WO2000066978A2

WO2000066978A2 - Position sensor

Info

Publication number: WO2000066978A2
Application number: PCT/US2000/007414
Authority: WO
Inventors: Paul A. Delong
Original assignee: Honeywell Inc.
Priority date: 1999-04-30
Filing date: 2000-03-20
Publication date: 2000-11-09
Also published as: WO2000066978A3

Abstract

An electronic transducer used in a positioner apparatus is disclosed, comprised of an input section that includes a pulse width modulator circuit that generates digital signals of a specific pulse width. Each pulse width is representative of the location of a movable member along a defined path. An output section includes a pulse amplitude modulator that alters the amplitude of the digital signals received from the pulse width modulator in accordance to the magnitude of a reference signal provided by the positioner apparatus. A pulse width demodulator receives the now amplitude-modulated digital signals and converts them into a variable output signal having a magnitude representative of the location of the movable member along the defined path. The variable output signal is output to the positioner apparatus where it is used in the control of the movable member.

Description

ELECTRONIC TRANSDUCER FOR COMMUNICATING DISPLACEMENT

OF A PHYSICAL PROPERTY

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates generally to electronic transducers and, more specifically, to a solid-state electrical device having no movable parts, that produces an output signal representing displacement of a physical property.

DISCUSSION OF THE RELATED ART

Variable resistance elements, or slide-wire assemblies, function as rheostats or voltage dividers to provide a variable output signal for the measurement of mechanical displacement. Such slide-wire assemblies are typically electro-mechanical devices comprised of a resistance element made from conductive film or a wire wound around a form. The ends of the resistance element are electrically terminated to a source of voltage, externally supplied by the user. The physical property that is being measured is mechanically connected to a wiper-element or brush that travels along the resistance element. The wiper-element is used to return to the user a portion of the voltage that is supplied. This variable output signal fed back to the user provides a measurable representation of the movement of the physical property. A better understanding of such electro-mechanical slide-wire assemblies can be had by reference to US Patent No. 4,260,995, to Strauss, assigned to the same assignee as the present invention.

Such measuring, or electrical transducer, slide-wire assemblies are used in various mechanical devices whose position is being controlled by an actuator or by some other means. For example, in the controls industry, devices such as valves having a valve stem or valve shaft which is movable by an actuator are used to control the flow of liquids and gasses associated with industrial processes of various types. In these applications it is common to provide a variable output signal to the actuator that represents the position of the movable valve stem or valve shaft. This information is used to ascertain the position of the valve stem at any given time corresponding to a command displacement, or a user initiated set point, in order to effect control of the process.

However, the presently known electro-mechanical slide- wire assemblies just described have characteristics which make them less than ideal in many applications.

These characteristics include: a short operational life due to wear of the resistance element from movements of the wiper-element; intermittent operation with excessive movements of the wiper-element in small regions of the resistance element; and susceptibility to dirt and contamination normally found in many industrial processes and industrial process plants. Additionally, these devices also suffer from problems associated with electro-mechanical devices, such as high power requirements and high original and replacement costs.

BRIEF SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a new and novel solid-state electrical device that emulates an electro-mechanical slide-wire assembly and which provides a variable output signal that represents a physical property being measured.

It is also an object of the present invention to provide a new and novel solid-state electrical device that exhibits an economical operation by consuming less power, has no moving parts to wear out and, thus, provides a longer operating life and is easily installed and set up.

In accomplishing these and other objects, there has been provided, in accordance with the present invention, an electronic transducer for use with a positioner apparatus. The electronic transducer of the present invention is arranged to generate and apply to the positioner apparatus a variable output signal, the magnitude of which is proportional to the magnitude of a reference signal provided by the positioner apparatus and also representative of the location of a movable member along a defined path. The movable member is controlled by the positioner apparatus along the defined path. The movable member also includes means for communicating its location along the defined path to the electronic transducer.

The electronic transducer of the present invention comprises in combination an input section that includes a pulse width modulator circuit that generates digital signals of a specific pulse width, each pulse width representing the location of the movable member along the movable member's defined path.

