electrical design and properties of the cable connection and reduces the number of connection wires from three to two. In addition it changes the mode of the analog interface from single ended voltage to a current loop, which provides both the dynamic signal transmission and power for the proximity transducer. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention there is provided a two wire-current interface for use in a proximity transducer system. In accordance with another aspect of this invention a proximity transducer system making use of such an interface is provided. 
     The proximity transducer system basically comprises a non-contacting proximity probe (e.g., an eddy current effect based proximity detecting probe), a proximity electronics module coupled to the probe and a monitoring system for measurement of position and vibration of a component to be monitored. The probe is arranged to measure the distance (i.e., length of the gap) between itself and the component being monitored 
     The two wire-current interface basically comprises a pair of wires providing power from the monitoring system to the proximity electronics module. The current interface is also adapted for providing an electrical signal from the proximity electronics module which is representative of (e.g., proportional to) the instantaneous value of the length of the gap (e.g., the signal has a DC component that is indicative of the steady-state position of the component monitored, and an AC component that is indicative of the vibration of the component monitored). 
     In accordance with one exemplary aspect of this invention the monitoring system delivers a constant current to the proximity electronics via the current interface and the proximity electronics module includes a variable impedance that changes impedance proportional to the distance of the probe from the monitored component. 
     In accordance with another exemplary aspect of this invention a small resistor is connected to the current interface and the monitoring system delivers a constant voltage through the small resistor to the proximity electronics module. In such a case the proximity electronics module includes a current source that modulates the current it consumes proportional to the distance of the probe from the monitored component. 
    
    
     
       DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of an exemplary prior art proximity transducer system; 
         FIG. 2  is a schematic diagram of one exemplary proximity transducer system constructed in accordance with this invention making use of a two-wire current interface; 
         FIG. 2A  is a schematic diagram of a portion of the exemplary transducer system shown in  FIG. 2 ; 
         FIG. 3  is a schematic diagram of another exemplary proximity transducer system constructed in accordance with this invention making use of a two-wire current interface; and 
         FIG. 3A  is a schematic diagram of a portion of the exemplary transducer system shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the various figures of the drawing wherein like reference characters refer to like parts, there is shown in  FIG. 1  a conventional prior art proximity transducer system  10  making use of a three-wire cable interface. The system  10  basically comprises an eddy current based proximity probe  12 , a proximity electronics module  14 , a monitoring system  16 , and a three-wire interface cable  18  interconnecting the electronics module and the monitoring system  16 . The probe  12  is any conventional device, such as those sold by General Electric, Bently Nevada™, under the trademark Proximitor® and includes a probe tip  12 A in which a coil (not shown) is disposed. 
     The electronics module  14  is any conventional device, such as those sold by General Electric, Bently Nevada™, under the trademark Proximitor® The probe  12  is connected to the electronics module via a conventional coaxial extension cable  12 B. As mentioned earlier, the electronics module may be integral with the probe  12 . In such a case no cable  12 A is necessary. An example of a commercially available integrated probe and electronics module is the IN series sold by Brüel &amp; Kjær Vibro. 
     The probe tip  12 A is arranged to be placed in close proximity of the component C (e.g., a machine shaft, thrust collar, etc.), to observed/monitored. Thus, the probe body not only supports the tip  12 A, but also allows setting the static distance (i.e., the length of the gap) from the probe tip to the target C. The proximity electronics module  14  contains the electronics for powering the probe tip  12 A and for converting the output signal from the probe  12 , i.e., the measured distance, to a linear voltage signal which is provided via the cable  18  to the monitoring system  16 . To that end the cable  18  is a conventional three-wire cable. As can be seen in  FIG. 1 , one of its three conductors provides +24 VDC to −24 VDC power to the proximity electronics module  14 . Another of its three conductors also provides power (e.g., it serves as the common or ground) for the proximity electronics module, while also being the ground for the transducer signal (e.g., DC to 20 KHz), while the last of its three conductors also carries the transducer signal. That transducer signal is indicative of the distance D (both static distance and dynamic distance). 
