CA1330833C - Transmitter with magnetic zero/span actuator - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A transmitter for use in a process control system has externally accessible actuators for permitting external adjustment of the zero and span (or full scale) settings of the transmitter . the transmitter has an explosion-proof housing which includes an interior chamber in which transmitter circuitry is located.
Each of the actuators includes a movable magnet which operates a magnetic reed switch located within the interior chamber of the housing through a wall of the housing . Each of the magnets is mounted on a movable actuator which extends into a blind hole in a wall of the housing .
By moving the magnet within its hole , the corresponding reed switch can be changed from a non-actuated to an actuated state. When the zero reed switch is actuated, the transmitter circuitry adjusts its output so that the present value of the parameter represents a process zero. When the span (or full scale) reed switch is actuated, the transmitter circuitry adjusts its output so that the present value of the sensed parameter represents a process maximum.

Description

1330~33 TRANSMITTER WITH MAGNETIC ZERO/SPAN ACTUATOR
BACKGROUND OF THE INVENTION

l. Field of the Invention The present invention relates to transmitters used in industrial process control systems.

2. Description of the Prior Art Two-wire transmitters (as well as three-wire and four-wire transmitters) find widespread use in industrial process control systems. A two-wire transmitter includes a pair of terminals which are connected in a current loop together with a power source and a load. The two-wire transmitter is powered by a loop current flowing through the current loop, and varies the magnitude of the loop current as a function of a parameter or condition which is sensed. Three and four wire transmitters have separate leads for supply current and outputs. In general, the transmitters comprise energized electrical circuits which are enclosed in a sealed housing such that ignition of any combustible atmosphere by faults or sparks from the energized circuit is contained in the housing.
Although a variety of operating ranges are possible, the most widely used two-wire transmitter output varies from 4 to 20 milliamperes as a function of the sensed parameter. It is typical with a two-wire transmitter provide adjustment of the X

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transmitter so that a minimum or zero value of the parameter sensed corresponds to the minimum output (for example a loop current of 4 milliamperes) and that the maximum parameter value to be sensed 05 corresponds to the maximum output (for example 20 milliamperes).
The minimum and maximum parameter values will vary from one industrial process installation to another. It i8 desirable, therefore, to provide some means for ~etting the maximum and minimum output levels in the field, and this i5 done typically with electrically energized zero and span potentiometers sealed in the housing. With so~e trans~itters, a housing cover must be removed to gain access to the potentiometers for adjustment, undesirably exposing the atmosphere surrounding the transmitter to the live circuits in the transmitter. A variety of techniques, however, are available for adjusting the potentiometers while sealing potentially explosive atmospheres surrounding the transmitter from the electrically live circuits in the transmitter. In some transmitters, a rotary adjustment shaft for adjusting a potentiometer i~ closely fitted through a bore in the housing to provide a long flame path for quenching ignition in the housing before it reaches the atmosphere surrounding the housing. In yet another arrangement, the potentiometers are mechanically coupled to a relatively large bar magnet which is then rotated magnetically by another bar magnet outside the live circuit' 8 enclosure. This arrangement with bar magnets can have the di~advantage of mechanical hysteresi~, making precise ~pan and zero setting difficult. Actuated switches are also used 3 ~

for setting span and zero in transmitters, such switches requiring an opening through the wall of the transmitter's housing to provide for mechanical coupling to the switch.
05 For many process control environments, the transmitter itself is required to have an explosion-proof enclosure. This means that, if a spark takes place inside of the transmitter housing which ignites gases within the housing, no hot gases should be propagated from the interior of the transmitter to the exterior which could cause any surrounding combustible atmosphere to ignite.
Providing ~or zero and span adjustments which are accessible from outside the transmitter (BO that the housing would not have to be opened) i8 desirable, but makes it difficult to maintain the explosion-proof characteristic6 of the transmitter. External span and zero actuators have, in the past, needed either bulky magnet pairs for transmitting rotational force or passages formed through the transmitter housing wall, so that one end of the actuating mechanism extends into the chamber which contains the transmitter electronics, while the other end is accessible from the exterior of the transmitter. In order to maint~in explosion-proof characteristics, very long flame paths must be created with very tight tolerances. It i8 also important that the passages be sealed so that moisture cannot enter the transmitter housing through the span and zero actuator passages.
There i8 a continuing need for improved zero and span actuators which are easier to fabricate, require less critical tolerances, and are less expensive than prior ar1: actuators.

