United States Patent
`
`[19]
`
`Wheeler et a1.
`
`USOOS341036A
`Patent Number:
`
`[11]
`
`5,341,036
`
`[45] Date of Patent:
`
`Aug. 23, 1994
`
`[54] TWO HAND OPERATED MACHINE
`CONTROL STATION USING CAPACITIVE
`PROXIMITY SWITCHES
`
`Assistant Examiner—Fritz M. Fleming
`Attorney, Agent, or Firm—Larry I. Golden; D. Russell
`Stacey; Larry T. Shrout
`
`[75]
`
`Inventors: Keith D. Wheeler; Stanley H.
`Edwards, Jr., bOth Of Raleigh, NC'
`Square D Company, Palatine, 111-
`[73] Assignee:
`[21] App]. No.: 805,422
`_
`Dec. 119 1991
`[22] Filed:
`[51]
`Int. Cl.5 ............................................... H01H 9/26
`[52] us. Cl. .................................... 307/328; 361/ 179;
`361/189; 340/573; 192/131 R; 192/129 A
`[58] Field of Search ................................ 307/ 1 16—1 18,
`307/308, 326, 328, 140, 141.4; 361/179—181,
`189, 280; 200/600, 61-35; 340/551, 552, 561,
`563, 573; 328/5, 7, 1; 192/129 R432 R
`References Cited
`US“ PATENT DOCUMENTS
`3,089,985
`5/1963 Camfleld et a1.
`................... 361/ 189
`
`3:322:33 1323;; nfifeflfeld -------
`-- ggéjfig
`..
`er ...........
`,
`,
`
`.. 361/189
`3,895,269
`7/1975 Geremia
`6/1977 H012 ................ 307/116
`4,031,408
`
`.. 361/189 X
`4,074,602 2/1973 Brower .....
`
`4,412,268 10/1983 Dassow ........... 361/131
`
`.. 307/116
`4,767,940
`8/1988 Tuttle ........
`.. 307/139
`5,168,173 12/1992 Windsor ....
`5,212,621
`5/1993 Panter ................................. 361/181
`
`[56]
`
`ABSTRACT
`[57]
`A machine operator control station circuit which uses
`capacitive proximity switches to replicate the function
`of mechanical switches used in prior art control sta-
`tions. A monitoring circuit for monitoring the “on”
`state of the capacitive proximity switches used in a two
`hand industrial machine control station is disclosed. The
`circuit also reduces the chance of unintended Operation
`of the machine being controlled as a result of emitted or
`conducted radio frequency interference which might be
`detected by the capacitive proximity switches. An isola-
`tion transformer provides an isolated power supply for
`the monitoring circuit. The monitoring circuit includes
`a first capgcitivefpfloximity switch for dfitecting the
`presence 0 one o t 6 machine operator’s ands, and a
`second capacitive proximity switch for detecting the
`presence of the other of the machine operator’s hands.
`Each proximity switch has a monitoring circuit for
`-
`-
`-
`-
`-
`-
`momionflg the letngth of Ema the mommy .SW‘FCh
`“man‘s "1 the “mated 0‘ °“ State- The m°mt°mg
`circuits of the first and second proximity switches are
`connected together such that both switches must be
`activated within a set time window in order to activate
`the machine control circuit.
`
`Primary Examiner—A. D. Pellinen
`
`10 Claims, 6 Drawing Sheets
`
`i
`
`r35
`
`
`
` EUSWIALMACHINE
`
`13
`
`1
`
`APPLE 1010
`
`APPLE 1010
`
`1
`
`

`

`US. Patent
`
`Aug. 23, 1994
`
`Sheet 1 of 6
`
`5,341,036
`
`...............
`
` 615131315)
`
`-ACH|NE
`
`INDUSTRIAL
`
`2
`
`

`

`US. Patent
`
`Aug. 23, 1994
`
`Sheet 2 of 6
`
`5,341,036
`
` INDUSTRIAL
`
`MACHINE
`
`13
`
`FIG. 2
`
`3
`
`

`

`US. Patent
`
`Aug. 23, 1994
`
`Sheet 3 of 6
`
`5,341,036
`
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`US. Patent
`
`Aug. 23, 1994
`
`Sheet 4 of 6
`
`5,341,036
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`

