CA1224260A - Pneumatic servo assembly for an electro-pneumatic converter - Google Patents
Pneumatic servo assembly for an electro-pneumatic converterInfo
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- CA1224260A CA1224260A CA000521404A CA521404A CA1224260A CA 1224260 A CA1224260 A CA 1224260A CA 000521404 A CA000521404 A CA 000521404A CA 521404 A CA521404 A CA 521404A CA 1224260 A CA1224260 A CA 1224260A
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- pneumatic
- assembly
- signal
- pneumatic servo
- servo assembly
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Abstract
PNEUMATIC SERVO ASSEMBLY FOR AN ELECTRO-PNEUMATIC CONVERTER
ABSTRACT OF THE DISCLOSURE
A pneumatic servo assembly (20) is provided for an electro-pneumatic control system (10) having closed loop control of a D.C. motor (12) operated from an error signal (56) developed from an electrical set point signal (4) and a pneumatic feedback signal (44).
The feedback signal (44) is made compatible by a pressure transducer (26) monitoring the pressure output signal (223 of the pneumatic servo assembly (20) and establishing an electrical signal indicative thereof which is scaled, zeroed, and amplified by an amplifier unit (46).
ABSTRACT OF THE DISCLOSURE
A pneumatic servo assembly (20) is provided for an electro-pneumatic control system (10) having closed loop control of a D.C. motor (12) operated from an error signal (56) developed from an electrical set point signal (4) and a pneumatic feedback signal (44).
The feedback signal (44) is made compatible by a pressure transducer (26) monitoring the pressure output signal (223 of the pneumatic servo assembly (20) and establishing an electrical signal indicative thereof which is scaled, zeroed, and amplified by an amplifier unit (46).
Description
~2 6 ~
~ ss~ 4537 PNEUMATIC SERVO ASSEMBLY FOR ~N ELECTRO-PNEUMATIC CONVERTER
TECMNICAL FIELD
The present invention relates to control systems for electro-pneumatic converters in general and parti-cularly to pneumatic servo assemblies for such control systems utili~ing a variable restrictiDn cam and back-pressure nozzle feeding an output bellows which also provides a feedback signal to the control system.
BACKGROU~ID ART
Control systems for electro-pneumatic converters are known. Usually a 4 to 20 milliamp electrical sig-nal is used to actuate a solenoid-like motor. The 4 to 20 milliamp lelectrical signal causes a proportionate displacement in the spring-Ioaded core of the solenoid-like motor which displacement is used to control the restriction of an associated pneumatic valve producing a pressure clange proportional to the motion of the core. ~n example of such a device may be found in U.S.
Patent No. 3,334,642 issued August 8, 1967 to P. G.
Borthwick.
There are certain disadvanta~es to the pneumatic servo assembLies of such electro-pneum~tic control systems. Fi;rstly, they are unable to hold positions on 4~6C) loss of power. Should power be removed from th~ coil, the core moves back to a position where it is in eq~1ilibrium with its associated spring. This causes the pn~umatic output signal to go off scale, resulting in the movement of control devices actuated by the electro-pneumatic system to either the fully-opened or fully-closed positions which may be catastrophic under certain circumstances. Secondly, such pneumatic servo assemblies are vibration sensitive.
Since the cores are suspended from springs which act as range and zero limiters, vibration of the core causes a variation in the pneumatic output signal. Also, there usually is no feedback signal of the pneumatic output sianal to the input of the control systems.
SUMMARY OF THE INVENTION
The present invention overcomes these problems of known electro-pneumatic control systems as well as others by, in a preferred embodiment, providing a pneumatic servo assembly for such systems which is dependent upon an electrically-driven D.C. motor to provide a variable restriction to a pneumatic nozzle, thereby providing a fail-safe device which will maintain the last electrical signal to the pneumatic assembly upon a loss of electrical power since the motor will stop in its last driven position.
The pneumatic servo assembly of the present invention preferably utilizes a D.C. motor-driven cam member to provide a variable restriction to a pneumatic backpressure nozzle thus allowing the nozzle to supply a spring-loaded bellows assembly which produces a 3 to 15 psi pneumatic output signal also providing a feedback signal to the electrical input signal.
The feecdback signal is used to produce an error signal between a set point signal determined by t:he 4 ~4~260 5 to 20 milliamp electrical input and the feedback signal of the pneumatic output as sensed by a pressure transducer changing this pneumatic feedback signal to an electrically-equivalent signal.
Thus, one object of a preferred embodiment of the 10 present invention is to provide a pneumatic assembly for an electro-pneumatic control system which will maintain the last pneumat:ic output upon a loss of electric power.