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' ~ 33~33 SUMMARY OF THE INVENTION
The present invention relates to a process control ~ransmitter which provides for external actuation for calibration purposes such as zero or 05 span setting without requiring a passage through the housing wall to the interior chamber in which the transmitter circuitry i~ located. In the present invention, the actuator includes a magnetically actuated switch located within the interior chamber of the transmitter adjacent to a wall of the transmitter housing. A magnet i5 mounted within a blind hole in the wall and i~ movable between a position in which the switch is not actuated and a position in which the switch is actuated. The blind hole opens to the exterior of the transmitter, 80 that means for selectively moving the magnet between the non-actuating and actuating positions is accessible from the exterior of the transmitter.
With the pre~ent ~nvention, a ~ignal is provided from the exterior of the transmitter without requiring a passage through the housing wall or the presence of a bulky permanent magnet inside the main cavity in the housing. As a result, the need for a long flame path and very tight tolerance3 is eliminated, because there is no connection between the blind hole and the interior chamber of the transmitter housing.
BRIEF DESCRI~TION OF TH~ DRAWINGS
Figure l i6 a partially exploded perspective view of a transmitter with the magnetic zero/6pan actuator of the present invention. ~ ~
Figures 2A and 2B are sectional views, along ~-section 2-2 of Figure 1, showing a preferred ;~;

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embodiment of the magnetic actuator in its non-actuating and its actuating position, respectively.
Figure 3 is an electrical block diagram of a 05 preferred embodiment of the transmitter circuitry used in conjunction with the magnetic zero/span actuator of the present invention.
Figure 4 i~ a flow chart showing operations of a microcomputer of the transmitter circuitry of Figure 3 when one of the zero/span actuators is actuated.
DETAILE~_p~SCR~ Ç!L5~L~ REFERR~ EMBODIMENTS
Figure 1 6hows pressure transmitter 10, which includes the magnetic zero and span actuator of the present invention. Tran~mitter 10 has a main housing 12 which defines a pair of internal chambers 14 and 16 separated by center wall 17 (as shown in Figures 2A
and 2B). The transmitter's energized electronics and terminals are housed in chambers 14 and 16 respectively. End cape 18 and 20 close chambers 14 and 16 to seal cha~bers 14 and 16 from the external environment and provide explo~ion-proof characteri6tics to the housing. End cap5 18 and 20 are threaded and ~crewed into mating threads on housing 12 80 that the threadfi provide a long, narrow path for quenching flames. As shown in Figure 1, o-ring 22 associated with end cap 18 provides a fluid-tight seal with transmitter housing 12, and a similar O-ring (not Qhown) provide6 a seal between end cap 20 and housing 12.
Tran~mitt~r housing 12 hae a relatively flat ~urface 24 which ~s located near its top.
Identification plate 26 (which typically includes an ~','.~' ''~, ' ' ' .

~ , ~33~33 identification of the manufacturer, the model number and the serial number of the transmitter) is removably attached to surface 24 by a pair of screws 28 and 30.
A recess 32 is formed in surface 24. A pair 05 of blind holes 34A and 34B extend downward from recess 32 into center wall 17 of housing 12. Internally threaded inserts 36A and 36B are press-fitted into the upper ends of holes 34A and 34B, respectively. There is no flame path between the blind holes 34A and 34B
and the chambers 14 and 16.
Screws 40A, 40B extend down into bl~nd holes 34A, 34B through threaded inserts 36A, 36B, respectively. Screws 40A, 40B have screw heads 42A, 42B at their upper ends; upper threaded portions 44A, 44B; lower threaded portions 46A, 46B; intermediate unthreaded portions 48A, 48B of smaller diameter than the threaded portions; and recesses 50A, 50B in their lower ends. Per~anent magnets 52A, 52B have their upper ends inserted with a press-fit in recesses 50A, 50B so that permanent magnet 52A moves in an axial direction with screw 40A, and permanent magnet 52B
moves in an axial direction wi~h screw 40B. Return springs 54A, 54B are mounted coaxially on the lower end of permanent magnets 52A, 52B, with their lower ends engaging the bottoms of blind holes 34A, 34B and their upper ends engaging the lower ends of screws . 40A, 40B, respectively.
Rubber washers 58A, 5~B are positioned below heads 42A, 42B, respectively. They provide an environmental seal for holes 34A and 34B.
Positioned within interior chamber 14 is circuit board 60, which carries some of the energized transmitter circuitry. The energized transmitter ~.,`' '`