An output section includes a pulse amplitude modulator for altering the amplitude of the digital signals received from the pulse width modulator in accordance to the magnitude of the reference signal provided by the positioner apparatus. A pulse width demodulator receives the now amplitude-modulated digital signals and converts them into a variable output signal having a magnitude representative of the location of the movable member along the defined path.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

A better understanding of the present invention may be had from the following description of a preferred embodiment thereof, taken in conjunction with the sheets of drawings, in which:

Fig. 1 is a prior art schematic block diagram of a typical electropneumatic valve positioning apparatus using a slide-wire assembly;

Fig. 2 is a schematic block diagram of a typical electropneumatic valve positioning apparatus using the electronic transducer of the present invention;

Fig. 3 is a schematic block diagram of the electronic transducer circuit in accordance to the present invention;

Fig. 4A is diagram illustrating the digital signals developed by the pulse width modulator in accordance to the present invention; and

Fig. 4B is a diagram illustrating the digital signals developed by the pulse width modulator amplitude modulated by the Vref supply signal.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to Fig. 1 of the included drawings, a typical electropneumatic valve positioning apparatus is shown in block diagram, using a prior art electro-mechanical slide-wire assembly. The electropneumatic valve positioning apparatus sets up a connection between electrical or electronic devices and pneumatic actuators. The input signal representing a command variable or desired position signal (set point) is a small load independent current in the range of 4-20 mA, that is input to a valve positioning apparatus directly by the user, or from a connected process control system. This signal is used to control and regulate the application of air pressure to a valve actuator. The air pressure applied actuates a pneumatic diaphragm or piston located in the valve actuator, which in turn drives a valve rod that moves a final control device.

A commercially available electropneumatic (E/P) positioner 10 is shown, comprised of a comparator 15, an electrical controller 12 and a pneumatic controller 14. The E/P positioner 10 receives the aforementioned command variable or desired position signal via line 11 and pressurized air from a supply of pressurized air, via air pressure line 16. An output air pressure line 17 is connected from E/P positioner 10 to a valve actuator 21 of valve 20. The valve actuator 21 is mechanically connected to a first end of a valve rod 22, which is arranged to be linearly displaceable, as represented by valve stroke 27. The second end of valve rod 22 is connected to the final control device 24, which may be a valve body that opens or closes to regulate the flow of a process medium flowing therethrough. The valve rod 22 is also mechanically connected to linkage 26, which transfers the movement of valve stroke 27. A slide-wire assembly 30. including a resistance element 31 , is connected to a voltage reference (Vref) output from electrical controller 12 via line 34. A wiper-element 32 is connected to linkage 26 and is displaced along resistance element 31 responsive to valve stroke 27. A voltage signal representing the position of the valve rod 22 is input to comparator 15 of E/P positioner 10 via a feedback signal line 35.

The position of valve rod 22 is converted into an electrical feedback signal by the slide-wire assembly which is compared with the desired position signal input from line 1 1. The output from comparator 15 is a difference signal that is input to electrical controller 12. The electrical controller 12 calculates an electrical control deviation signal with the input difference signal and outputs the control deviation signal it calculates to pneumatic controller 14. A device within the pneumatic controller 14 operates as an E/P transducer, converting the control deviation signal into a pneumatic control signal. The pneumatic controller regulates the pressure of the air supplied from output pressure line 17 in accordance to the magnitude of the applied control deviation signal, thereby displacing the valve actuator 21 and the stroke 27 of valve rod 22.

As will be understood by those skilled in the art, the slide-wire assembly provides a means of feeding back a voltage signal to the E/P positioner 10 that represents the position, at a given time, of the valve rod 22 and, ultimately, the position of the final control device 24. The current position of the valve rod 22 is compared to a signal representing a desired position 11 , or set point, and a deviation signal generated to control the application of pressurized air to the valve actuator 21 to position controlled device 24 in the desired position.