Another object of the present invention is to provide a pneumatic assembly for an electro-pneumatic control system 15 which is insensitive to vibration of the pneumatic assembly.
Yet another object of a preferred embodiment of -the present invention is to provide a pneumatic assembly for an electro-pneumatic control system which provides a feedback signal to the electronic part of the control system producing 20 an error signal clriving the restriction of the pneumatic assembly.
In accordance with the present invention, there is provided a pneumatic servo assembly for an electro-pneumatic closed loop control system providing a pneumatic output 25 signal in response to a corresponding input signal comprising variable restxiction means for producing different pneumatic output signals from the pneumatic servo assembly; motor means for moving said variable restriction means to vary the pneumatic output from the pneumatic servo assembly;
30 means for actuating said motor means in response to a control signal; means for establishing a set point signal in response to the electrical input signal indicative of a desired pneumatic output signal; means for establishing a feedback signal indicative of the pneumatic output signal; combining 35 means for comparing the set point signal with the feedback signal to establish a control signal to said actuating means.
.1. ;~ 4 Z 6 0 .
- 3a -The above and other objects of the present invention will be more clearly understood from a review oi the following detailed description of the invention when considered with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a functional block diagram of the control system of the present invention.
Fig. la is an expanded view of the motor-driven cam - and backpressure nozzle of the mechanical servo assembly of Fig. 1.
Fig. 2 is a functional block diagram of the pneumatic servo assembly of the Fig. 1 control system.
Fig. 3 is a detailed side view of the bellows and spring assembly of Fig. 2.
Fig. 4 is a detailed end view of the Fig. 3 ~Z~60 . 4 -bellows and spring assembly.
Fig. 5 is a detailed end view of the carn assembly 23 of Fig. 2.
5 DESCRIPTION OF THE PREFERRFD EM~Ol)IMENT
Referring now to the drawings wherein the showings - are made i.or purposes of illustrating the preferred embodiment: of the present invention and are not intended to limit t:he invention thereto Fi~s. 1 and la show an electro-pneumatic control ~ystem 10 wherein a D.C. motor 12 is cont:rolled by an electronically-controlled motor servo circuit 14 which is powered by a power supply 16 operated i~rom a 4 to 20 milliamp input control si~nal connected to the power supply 16 along line 18. The D.C. motor 12 is mechanically constrs;ned to a pneumatic servo assembly 20 which has a backpressure noz~le 21 variably r.estricted by a cam assembly 23 connected to and driven by the D.C. motor 12 to thus ~rovide a vari-able backpressure output along output line 22 of the pneumatic servo assembly 20 no~mally in the 3 to 15 psi output range. This 3 ~o 15 psi output is linear and corresponds to the linear 4 to 20 milliamp electrical input provided along input line 18. This same 3 to lS
psi outpul is also sent along line 24 to a pressure transducer 26 which provides a feedback signal used in determining control of the D.C. motor 12 as will be described more fully later.
To allow th~ D.C. motor 12 to be operate~ bi-directionally without the need for dual polarity volt-ages, the power supply 16 es~ablishes dual voltages V+and Vref along power lines 28 and 30 respectively.
The V+ voLtage is in the range of 6.4 volts nomi.nal and powers the motor servo circuit 14 as well as a desired position amplifier 38 and an amplifier circult 46 alon~
o input lines 31 and 33 respectively. The Vref portion of power supply 16 is transmitted along line :30 to bias up the motor servo circuitry 14 and power the pressure transducer 26 along line 32.
To establish the set point from the 4 to 20 milli-a~p inpu~ signal to which the appropriate 3 to 15 psl output will ha~e to be supplied from output line 22, the particular electrical input signal is sent along line 34 to a 10 ohm precision resistor located between the circuit co~mon at line 36 and the input to a position amplifier 38. The precision 10 ohm resistor senses the particular current level and establishes a voltage drop across itslelf with that voltage drop providin~r the counterpart voltage input to the position amplifier 38.
The position amplifier 38 raises the input signal level to a predetermined level and sends this along line 40 as a set point signal to a difference amplifier 42 compatible with the level of the feedback signal also provided to the difference amplifier 42. The difference amplifier 4~ is the first stage of the motor servo cir-cuit 14. 'Fhe second input to the difference amplifier 42 is the :Eeedback signal provided along line 44 from amplifier circuit 4~ which scales and zeroes the pressure signal provided by pressure transducer 26 which acts as the pneumatic-to-electric converter for ~he 3 to 15 psi output signal established at output line 22.