~.3~833 terminals loo, 102 and a portion of the loop circuit 101 are located in the chamber 16.
Magnetically actuated reed switches 62A and 62B are electrically connected to the circuitry on 05 circuit board 60 and are thus energized. Support posts 66A and 68A support reed switch 62A so that it ic parallel to blind hole 34A and is positioned adjacent center wall 17. Similarly, support posts 66B
and 68B extend from circuit board 60 and support reed switch 62B parallel to blind hole 34B.
Reed switches 62A and 62B are actuated by magnets 52A and 52B, respectively. Reed switches 62A
and 62B are normally open, and do not close until the centerline of their respective magnets 52A, 52B
approaches the centerline of the switches. For reference, in Figures 2A and 2B, centerline 70A of reed switch 62A and centerline 72A of magnet 52A are shown.
Figures 2A and 2B show maqnet 52A and reed 6witch 62A. The operation of magnet 52B and reed switch 62B is essentially identical, and will not be discu~sed ~eparately.
Each of the reed switche~ 62A and 62B
comprises a pair of narrow 6trip3 formed of a material which i8 electrically conductive and magnetically soft, such as permalloy. The strip6 are sealed into opposite ends of a glass tube and overlap one another near the centerline 70A of the reed switch. When the centerline of the magnet 72A and the centerline of the reed ~witch 70A are substentially aligned, the two narrow strips are magnetically attracted toward one another and bend to contact each other, closing an electrical circuit between them. When the upper pole . ~

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~ 3 ~ '' 3 or end of the magnet 72 is near the centerline 70A of the reed switch 70A, the overlapping ends of the narrow strips are held apart, and the circuit between the strips is open. The arcing or sparking contact of 05 each reed switch is thus sealed from the atmosphere in the chamber 14. Both the glass tube and the wall 17 thus separate the contacts from the atmosphere surrounding the transmitter 10. Wall 17 is formed of a substantially non-magnetic material so that the magnetic flux from magnets 52A, 52B can couple effectively to the reed switches 62A, 62B. When the two switch assemblies are close together, it is desired that the north poles of the two magnets be oriented in the same direction to prevent undesired interaction.
As shown in Figure 2A, identification plate 26 is mounted on surface 24 and covers recess 32. In this condition, which is the normal operating condition for transmitter 10, upper threads 44A of screw 40A are fully threaded into threaded insert 36A, and magnet 52A is in its lowermost position within blind hole 34A. Spring 54A is compressed, but the bias force being applied is counteracted by the threaded connection between upper threads 44A of screw 40A and the internal threads of insert 36A. In the position shown in Figure 2A, the centerline 72A of magnet 52A is well below centerline 70A of reed switch 62A, and reed switch 62A remains in its normally open i~
state.
In order to move magnet 52A up and actuate reed switch 62A, identification plate 26 is removed by removing screws 28 and 30. This exposes the upper ends of screws 40A and 40B. Using a screw driver (not shown~, a technician backs screw 40A out until upper threads 44A clear the internal threads of insert 36A.
At this point, spring 54A, which has been compressed, pushes actuation screw 40A up until lower threads 46A

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contact threaded insert 36A. At this point, movement is stopped and the magnet centerline 72A is essentially aligned with reed switch centerline 70A.
This causes reed switch 62A to clo~e.
05 As will ~e described in further detail later, the transmitter circuitry then waits a predetermined amount of time before responding to the change-of-state of reed switch 62A or 62B. In response to a chanqe--of-state, the circuitry adjusts itself to indicate either a zero reading (such afi 4 milliampere output) or a full scale reading (~uch as milliamperes~ depending on which of the two actuator screws 40A or 40~ was used. Thereafter, whenever the value of the sensed parameter is the lS same, the zero reading (or full ~cale reading) will be provided by transmitter 10 as its output.
Figure 3 shows an electrical block diagra~ of two-wire trans~itter 10. Transmitter 10 of Figure 3 includes a pair of electricai terminals 100 and 102 which are connected to a two-wire current loop 101.
~he loop current IL flows in through terminal 100 and out through terminal 102. The magnitude of loop current IL is controlled to be representative of the sensed parameter by current control 104 based upon a control signal received from digital-to-analog (D/A) converter 1060 The control signal provided by D/A
converter 106 is based upon a digital value representative of the sensed parameter and adjusted for span and zero settings supplied by microcomputer system 108. Sensor 110 sense~ the parameter (e.g., pressure or temperature) and provides an analog signal representative of the sensed parameter to analog-to-digital (A~D) converter 112. The digital . - . . i;