`

`US. Patent
`
`Aug. 23, 1994
`
`Sheet 5 of 6
`
`5,341,036
`
`RF
`FIELD
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`10
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`

`

`US. Patent
`
`Aug. 23, 1994
`
`Sheet 6 of 6
`
`5,341,036
`
`
`
`7
`
`

`

`1
`
`5,341,036
`
`TWO HAND OPERATED MACHINE CONTROL
`STATION USING CAPACITIVE PROXIlVIITY
`SWITCHES
`
`FIELD OF THE INVENTION
`
`The present invention relates to industrial machine
`controls which require two hand operation to meet
`O.S.H.A. standards, and particularly to industrial ma-
`chine controls using capacitive proximity switches as a
`means for the machine operator to activate the machine.
`BACKGROUND OF THE INVENTION
`
`O.S.H.A. requirements for many industrial machine
`controls specify that the operator must use both hands
`to initiate operation of the machine. This requirement
`was established to prevent possible hand injuries to the
`machine operator. Most machine control stations of this
`type employ two palm button mechanical switches. The
`two palm buttons must close the operating circuit to
`cause the machine to operate. The force required to
`press the palm buttons is variable with each palm but-
`ton, however, it is sufficient enough to give the operator
`a “tactile feel” indicating to him that the switch has
`been activated. U.S. Pat. No. 4,412,268 to Dessow dis-
`closes an industrial machine control station employing
`two proximity switches which replicate the function of
`the more
`conventional palm button mechanical
`switches for controlling an industrial machine. In Das-
`sow’s device the machine operator is required to touch
`a sensing plate to operate the machine. The control
`circuit of Dessow does not provide for proximity
`switch failure monitoring. It is also noted that the ma-
`chine control station disclosed in the patent to Dassow
`has output contacts which are connected in series with
`the line power to the machine being controlled. As a
`result, the high current carried by the output contacts
`significantly increases the chance of welding such
`contacts closed which could result in the machine being
`controlled continuing to run after the operator has re-
`moved his hands from the operator station. Dassow’s
`output circuit is also controlled by an anti-cheat timer
`which would appear to provide a pulsed output to the
`machine. A pulsed output would require adjustment to
`match the machine’s operating time cycle. The patent to
`Dassow also discloses the provision of proximity
`switches which are tuned to different frequencies for
`the purpose of preventing “inadvertent operation as a
`consequence of operating the first switch”, i.e. unin-
`tended activation of one of the two proximity switches
`caused by interference from the other switch. However,
`the patent to Dassow does not address or even recog-
`nize the problem of unintended simultaneous activation
`of both capacitive proximity switches which is caused
`by the presence of emitted or conducted radio fre-
`quency interference.
`SUMMARY OF THE INVENTION
`
`A first object of the present invention is to provide a
`more ergonomic operator station by eliminating the
`physical contact pressure required to activate prior art
`machine controls of the type using two palm button
`switches. This is achieved by incorporating two capaci-
`tive proximity switches, spaced apart one from the
`other at a distance comfortable for the machine opera-
`tor, and mounted in a machine control station. The
`capacitive proximity switches do not require physical
`contact by the machine operator to be activated or
`
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`turned “on”. To activate the capacitive proximity
`switch the machine operator places his hand in the
`sensing field of the switch. Therefore,
`the machine
`operator is not subjected to the repetitive pressing of
`palm button switches required with prior art devices.
`Since there is no physical pressure required to acti-
`vate a capacitive proximity switch, the machine opera-
`tor will have no tactile feedback to indicate that the
`
`switch has been activated. Accordingly, a second ob-
`ject of the present invention is to provide an indicator
`light source in the capacitive proximity switch circuit
`for providing the machine operator with a visual indica-
`tion that the capacitive proximity switch has been acti-
`vated. In the preferred embodiment of the present in-
`vention, this indicator light is placed at a point in the
`machine control station that is easily visible to the ma-
`chine operator.
`A third object of the present invention is to provide a
`capacitive proximity switch monitoring circuit which
`will prevent the machine control station of the present
`invention from operating 'the machine to which it is
`connected if either capacitive proximity switch should
`fail in the “on” state, or have an object placed in its
`sensing field for an extended period of time. In the
`preferred embodiment of the present invention, a moni-
`toring circuit is provided for each of the two capacitive
`proximity switches. Each monitoring circuit includes an
`interval timer relay which monitors the length of time
`the capacitive proximity switch is in the “on” state. The
`interval timer relay has a normally open contact which
`controls an output control relay. The output control
`relay provides normally open and normally closed
`contacts to the input circuit of the machine control. If
`the capacitive proximity switch is “on” for a period of
`time longer than the interval timer relay is programmed
`for, its normally open contact will open. This in turn
`will deenergize the output control relay causing its
`normally open contact to open and thereby open the
`output circuit of the machine control station. The nor-
`mally open contact of the interval timer relay will re-
`main open until the interval timer relay has been reset.
`The interval timer relay can only be reset when the
`capacitive proximity switch is returned to the “off"
`state or a failed “on” switch has been replaced with a
`new capacitive proximity switch. Therefore, if someone
`places an object in the proximity sensing field or the
`capacitive proximity switch should fail in the “on” state
`the interval timer relay normally open contacts will
`open causing the output control relay to deenergize and
`therefore 'open the machine control output circuit pro-
`hibiting machine operation.
`Each of the sensor monitoring circuits are electrically
`coupled together such that the machine operator must
`activate both capacitive proximity switches within a
`specific time window in order to initiate the machine
`operation. This time window is determined by the first
`interval timer relay to be activated. If both capacitive
`proximity switches are not activated within this time
`window they must both be returned to the “off” state to
`reset their respective interval timer relays. If both inter-
`val timer relays are not reset the machine control output
`circuit cannot be activated.
`A fourth object of the machine control station of the
`present invention is to provide an isolated output from
`the machine control station to the machine such that
`any voltage required to operate the machine may be
`controlled by the operator at the machine control sta-
`
`8
`
`8
`
`