     In  FIG. 2  there is shown one exemplary proximity transducer system  20  which overcomes the disadvantages of the prior art by making use of a two-wire, current interface constructed in accordance with one exemplary aspect of this invention. The system  20  is identical to the system  10 , except for the proximity electronics module  22 , the interface  24  and the monitoring system  26 . In the interest of brevity, those components which are common to systems  10  and  20  will be given the same reference numbers and the details of their construction and operation will not be reiterated. Thus, the system  20  comprises an eddy current based proximity probe  12 , a proximity electronics module  22 , the monitoring system  26 , and the interface  24 . It should be pointed out at this juncture that other non-contacting type proximity probes (e.g., inductive or capacitive based devices), can be used in lieu of an eddy current based probe described heretofore. The interface  24  interconnects the proximity electronics module  22  and the monitoring system  26  and is in the form of a twisted, two-wire  24 A and  24 B cable for carrying loop current in the range of DC to 10 KHz. The cable can be of any desired length, e.g., from a few meters to up to about a 1000 meters. As is well known, current loops operating in the range of 4 to 20 mA have been used in the prior art for other applications to communicate measured parameters. They have not been used for transmitting a dynamic proximity transducer signal which is proportional to the actual physical gap voltage, without any position offset or peak detection to the signal prior to transmission. Moreover, 4 to 20 mA current loops having a bandwidth from DC to 10 KHz have not been used to provide power to the sensing transducer. 
     Since the proximity electronics module  22  is connected to the monitoring system  26  by the two wires  24 A and  24 B of the cable, instead of the prior art three-wire cable, the proximity electronics module  22  is modified slightly from a conventional one (like that shown in  FIG. 1 ) to form a portion of the current interface of this invention. The modifications to the proximity electronics module are best seen in  FIG. 2A  and will be described later. The monitoring system  26  is also modified slightly from a conventional monitoring system  16  (shown in  FIG. 1 ). The modifications to the monitoring system  26  are best seen in  FIG. 3A  and will also be described later. The proximity electronics module includes a pair of input/output terminals- 22 A and  22 B for connection to respective wires  24 A and  24 B of the interface cable. In a similar manner the monitoring system  26  includes a pair of input/output terminals  26 A and  26 B for connection to respective wires  24 A and  24 B of the interface cable. 
     As should be appreciated by those skilled in the art from the drawings and the description to follow, the current interface  24  of the embodiment of  FIG. 2  is arranged so that the monitoring system  26  delivers a constant current to the proximity electronics module  22  via the monitoring system&#39;s input/output terminals  26 A and  26 B. The electronics of the proximity electronics module  22  includes a variable impedance that changes the voltage at the monitoring system&#39;s constant current output terminals  26 A and  26 B. The impedance of the proximity electronic module is proportional to distance D to the target to be monitored, so that the monitoring system  26  will measure a voltage created by the constant current and the variable impedance established by proximity electronics module. Thus, that voltage will be proportional to distance (static and dynamic). Moreover, the monitoring system  26  is arranged to detect a properly connected proximity electronics module  22  and proper distance to the observed target C by verifying that the measured voltage is within a specified voltage window. 
     The modification to the electronics of the proximity electronics module  22  is best seen in  FIG. 2A . To that end, the proximity electronics module includes sensing elements  22 C and a variable impedance  22 D having a control input provided via line  22 E. The input/output terminals  22 A and  22 B of the proximity electronics module  22  are connected across the variable impedance. The modification to the electronics of the monitoring system  26  is best seen in  FIG. 3A . To that end, it includes a current source  26 C and an associated operational amplifier  26 D. One side of the current source is connected to the common junction of the input/output terminal  26 A and one input of the operational amplifier. The other side of the current source is connected to a +24 to −24 VDC loop supply. The other input of the operational amplifier is connected to the common junction of the input/output terminal  26 B and ground. The other components and circuitry making up the monitoring system  26  (as well as the other components of the proximity electronics module  22 ) are conventional and have not been shown and will not be described in the interest of brevity since they are conventional. 
     The proximity electronics module  22  is arranged so that its sensing elements  22 C use some amount of current from the current source  26 C of the monitoring system  26  to power its circuits. The sensors provide a signal on line  22 E which controls the variable impedance  22 D to make the voltage between the terminals  26 A and  26 B proportional to the probe distance D to the target C. Accordingly, the output signal from the operational amplifier  26 D, which is provided on line  26 E, is a voltage which is proportional to the distance D sensed by the probe&#39;s tip. 