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~L331~33 output of A/D converter 112 is provided as an input to microcomputer system 108.
Reed switches 62A and 62B are connected to input ports of microcomputer system 108. Switches 62A
05 and 62B are connected to supply potential V+, so that when they are closed they provide high logic levels to their respective input ports. Biasing resistors are coupled between the input ports and a DC common level so that when the switches are open, a low logic level is provided to the input ports.
Power supply 114 provides the necessary supply voltages to the other components of the transmitter shown in Figure 3. In this particular embodiment, all power used by the transmitter circuitry is derived from the loop current IL.
Microcomputer system 108, during each pass through its operating cycle or update loop, performs a routine whlch determines whether reed switches 62A and 62B are closed. Thi6 routine is shown in Figure 4.
Microcomputer system 108 first checks to see whether either of the switches 62A, 62B is closed as shown at 120 in Figure 4. If the answer i5 no, a running switch history (described below) i5 reset as shown at 122 and microcomputer system 108 returns to its normal cycle.
If, on the other hand, a switch is closed, microcomputer system 108 then checks to see whether this is the same switch which was closed the last time the routine was performed as shown at 124. If the answer is no, the identity of the switch which was closed is put in a buffer and a two-second timer is initialized as shown at 126 and then decremented by one as shown at 128. On the other hand, if the same . , ,,. .. . , - :.: -:

1331~3 switch was closed the last time the routine was performed, the two-second timer is simply decremented as shown at 128.
Once the two-second timer hac been os decremented, microcomputer system 108 checks to see whether the two-~econd timer has reached zero as shown at 130. If the answer is no, microcomputer system 108 returns to its normal operating cycle. If the answer is yes, microcomputer system 108 then checks to see whether it has already executed a span or zero function based on th~ particular switch having timed out as shown at 132. If the answor is yes, it means that the actuator ~crew 40A or 40B ha~ not been screwed back in yet, but microcomputer system 108 does not need to perform the zero or span calibration function another time.
If the two-6econd timer has timed out for the first time, microcomputer system 108 then checks to see whether it i8 the zero or the span ~witch which is closed as ~hown at 134. If it i8 the zero switch that is closed, then microcomputer system 108 takes the then current 8en80r rQading which it received from A/D
converter 112 and u8e8 that v~lue thereafter as the "zero" point and 8et8 thst zero point to correspond to a 4 milliampere value of loop current IL at the then sensed parameter value as ~hown at 136. Microcomputer system 108 output~ the digital value to D/A converter 108 which will cause current control 104 to produce a 4 milliampere output.
If th~ span switch has been actuated, microcomputer 6ystem 108 takes the then-current sensor reading and correlates that to the 20 milliampere output level for loop current IL. Microcomputer 0-- .
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~ 3 ~ 3 system 108 provides the appropriate digital value to D/A converter 106 which will provide the necessary control signal to current control 104 to cause IL to equal 20 milliamperes as shown at 138. The digital 05 value from A/D converter 112 which corresponds to that milliampere output is stored by microcomputer system 108 and used subsequently. The span of the transmitter is adjusted accordingly so that there is a linear relationship between the sensed variable and the output current.
In this particular embodiment, the "zero"
setting is an offset adjust~ent in that it effects all points equally. It indicates to microcomputer system 108 that a particular sensor reading is the proces zero and should result in a 4 milliampere loop current.
The span switch in this particular embodiment actually sets the process maximum or full scale value.
Microcomputer system 108 is adjusted by the technician, by actuating the span reed switch, 80 that the current sensor reading corresponds to a process maximum value and therefore should correlate to a 20 milliampere output.
In a preferred embodiment of the present invention, the update loop or cycle for microcomputer system 108 i8 on the order of forty milliseconds long.
To produce a two-second time-out, the routine shown in Figure 4 must he performed approximately fifty times.
If either switch 62A or 62B opens for 40 millisecond~
sometime during the two-second interval and then re-closes, the switch history is reset and the two-second timer has to be reinitialized. This provides some protection against brief actuations of 133~833 switch 62A or 62B due to vibration, or accidental re-actuation of switch 62A or 62B while the actuator screw 40A
or 40B is being threaded back into its normal "down"
position.

The particular embodiment shown in Figures 3 and 4 is, of course, only one example of a transmitter circuit which can make use of the magnetic zero/span actuator of the present invention.
The transmitter 10 of Figure 1 can also be fabricated with a single actuator rather than two actuators for setting span and zero. This can be implemented in several different ways depending on the control algorithm entered in microcomputer system 108.

In one algorithm, no adjustment for span or zero is made until the actuator is up for at least 2 seconds. If the actuator is pushed back down between 2 and 4 seconds after the actuator is let up, then the zero setting is adjusted to the current value of the process variable when the actuator is pushed back down. If the actuator is pushed back d~wn more than 4 seconds after the actuator is let up, then the full scale setting is adjusted to the current value of the process variable when the actuator is pushed back down.