`

`3
`tion. This is accomplished by providing isolated nor-
`mally closed and normally open output terminals associ-
`ated with each capacitive proximity switch for the con-
`trol power to the machine being operated. The isolated
`outputs also permit this machine control circuit to be
`retrofitted to machines having O.S.H.A. approved anti-
`tie down and anti-repeat circuits.
`A fifth object of the machine control station of the
`present invention is to provide a continuous, non-pulsed
`output to the machine being controlled. This is accom-
`plished by using a timer bypass relay to control the
`isolated output contacts. The timer bypass relay is ener-
`gized only when both capacitive proximity switches are
`activated within the time window. The timer bypass
`relay will remain energized as long as both capacitive
`proximity switches remain activated.
`A sixth object of the present invention is to provide a
`machine control station employing capacitive proximity
`switches which has an increased immunity to interfer-
`ence from most conducted or emitted radio frequencies.
`It has been determined that a capacitive proximity sen-
`sor can be activated by conducted or emitted radio
`frequencies that are at or near the internal oscillator
`frequency of the sensor. Certain types and shapes of
`enclosure enhance the detecting capabilities of capaci-
`tive sensors and thereby increase the probability of
`inadvertent activation by radiated radio frequency in-
`terference. In the machine control station of the present
`invention, the possibility of radiated radio frequency
`interference activating both sensors simultaneously is
`minimized by providing for the two capacitive proxim-
`ity switches having internal operating frequencies
`which are sufficiently separated as discussed in greater
`detail in the detailed description of the preferred em-
`bodiment hereinbelow. Capacitive proximity switches
`may also be inadvertently activated by conducted radio
`frequency interference. The possibility of simultaneous
`activation of the two capacitive proximity switches by
`conducted radio frequency interference is minimized in
`the present invention by the provision of an isolation
`transformer placed in its power input circuit. In either
`case the status of the capacitive proximity switch is
`displayed to the operator by the indicator lights pro-
`vided by the present invention.
`Other features and advantages of the invention will
`become apparent to those skilled in the art upon review
`of the following detailed description, claims and draw-
`ings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic of an AC capacitive proximity
`switch monitoring and indicator light circuit for a ma-
`chine operator control station constructed in accor-
`dance with the present invention.
`FIG. 2 is a schematic of a second embodiment of the
`
`capacitive proximity switch monitoring circuit of a
`machine operator control station constructed in accor-
`dance with the present invention for use with DC cir-
`cuits.
`FIG. 3 is a graph of the relationship of the frequency
`sensitivity curves of the two capacitive proximity
`switches utilized by the present invention with respect
`to a radio frequency field strength expressed in volts per
`meter.
`
`FIG. 4 is a cross-sectional view of a portion of a
`machine operator control station constructed in accor-
`dance with the present invention which is illustrative of
`
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`5,341,036
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`4
`the relative positioning of the enclosure and sensing pad
`embodied by the control station.
`FIG. 5 is a graph of the relationship of the frequency
`sensitivity with respect to the field strength, expressed
`in volts per meter, of a capacitive proximity switch
`having its sensing plate positioned in approximately the
`same plane as the surface of the enclosure of a machine
`operator control station constructed in accordance with
`the present invention as compared to having the sensing
`plate positioned above (outboard) the surface of the
`enclosure or below (inboard) the surface of the enclo-
`sure as it is positioned in the preferred embodiment of
`the surface of the enclosure or below (inboard) the
`surface of the present invention.
`FIG. 6 is a graph of the relationship of the frequency
`sensitivity with respect to the field strength, expressed
`in volts per meter, of the two capacitive proximity
`switches having their sensing plates positioned below
`(inboard of) the plane of the surface of the enclosure, as
`they are positioned in the preferred embodiment of a
`machine operator control station constructed in accor-
`dance with the present invention, as compared to hav-
`ing their sensing plates positioned in approximately the
`same plane as the surface of the enclosure.
`FIG. 7 is an isometric View of an enclosure for a
`machine operator control station constructed in accor-
`dance with the present invention.
`Before one embodiment of the invention is explained
`in detail, it is to be understood that the invention is not
`limited in its application to the details of construction
`and description or illustrated in the drawings. The in-
`vention is capable of other embodiments and of being
`practiced or being carried out in various other ways.
`Also, it is to be understood that the phraseology and
`terminology used herein is for the purpose of descrip—
`tion and should not be regarded as limiting.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`
`An AC electrical circuit 10 for a two hand industrial
`machine operator control station having capacitive
`proximity switches which replicate the function of palm
`button mechanical switches used in prior art control
`stations is shown in FIG. 1. The circuit 10 is designed to
`prevent an industrial machine 13 from operating if ei-
`ther of the two capacitive proximity switches 14, 42
`should fail in the “on” state, or if an object is placed in
`the sensing field of either capacitive proximity switch
`for an extended period of time. The circuit 10 is also
`designed to reduce the chance of inadvertent simulta-
`neous activation of the capacitive proximity switches 14
`and 42 by an emitted or conducted radio frequency.
`Referring to FIG. 1, the circuit 10 includes an isola-
`tion transformer 12 having terminals L1 and L2 for
`connecting to an external AC power source and a pri-
`mary 12’ and secondary 12”. The isolation transformer
`12, through the secondary 12", provides an isolated AC
`power supply to the circuit 10, a first AC 2-wire capaci-
`tive proximity switch 14 which can be activated (closed
`or placed in the “on” state) by the presence of one of the
`machine operator’s hands within its sensing field, a
`capacitive proximity switch monitor comprising a first
`interval timer relay 18 having a normally open electri-
`cal contact 22, a first output control relay 26 having a
`first normally open electrical contact 30, a second nor-
`mally open electrical contact 34 and a normally closed
`electrical contact 36, a first indicator light source 38, a
`second AC 2-wire capacitive proximity switch 42
`
`9
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`9
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`