The difference amplifier 42 senses any deviation of the feedback signal from line 44 to the established set point signal 40 and ~stablishes an error signal along line 48 which is an amplified difference signal so long as such difference between set point and eedback is maintained. This amplified error signal i9 inputed into a proportional and integral controller 50 where it is integrated and scaled up or down with respect to Vref .
i~426 Thu~ bi-directional rotation of the motor 12 iR
achleved by the voltage output of the propor~ional plus integral controller 50 rising or falling below ~he volt-age reference Vref. When the output signal ls abovevoltage reference Vref , the current ~hrough the ~otor 12 will drive the motor 12 in a first rotationsl direc-tion. When the voltage output of the proportional plus - i.ntegral controller 50 is equal to Vref , no current flows through the motor and the motor is stationary. If the output voltage drops ~elow Vref , the ro~:ation of the motor 12 will reverse to a second rotational direc-tion due to the vol~age level applied to it crossing the Vref point.
Turning back now with particular reference to Fig.
1, the amplifier circuit 46 has both a zero adjustmeTIt 60 and a span adjustment 62. The zero and span adjust-ment allows ~he feedback signal to be adjusted to respond over a variety of ranges. The predominant pres-20 sure range and pressure starting or zero pOill~ that t~e feedback will be adjusted for is the 3 ~o 15 psi signal which is the standard for pneumati~ instrumentation as 4 to 20 milliamp is the standard for electrical instru-mentation. Other ranges are also available ~md may be 2S set, including any 50 per cent split range desired ~i.e., 0 per cent is 3 psi, 100 per cent is 9 psi~.
Should an electrical failure occur in t.he system 10, the D.C. motor 12 would stop with the cam 23 remain-ing in its last position to provide the same backpressure restrictiDn from the nozzle 21 to the pneumatic servo assembly 20 and the last conforming pressure output sig-nal would be maineained along outpu~ line 22 by the pneumatic servo assembly 20.
Referring now to Fig. 2, the pneumatic servo mechanism 20 is seen to include a regulator 64 whieh is connected to an air supply of unregulated high pressure 4~GO
air and acts to reduce the ~ir supply pressur~ to a constant clean, low pressure of 22 ~ 2 psi. l~e filtered and regulated air from re~ulator 64 is piped to the backpressure nozzle 21 by way of an orifice 66. AB
is known to those in the pneumatic control art:s area, ~he ~ize of the orifice 66, in conjunction wit:h the opening of the backpressure nozzle 21, act ~o determine - the air consumption as well as response time of the pneumatic servo mechanism 70.
As was previously described, ~he motor servo as-sembly 14 causes the D.C. mo~or 12 to be rota~ed in either a clockwise or counterclockwise direction which direction is dictated by a comparison of the set point and the feedback signals inputed to the control circuit 14 which acts to thus control the D.C. motor 12. The rotation of the motor 12 causes the cam assembly 23, which may be best seen at Fig . 5, to rotate with respect to the backpressure nozzle 21 causing a relative block-age or opening of the backpressure nozzle 21.
With particular reference to Fig. 5, it will beseen that the cam assembly 23, shown as the typical 3 to 15 psi output cam assembly 23, is formed as a sPiral-generated plane 68 having a notched portion 70 with a hub 72 loci~ted in the center of the spiral plane section 68. The spiral is formed to produce a linear function pressure output from output line 22 from the backpressure nozzle 21. By way of example, when the nozzle, which will alway~ seek the edge of the plane 68 aligned there-with, is positioned with point A a 3 psi output will beproduced. Similarly, at point B, a 15 psi output will be produced. Angularly linear outpu~s will be produced between points A and B. Thus, the height of ehe notch 70 is the ran~e of the output signal. The hub 72 is used to mount the cam assembly 23 ~o the shaft of the D.C. motor. Turning next to Figs. ~ through 4, it will o be seen that restricting the b`ackpressure noz~.le 21 causes an increase in backpressure which is piped to the bellow~ spring assembly 74 through line 76. There is a directly-proportional relationship between the pressure in the bellows spring assembly 74 and ~he height to which it will expand. This is dete~ined by the construction of the bellows 78 as well as the spring 80 which ir, mounted in parallel with the bellows 78.
The spring 80 acts to limit the motion of the bellows 78, thereby limiting the output range of the pneumatic output signal along line 22 to a desired ranRe which is determinable by adjusting the spring pressure of the spring 80 by either extending or loosening the spring 80 and setting it in that particular position by way of adjusting nuts 82. Thus, nuts 82 may be used to proYide fine adjustment to the particular output pressure range desired. Should a differen~ pressure range be desired, such as a :3 to 27 psi, different spring 80 having a different spring coefficient may be replaced.