Turning now to Fig. 2, the electropneumatic valve positioning apparatus of Fig. 1 is shown having a position sensor device 40 substituted for linkage 26 and the electronic transducer circuit 50 of the present invention substituted for the electromechanical slide-wire assembly 30. The electropneumatic valve positioning apparatus functions substantially as described in Fig. 1 ; however, the valve stroke 27 is detected by a position sensor 40. Position sensor 40 is disposed to determine the position of valve rod 22 and output a voltage signal, the magnitude of which represents the position of valve rod 22. Such sensor devices using either electro-optical and/or magnetic techniques to ascertain the position of a member movable along a defined path are well known in the art. Such devices are shown and taught by US Patent No. 5,589,769 to Donald R. Krahn, and US Patent No. 4,698,996 to H. D. Kreft et al.

A signal representing the present position of the valve rod 22 is output by position sensor 40 and is applied to the input of the electronic transducer circuit 50 of the present invention. Circuit 50 produces and outputs a voltage signal to comparator 15 indicative of the present position of valve rod 22, where a desired position is calculated to control the application of pressurized air to the valve actuator 21.

The electronic transducer circuit of the present invention is shown in Fig. 3 and includes in combination a pulse width modulator 52 having its input connected to the position sensor 40. This input side of circuit 50 is isolated from the output side of the circuit by optical isolator 54. The voltage reference 34 output from electrical controller 12 is applied to the output side of circuit 50 that also includes buffer/amplifier 56, pulse width demodulator 57 and buffer/amplifier 58. The output of buffer/amplifier 58 is the feedback signal 35 that is applied to comparator 15. The output signal from position sensor 40 in this embodiment is a voltage signal that is linearly proportional to stroke 27. This voltage signal is used to modulate the pulse width modulator 52 that operates on a fixed frequency. For example, the proportional input voltage from position sensor 40 could be an input voltage varying in magnitude from a minimum of 0 volt (SI_mιn ) to a maximum of 1.25 volt (SI _max ). As can be seen in Fig. 4A, pulse width modulator 52 generates digital signals in the form of a pulse train, with each pulse having a pulse width varying from PW_mιn responsive to the SI_mιn input signal to PW_max responsive to the SI_max input signal. The pulse width modulator 52 will output pulses having a pulse width between PW_mm and PW_max dependant on the position of valve rod 22 along stroke

27. The digital signals output from modulator 52 are next applied to optical isolator 54 and optically coupled to the isolated output side of circuit 50.

Since the electronic transducer circuit 50 of the present invention is to be used to substitute for an electro-mechanical slide-wire apparatus used with E/P positioners currently in production, the Vref signal is used to further modulate the amplitude of the digital signals produced by modulator 52 in order to provide feedback signals of a voltage level that are expected by the comparator 15. E/P positioners of various manufacturers use slide -wire supply voltages that vary in magnitude between 1.0 volts and 24 volts. Therefore, the digital pulses shown in Fig. 4A are amplitude modulated by the Vref voltage normally used by the E/P positioner. The amplitude modulation of the digital pulses, as shown in Fig. 4B, produces a feedback signal having a voltage range expected by comparator 15. This now double- modulated digital signal is buffered and amplified by device 56 and applied to a pulse width demodulator 57 to produce a voltage signal that is linearly proportional to both the input signal from position sensor 40 and the Vref supply from electrical controller

12. This resultant feedback signal is output from buffer/amplifier 58 and transmitted via line 35 to comparator 15. Thus, a feedback signal is output from circuit 50 to the comparator 15 that, like the voltage output from the electro-mechanical slide-wire assembly, is a variable voltage that represents the position of valve rod 22 along stroke 27.

It will be appreciated by those skilled in the art that the demodulator 57 can also be constructed as a low pass filter that is arranged to produce and output variable voltage signals indicative of the pulse widths received from the input side of circuit 50. Additionally, an RC circuit can be employed in the pulse width demodulator 57 (not shown) that provides a variable output signal based on a representative sample of pulses received over a fixed period of time. Further, it would also be within an artisan's ordinary skill to construct the functional blocks of the present invention such as the pulse width modulator 52, optical isolator 54, buffers 56, 58 and pulse width demodulator 57 in combination as a single monolithic integrated circuit or from individual discrete components and, therefore, the present invention is not limited to the type of construction used.