`

`5
`which can be activated by the presence of the machine
`operator’s other hand within its sensing field, a second
`interval timer relay 46 having a normally open electri-
`cal contact 50, a second output control relay 54 having
`a first normally open electrical contact 58, a second
`normally open electrical contact 62 and a normally
`closed electrical contact 64, a second indicator light
`source 66, and a timer bypass relay 70 having a first
`normally open electrical contact 74 and a second nor-
`mally open electrical contact 78.
`Still referring to FIG. 1, the first capacitive proximity
`switch 14 is connected in series with the first interval
`
`timer relay 18 such that the relay 18 is energized when
`the first capacitive proximity switch 14 is activated or
`turned “on” by the presence of one of the machine
`operator’s hands. The first interval timer relay 18 moni-
`tors the length of time the first capacitive proximity
`switch 14 is activated, or in the “on” state. When the
`first interval timer relay 18 is energized, its normally
`open contact 22, which is connected in series with and
`between the first capacitive proximity switch 14 and the
`first output control relay 26,
`is closed and remains
`closed for a preselected interval of time; however,
`contact 22 will immediately open if relay 18 becomes
`deenergized. Closing contact 22 energizes the first out-
`put control relay 26 which causes its first and second
`normally open contacts 30 and 34, respectively, to close
`and its normally closed contact 36 to open. The first
`indicator light source 38, a neon light in the preferred
`embodiment, is connected in series across the line with
`the capacitive proximity switch 14 and is energized
`when the first capacitive proximity switch 14 is acti-
`vated and remains illuminated as long as the first capaci-
`tive proximity switch 14 is in the “on” state.
`Still referring to FIG. 1, the second capacitive prox-
`imity switch 42 is connected in series with the second
`interval timer relay 46 such that the second interval
`timer relay 46 is activated when the second capacitive
`proximity switch 42 is activated or turned “on” by the
`presence of the other of the machine operator’s hands
`within its sensing field. The second interval timer relay
`46 monitors the length of time the second capacitive
`proximity switch 42 is activated, or in the “on” state.
`When the second interval timer relay 46 is activated its
`normally open electrical contact 50, which is connected
`in series with and between the second capacitive prox-
`imity switch 42 and the second output control relay 54,
`is closed and remains closed for a preselected interval of
`time; however, contact 50‘ will immediately open if
`relay 46 becomes deenergized. Closing contact 50 ener-
`gizes the second output control relay 54, causing its first
`and second normally open contacts 58 and 62, respec-
`tively, to close and its normally closed contact 64 to
`open. The second indicator light source 66, a neon light,
`is also energized when the second capacitive proximity
`switch 42 is activated and remains illuminated as long as
`the second capacitive proximity switch 42 is in the “on”
`state.
`
`The first normally open contact 30 of the first output
`control relay 26 and the first normally open contact 58
`of the second output control relay 54 are connected in
`series with the timer bypass relay 70. The first normally
`open contact 74 of the timer bypass relay 70 is con-
`nected in parallel with the normally open contact 22 of
`the first interval timer relay 18 and the second normally
`open contact 78 of the timer bypass relay 70 is con-
`nected in parallel with the normally open contact 50 of
`the second interval timer relay 46.