With particular reference to Figs. 3 and 4, it will be seen that the backpressure nozzle 21 is rigidly-mounted to a bracket assembly 84 to which the bellows 78 and the spring 80 are also mounted. The bracket assembly 84 is then mounted to a stationary frame member 86 through a hinge 88 to thus allow rotational motion of the backpressure nozzle 21 and the bellows 7~ and spring 80 around the pivot point 88.
This mounting of the backpressure nozzle 21, bellows 78, and spring 80 as a gingle unit makes the pneumatic servo assembly impervious to ~ibration in-duced errors by allowing the entire assembly to move as a single unit in response to any vibration induced by external sources into the pneumatic servo asse~bly 20.
In operation, it will be seen that as the motor ~ ~ 4Z 6 ~
12 and cam assembly 23 rotate to variably restrict the backpressure nozzle 21, the bellows 78 will expand causing a pivoting of the previously-mentionecl assembly aro~nd pivot point 88 until the backpressure nozzle 21 reaches a position along the edge of the cam plane surface 6 producing a backpressure feedback si.gnal which will balance the set point signal and stop the rotation of the mo~r 12 and cam assembly 23. This will result in an outpu~ pressure signal along line 22 which 1~ pro-portional to the electrical input signal which originally had caused the D.C. ~otor 12 to rotate.
As may be seen, there is no mechanical c:ontact be-tween the backpressure noz~le 21 and the cam assembly 23. Therefore, the D.C. mo~or 12 need only overcome its own internal friction to rotate along with a small amount of drag on the cam assembly 23 which may be caused by the nozzle clamp 86. The nozzle clamp 86 rides on the cam assembly 23 loading it away from the motor 12 to thus take up any end play in the shaft of the motor 12.
This force is minimal and there are no forces developed which will turn the motor 12 off if the motor 12 and the cam assembly 23 do not turn clearly the backpressure output along line 22 will not change.
Certain modifica~ions and improvements Will QCCUr to those skilled in the art upon reading the foregoing Specificat:ion. It will be understood that all such improvements and modifications are deleted he~-ein for the sake oE conciseness and readability but are properly intended to be within the scope of the following claims.
~ ss~ 4537 PNEUMATIC SERVO ASSEMBLY FOR ~N ELECTRO-PNEUMATIC CONVERTER
TECMNICAL FIELD
The present invention relates to control systems for electro-pneumatic converters in general and parti-cularly to pneumatic servo assemblies for such control systems utili~ing a variable restrictiDn cam and back-pressure nozzle feeding an output bellows which also provides a feedback signal to the control system.
BACKGROU~ID ART
Control systems for electro-pneumatic converters are known. Usually a 4 to 20 milliamp electrical sig-nal is used to actuate a solenoid-like motor. The 4 to 20 milliamp lelectrical signal causes a proportionate displacement in the spring-Ioaded core of the solenoid-like motor which displacement is used to control the restriction of an associated pneumatic valve producing a pressure clange proportional to the motion of the core. ~n example of such a device may be found in U.S.
Patent No. 3,334,642 issued August 8, 1967 to P. G.
Borthwick.
There are certain disadvanta~es to the pneumatic servo assembLies of such electro-pneum~tic control systems. Fi;rstly, they are unable to hold positions on 4~6C) loss of power. Should power be removed from th~ coil, the core moves back to a position where it is in eq~1ilibrium with its associated spring. This causes the pn~umatic output signal to go off scale, resulting in the movement of control devices actuated by the electro-pneumatic system to either the fully-opened or fully-closed positions which may be catastrophic under certain circumstances. Secondly, such pneumatic servo assemblies are vibration sensitive.
Since the cores are suspended from springs which act as range and zero limiters, vibration of the core causes a variation in the pneumatic output signal. Also, there usually is no feedback signal of the pneumatic output sianal to the input of the control systems.
SUMMARY OF THE INVENTION
The present invention overcomes these problems of known electro-pneumatic control systems as well as others by, in a preferred embodiment, providing a pneumatic servo assembly for such systems which is dependent upon an electrically-driven D.C. motor to provide a variable restriction to a pneumatic nozzle, thereby providing a fail-safe device which will maintain the last electrical signal to the pneumatic assembly upon a loss of electrical power since the motor will stop in its last driven position.