The present invention has been described with particular reference to the preferred embodiments thereof. It will be obvious that various changes and modifications can be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

CLAIMSWhat is claimed is:

1. An electronic transducer for use with a positioner apparatus for generating and applying to said positioner apparatus a variable output signal the magnitude of which is proportional to the magnitude of a reference signal provided by said positioner apparatus and representative of the location of a movable member along a defined path, said movable member controlled by said positioner apparatus and movable along said defined path and said movable member including means for communicating its location along said defined path to said electronic transducer, said electronic transducer comprising:

a) an input section including a pulse width modulator for generating digital signals of a specific pulse width, each pulse width representing the location of said movable member along said defined path; and

b) an output section including a pulse amplitude modulator for altering the amplitude of the digital signals received from said pulse width modulator in accordance to the magnitude of the reference signal provided by said positioner apparatus, and a pulse width demodulator receiving said amplitude modulated digital signals and converting the amplitude modulated digital signals into said variable output signal having a magnitude representative of the location of said movable member along said defined path.

2. The electronic transducer as claimed in claim 1 wherein said electronic transducer further includes an optical isolator for isolating the input section from the output section.

3. The electronic transducer as claimed in claim 2 wherein said output section further includes a first buffer/amplifier connected between said pulse amplitude modulator and said pulse width modulator for amplifying the pulse amplitude modulated signal before it is applied to said pulse width demodulator.

4. The electronic transducer as claimed in claim 3 wherein said output section further includes a second buffer/amplifier connected between said pulse width demodulator and said position apparatus for buffering said variable output signal before it is output to said position apparatus.

5. An electronic transducer for use with a positioner apparatus for generating and applying to said positioner apparatus a variable output signal, the magnitude of which is proportional to the magnitude of a reference signal provided by said positioner apparatus and representative of the location of a movable member along a defined path, said movable member controlled by said positioner apparatus and movable along said defined path and said movable member including means for communicating its location along said defined path to said electronic transducer, said electronic transducer comprising:

a) an input section including electronic means for generating a plurality of location signals, each location signal having a specific duration, each specific duration representing the location of said movable member along said defined path; and

b) an output section including means for altering the amplitude of the location signals received in accordance to the magnitude of the reference signal provided by said positioner apparatus, and electronic means for converting said amplitude altered location signals into said variable output signal having a magnitude representative of the location of said movable member along said defined path

6. The electronic transducer as claimed in claim 5 wherein said electronic transducer further includes electronic means for isolating the input section from the output section.

7. The electronic transducer as claimed in claim 6 wherein said electronic means for generating a plurality of location signals is a pulse width modulator having an input connected to said means for communicating, whereby said pulse width modulator generates and outputs digital pulses having a plurality of pulse widths, with each pulse width representative of the location of said movable member along said defined path.

8. The electronic transducer as claimed in claim 7 wherein said electronic means for isolating is an optical isolator circuit having its input connected to said pulse width modulator and receiving the digital pulses output by said pulse width modulator, and said means for altering is a voltage reference signal of a specific magnitude that is connected to the isolation side of said optical isolator circuit, whereby the voltage reference signal modulates the amplitude of the digital pulses received by the optical isolation circuit to the magnitude of the voltage reference signal applied.

9. The electronic transducer as claimed in claim 8 wherein said electronic means for converting is a pulse width demodulator circuit having its input connected via a first buffer/amplifier to the isolation side of said optical isolator and said voltage reference signal and said pulse width demodulator circuit is arranged to receive said amplitude modulated digital pulses whereby said amplitude modulated digital pulses are converted by said pulse width demodulator circuit into said variable output signal having a magnitude representative of the location of the movable member along said defined path.

10. The electronic transducer as claimed in claim 9 wherein said variable voltage output signal from said demodulator circuit is applied through a second buffer/amplifier to said position apparatus.