In a second algorithm, if the actuator is let up and pushed down only once during a two second time period, then the zero setting is adjusted to the P~ ~, ,;
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current value of the process variable at the end of the two second interval. If the actuator is let up and pushed down three or more times during a two second interval, then the full scale setting is oS adjusted to the current value of the process variable at the end of the two second interval.
In yet another arrangement, the zero setting is adjusted to the current value of the process variable 50 milliseconds after the actuator is let up, and the full scale setting is adjusted to the current value of the process variable 50 milliseconds after the actuator is again pushed down.
The magnetic zero/span actuator of the present invention has a number of important advantages. First, it allows hysteresis-free setting of zero and span through external actuators, without compromise of the explosion-proof characteristics of the housing. ;
The present invention provides external actuation for setting zero and span without creating a flame path from the interior of the transmitter to the -~
exterior. As a result, the need for elaborate seals and very tight tolerances, as well as long passages to produce long flame paths, i8 avoided.
Another advantage of the present invention is that the actuator screws 4QA and 40B and magnets 52A
and S2B can be removed entirely without affecting the operation of transmitter 10, and without leaving an open passage to the interior of transmitter 10. This makes adjustment of transmitter 10 span and zero ~etting resistant to tampering. A software flag can also be set from a re~ote digital communicator 103 which will disable the s]pan and zero setting functions ~33~33 of switches 62A, 62B located at the transmitter providing redundant protection against tampering with span and zero settings.
In addition, it is possible to provide a OS transmitter which allows settings of only zero or only span simply by removing one of the actuator screws 40A, 40B and its corresponding magnet 52A, 52B.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

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Claims (10)

1. A transmitter comprising: transmitter circuitry for producing an output as a function of a sensed parameter; a housing having an exterior wall and having a sealed interior chamber in which the transmitter circuitry is located; the housing further including an interior wall connected to the exterior wall and which extends into the interior chamber, and a first blind hole which extends from an exterior surface of the exterior wall into the interior wall; a first magnetically-actuated switch located within the interior chamber and positioned adjacent to and separated from the first blind hole by the interior wall of the housing; a first magnet movable within the first blind hole between positions in which the first switch is in a first state and a second state; means for selectively moving the first magnet in the first blind hole; and means for adjusting the output of the transmitter circuitry in response to a change of state of the first switch.

2. The transmitter of Claim 1, wherein the means for selectively moving comprises: an elongated member having a head at an outer end and having means for carrying the first magnet at an inner end.

3. The transmitter of Claim 2, wherein the means for selectively moving further comprises: spring means positioned between a bottom of the first blind hole and the elongated member for applying a bias force to the elongated member in an outward direction.

4. The transmitter of Claim 3, and further comprising:
means for limiting movement of the elongated member in the outward direction.

5. The transmitter of Claim 4, wherein the elongated member has first and second threaded portions separated by an intermediate portion of smaller diameter.

6. The transmitter of Claim 5, wherein the means for limiting movement comprises: a threaded insert positioned in the first blind hole, the insert engaging the second threaded portion to limit movement in the outward direction.

7. The transmitter of Claim 6, wherein the insert has internal threads for engaging the first threaded portion to hold the elongated member in an innermost position.

8. The transmitter of Claim 1, wherein the means for adjusting makes an adjustment to the output only after the first switch has remained in a first state for a predetermined time interval.

9. In a transmitter having means for sensing a parameter, means for producing an output signal as a function of the sensed parameter, and a housing having an exterior wall and a closed interior chamber therein; the improvement comprising:
magnetically-actuated zero switch means positioned in the closed interior chamber within the housing; magnetically-actuated span switch means positioned within the closed interior chamber; zero actuator means for actuating the zero switch means, the zero actuator means being accessible from outside the housing and including a first movable magnet; the zero actuator means extending into and being movable within a first blind hole which extends from an exterior surface of the exterior wall into an interior wall which is attached to the exterior wall and extends into the closed interior chamber; span actuator means for actuating the span switch means, the span actuator means being accessible from outside the housing and including a second movable magnet; the span actuator means extending into and being movable within a second blind hole which extends from the exterior surface of the exterior wall into the interior wall; and means connected to the zero switch means, the span switch means and the means for producing an output signal for making zero and span adjustments in response to actuation of the zero switch means and the span switch means, respectively.

10. The transmitter of Claim 1, wherein the interior wall separates the interior chamber into a first chamber and a second chamber.