`
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`6
`The second normally open contacts 34 and 62 of first
`and second output control relays 26 and 54 respectively
`are the isolated output terminals for connecting the
`machine control station to the circuitry of the industrial
`machine 13 to be operated. The normally closed
`contacts 36 and 64 of first and second output control
`relays 26 and 54, respectively, are isolated outputs for
`connecting to an O.S.H.A. approved anti-tie down cir-
`cuit.
`
`It will be apparent to those skilled in the art that if a
`machine operator activates both capacitive proximity
`switches 14 and 42 within the preselected time interval
`determined by the first interval timer relay, 18 or 46, to
`be activated, the first and second output control relays
`26 and 54 respectively will be energized, closing the
`first normally open contacts 30 and 58 of the first and
`second output control relays 26 and 54, respectively,
`thereby energizing timer bypass relay 70. Energizing
`the timer bypass relay 70 closes its first and second
`normally open contacts 74 and 78, respectively, thereby
`keeping the first and second output control relays 26
`and 54, respectively, energized when the contacts 22
`and 50 open after the expiration of the time intervals
`determined by the relays 18 and 46, respectively. There-
`after, as long as both first and second capacitive proxim-
`ity switches 14 and 42, respectively, are simultaneously
`held in the “on” state,
`the second normally open
`contacts 34 and 62 of first and second output control
`relays 26 and 54 respectively will be closed and the
`normally closed contacts 36 and 64 of first and second
`output control relays 26 and 54 respectively will be
`open permitting a non-pulsed control signal to be sent to
`the industrial machine 13.
`It will also be apparent to those skilled in the art that
`if the machine operator does not activate both capaci-
`tive proximity switches 14 and 42 within the time win-
`dow defined by the first interval timer relay to be acti-
`vated, the first interval timer to be energized will cause
`its associated normally open contact 22 or 50 to open
`thereby deenergizing the associated first or second out-
`put control relay 26 or 54 respectively. This will in turn
`cause the first and second normally open contacts and
`the normally closed contact associated with the deener-
`gized output control relay 26 or 54 to return to their
`normal state, thereby preventing the industrial machine
`from operating.
`The interval timer relays 18 and 46 can only be reset
`by deactivating or turning off their respective capaci-
`tive proximity switches 14 and 42. It will be apparent to
`those skilled in the art that if either of the interval timer
`relays 18 or 46 is not reset after the relay 70 has been
`deenergized, the associated output control relay 26 or
`54, respectively, will remain deenergized while their
`respective associated normally open isolated output
`contacts 34 or 62 will not close nor will their respective
`normally closed contacts 36 or 64 open, thus the ma-
`chine 12 is prevented from operating.
`It will be noted that in the preferred embodiment the
`preselected time intervals during which contacts 22 and
`50 remain closed after the interval timer relays 18 and
`46, respectively, are energized are determined by the
`selection and adjustment of the timer relays 18 and 46
`and for most applications the intervals will be set to be
`the same for both relays.
`An alternate embodiment of the present invention for
`use in DC circuits is shown in FIG. 2. The circuit 82
`works in the essentially same manner as the AC moni-
`toring circuit 10 of FIG. 1 and uses the many of the
`
`10
`
`10
`
`