     In  FIG. 3  there is shown one exemplary proximity transducer system  100  which overcomes the disadvantages of the prior art by making use of a two-wire, current interface constructed in accordance with another exemplary aspect of this invention. The system  100  is identical to the system  20 , except for the interface, the proximity electronics module and the monitoring system. In the interest of brevity, those components which are common to systems  20  and  100  will be given the same reference numbers and the details of their construction and operation will not be reiterated. The system  100  comprises an eddy-current based proximity probe  12 , a proximity electronics module  104 , a monitoring system  106  and a current interface  102 . 
     The interface  102  also comprises a twisted two-wire  24 A and  24 B cable (like that of embodiment 20) for carrying loop current in the range of DC to 10 KHz. The interface  102  makes use of a small resistor, in the monitoring system  106  to create a constant voltage source. In particular, as best seen in  FIG. 3A  the monitoring system  106  has been modified from a conventional monitoring system (like shown in  FIG. 1 ) to include an operational amplifier  106 A and a typical resistance of 250 ohm, resistor  106 B, although the resistance could be anywhere between 0.1 to 100K ohm depending the system design, including factors such as current loop current value and the design of the front end signal conditioning electronics. One side of the resistor  106 B is connected to one of the inputs of the operational amplifier  106 A and to terminal  26 B. The other side of the resistor  106 B is connected to the common junction of the other input to the operational amplifier  106 A and ground. The output of the operational amplifier is provided on line  106 C. Terminal  26 A is connected to the +24 to −24 VDC loop supply. 
     When constructed as just described, the proximity electronics module  104  creates a current source that modulates the current it consumes proportional to the distance to the target C. The modulated current produces a voltage on the proximity electronics module&#39;s side of the resistor  106 B that is proportional to distance D (static and dynamic) to the target C being observed. In particular, the monitoring system  106  delivers a constant voltage on one wire  24 A of the cable, with the other wire  24 B of the cable being connected to ground through the resistor  106 B. The proximity electronics module  104  is arranged so that the sensing elements  104 A use some amount of current from the current controller  104 B to power its circuits. The sensors provide a signal on line  104 C, which controls the current controller  104 B to make the current at the output terminals proportional to the distance D. The current controller  104 B maintains the current at those terminals proportional to input from sensing elements. Since the operational amplifier of the monitoring system is connected between the terminals  26 A and  26 B, its output as provided on line  106 C is thus indicative of the distance D (static and dynamic) to the target C being observed. Moreover, the monitoring system  106  detects the proper connection of the proximity electronics module  104  and the proper distance to the observed target C by verifying that the loop current is within the specified limits, e.g., 4-20 mA, for proper operation, with current less than approximately 3.5 mA indicating a fault value. However, the current loop magnitude for a proportional amount of current versus gap distance could be set at any convenient value depending on design constraints. The fault current would be a current outside of the allowable linear design range of the gap distance. 
     As should be appreciated from the foregoing, the subject invention offers considerable advantages over prior art systems making use of three-wire interface cables. In particular, two-wire twisted shielded pairs are less expensive than equivalent three-wire cables. Only two wires must be terminated per channel. Two wires provide both transmission of the eddy current distance measurement from DC to 10 KHz and the power to operate the electronics in the proximity transducer. The proximity electronics can be made immune to the polarity of the connection, the system cannot be miswired. Moreover, the proximity transducer system making use of this invention should be significantly less susceptible to EMI and RFI (e.g., at least 40 times less). The primary reason for this is the monitoring system has only a 250 ohm terminating resistor compared to the prior art&#39;s current solution of 10K ohm. The cable bundles themselves will be approximately ⅓ less thick, thereby permitting fitting in smaller conduit and simplifying panel wiring and the monitoring system can accept more channels on a fixed amount of connector space. Moreover, retrofit installations can use existing twisted pairs, which are much more common. Lastly, the use of safety barriers for explosive or hazardous area applications with this invention will not result in a scale factor change or adversely affect total system range. 
     While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof and thus others may, by applying current or future knowledge, adopt the same for use under various conditions of service.