`

`5,341,036
`
`7
`same components described in the first embodiment
`with the following exceptions. A DC power supply 86
`provides DC power for the monitoring circuit 82. The
`first AC 2-wire capacitive proximity switch 14 of the
`previously described embodiment is replaced with a
`first DC 3-wire capacitive proximity switch 90. The
`first indicator light source 38 of the previously de-
`scribed embodiment is replaced with a first indicator
`light source 94, and in this embodiment is preferably a
`light emitting diode (LED). The second AC 2-wire
`capacitive proximity switch 42 of the previously de-
`scribed embodiment is replaced with a second DC 3-
`wire capacitive proximity switch 98. The second indica-
`tor light source 66 of the previously described embodi-
`ment is replaced with a second indicator light source
`102, an LED.
`It has been empirically determined that capacitive
`proximity sensors must be enclosed in a metallic hous-
`ing 135, partially shown in FIG. 4, to prevent activation
`by radio frequency fields above 27 Mhz. It has further
`been empirically determined that capacitive proximity
`switches are subject to activation, i.e. being turned on,
`by emitted radio frequencies having field strengths
`above a threshold level if the metallic enclosure in
`
`which the proximity switches are installed is un-
`grounded. It will be noted that the one side of the out-
`put of the isolation transformer 12 is electrically con-
`nected to the metallic housing 135, partially illustrated
`in FIG. 4, which is grounded in the preferred embodi-
`ment, as shown in FIG. 1. It will be understood that
`because the proximity switches and the enclosure 135
`are at the same potential, the enclosure 135 will act as a
`shield to radio frequency interference with the proxim-
`ity switches 14 and 42, so long as the enclosure is
`grounded. However, if the enclosure is ungrounded it
`will act as an antenna and an electrical potential can be
`induced in the enclosure 135 in the presence of a radio
`frequency field. Because it is anticipated that a user of
`an operator station constructed in accordance with the
`present invention might fail to provide a ground for the
`enclosure or the connection to ground might unknow-
`ingly become broken, means are provided, as described
`in greater detail hereinbelow, for preventing simulta-
`neous activation of the proximity switches 14 and 42 in
`the presence of radio frequency fields which could be
`expected to be present. It will be appreciated by those
`skilled in the art that fields in the frequency range of
`concern could be produced by a radio transmitter in the
`AM band located near the operating station of the pres-
`ent invention. In addition, it is anticipated that the pres-
`ent invention will be used in industrial environments
`where devices such as adjustable frequency drives will
`be operating which could produce fields in the fre-
`quency range of concern.
`FIG. 3 is a graph comparing the frequency sensitivity
`curve 106 of the first capacitive proximity switch 14 and
`the frequency sensitivity curve 110 of the second capac-
`itive proximity switch 42 to an emitted radio frequency
`having the field strengths plotted in the graph. The
`horizontal axis of the graph is a function of frequency
`and the vertical axis of the graph expresses the field
`strength of the emitted radio frequency in volts per
`meter. The horizontal line 114 indicates an emitted
`
`radio frequency having a field strength equivalent to 10
`volts/meter. It will be appreciated by those skilled in
`the art that the area above and including the curves 106
`and 110 represent the conditions under which the ca-
`pacitive proximity switches 14 and 42, respectively, are
`
`8
`subject to activation in the presence of an emitted radio
`frequency. The two points 118 and 118’ on the fre-
`quency sensitivity curve 106 indicate the intersection of
`the frequency curve 106 of the first capacitive proxim-
`ity switch 14 with the 10 volts/meter field strength line
`114. The two points 122 and 122’ on the frequency
`sensitivity curve 110 indicate the intersection of the
`frequency sensitivity curve 110 of the second capacitive
`proximity switch 42 with the 10 volts/meter field
`strength line 114. The points 126 and 130 on the fre-
`quency sensitivity curves 106 and 110, respectively,
`indicate the point of maximum sensitivity of the first and
`second capacitive proximity switches 14 and 42, respec-
`tively, to an emitted radio frequency. It has been deter-
`mined that the points 126 and 130 correspond approxi-
`mately, if not identically, with the internal operating
`frequencies ’of the capacitive proximity sensors 14 and
`42, respectively. The frequency sensitivity curves of a
`given model proximity switch can be empirically deter-
`mined. Accordingly, in the preferred embodiment of
`the present invention, the first and second capacitive
`proximity switches 14 and 42 are selected such that the
`sensitivity curves 106 and 110 of the first and second
`capacitive proximity switches 14 and 42, respectively,
`do not overlap any point below the 10 volt/meter
`strength line 114 and in an embodiment constructed in
`accordance with applicants’ best mode are spaced apart
`by approximately 75 Kilohertz at points 118 and 122
`where sensitivity curves 106 and 110, respectively, in-
`tersect the 10 volts/meter field strength line 114. It will
`be appreciated by those skilled in the art that, as the
`sensitivity of the first capacitive proximity switch 14
`increases from point 118 to point 126 on curve 106 and
`the sensitivity of the second capacitive proximity
`switch 42 increases from point 122 to point 130 on curve
`110, the required bandwidth, i.e. the horizontal distance
`(measured in terms of frequency) between the curves
`106 and 110, indicated by diagonal lines 134, of an emit-
`ted radio frequency which could cause unintended and
`simultaneous activation of both the first and second
`capacitive proximity switches 14 and 42, respectively,
`also increases. It has been empirically determined that
`capacitive proximity sensors having the characteristics
`illustrated by the graphs in FIG. 3 are an Efector Model
`KB-ZOZO-ABOW proximity switch having an internal
`frequency of approximately 300 Khz, available from
`Efector, Inc., Exton, Pa., and a Square D Class 9006
`Catalog No. DPJA21 proximity switch having an inter-
`nal operating frequency of approximately 1400 Khz,
`available from authorized distributors of the Square D
`Company, Palatine, Ill.
`Referring to FIG. 3, the method of selecting. the
`capacitive proximity switches 14 and 42 will include the
`steps of selecting a first proximity switch 14 having a
`frequency sensitivity curve 106 in which the bandwidth
`118/118’ at a preselected field strength, 10 Volts/Meter
`in the preferred embodiment, is relatively (i.e. consider-
`ing such factors as costs and availability) narrow, and
`selecting a second proximity switch 42 having a differ-
`ent (greater in the example illustrated in FIG. 3) internal
`operating frequency than the first switch 14, and having
`a frequency sensitivity curve 110 in which the band-
`width 122/122’ at a preselected field strength,
`10
`Volts/Meter in the preferred embodiment, is also rela-
`tively narrow, such that the sensitivity ranges, i.e. the
`points above and including the curves 106 and 110, do
`not intersect below a preselected field strength, 10
`Volts/Meter in the preferred embodiment, and the
`
`5
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`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`50
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`55
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`60
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`65
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`11
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`11
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`5,341,036
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`9
`bandwidth 118/122 at a preselected field strength, 10
`Volts/Meter in the preferred embodiment, between the
`sensitivity curves 106 and 110 below their point of inter-
`section is at least a threshold level. It will be appreciated
`by those skilled in the art that the threshold field
`strengths and bandwidth (i.e. frequency difference) of
`118 and 122 are a function of the environment in which
`
`the operator station of the present invention is expected
`to be safely utilized. It has been determined tha