The pneumatic servo assembly of the present invention preferably utilizes a D.C. motor-driven cam member to provide a variable restriction to a pneumatic backpressure nozzle thus allowing the nozzle to supply a spring-loaded bellows assembly which produces a 3 to 15 psi pneumatic output signal also providing a feedback signal to the electrical input signal.
The feecdback signal is used to produce an error signal between a set point signal determined by t:he 4 ~4~260 5 to 20 milliamp electrical input and the feedback signal of the pneumatic output as sensed by a pressure transducer changing this pneumatic feedback signal to an electrically-equivalent signal.
Thus, one object of a preferred embodiment of the 10 present invention is to provide a pneumatic assembly for an electro-pneumatic control system which will maintain the last pneumat:ic output upon a loss of electric power.
Another object of the present invention is to provide a pneumatic assembly for an electro-pneumatic control system 15 which is insensitive to vibration of the pneumatic assembly.
Yet another object of a preferred embodiment of -the present invention is to provide a pneumatic assembly for an electro-pneumatic control system which provides a feedback signal to the electronic part of the control system producing 20 an error signal clriving the restriction of the pneumatic assembly.
In accordance with the present invention, there is provided a pneumatic servo assembly for an electro-pneumatic closed loop control system providing a pneumatic output 25 signal in response to a corresponding input signal comprising variable restxiction means for producing different pneumatic output signals from the pneumatic servo assembly; motor means for moving said variable restriction means to vary the pneumatic output from the pneumatic servo assembly;
30 means for actuating said motor means in response to a control signal; means for establishing a set point signal in response to the electrical input signal indicative of a desired pneumatic output signal; means for establishing a feedback signal indicative of the pneumatic output signal; combining 35 means for comparing the set point signal with the feedback signal to establish a control signal to said actuating means.
.1. ;~ 4 Z 6 0 .
- 3a -The above and other objects of the present invention will be more clearly understood from a review oi the following detailed description of the invention when considered with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a functional block diagram of the control system of the present invention.
Fig. la is an expanded view of the motor-driven cam - and backpressure nozzle of the mechanical servo assembly of Fig. 1.
Fig. 2 is a functional block diagram of the pneumatic servo assembly of the Fig. 1 control system.
Fig. 3 is a detailed side view of the bellows and spring assembly of Fig. 2.
Fig. 4 is a detailed end view of the Fig. 3 ~Z~60 . 4 -bellows and spring assembly.
Fig. 5 is a detailed end view of the carn assembly 23 of Fig. 2.
5 DESCRIPTION OF THE PREFERRFD EM~Ol)IMENT
Referring now to the drawings wherein the showings - are made i.or purposes of illustrating the preferred embodiment: of the present invention and are not intended to limit t:he invention thereto Fi~s. 1 and la show an electro-pneumatic control ~ystem 10 wherein a D.C. motor 12 is cont:rolled by an electronically-controlled motor servo circuit 14 which is powered by a power supply 16 operated i~rom a 4 to 20 milliamp input control si~nal connected to the power supply 16 along line 18. The D.C. motor 12 is mechanically constrs;ned to a pneumatic servo assembly 20 which has a backpressure noz~le 21 variably r.estricted by a cam assembly 23 connected to and driven by the D.C. motor 12 to thus ~rovide a vari-able backpressure output along output line 22 of the pneumatic servo assembly 20 no~mally in the 3 to 15 psi output range. This 3 ~o 15 psi output is linear and corresponds to the linear 4 to 20 milliamp electrical input provided along input line 18. This same 3 to lS
psi outpul is also sent along line 24 to a pressure transducer 26 which provides a feedback signal used in determining control of the D.C. motor 12 as will be described more fully later.
To allow th~ D.C. motor 12 to be operate~ bi-directionally without the need for dual polarity volt-ages, the power supply 16 es~ablishes dual voltages V+and Vref along power lines 28 and 30 respectively.
The V+ voLtage is in the range of 6.4 volts nomi.nal and powers the motor servo circuit 14 as well as a desired position amplifier 38 and an amplifier circult 46 alon~
o input lines 31 and 33 respectively. The Vref portion of power supply 16 is transmitted along line :30 to bias up the motor servo circuitry 14 and power the pressure transducer 26 along line 32.
To establish the set point from the 4 to 20 milli-a~p inpu~ signal to which the appropriate 3 to 15 psl output will ha~e to be supplied from output line 22, the particular electrical input signal is sent along line 34 to a 10 ohm precision resistor located between the circuit co~mon at line 36 and the input to a position amplifier 38. The precision 10 ohm resistor senses the particular current level and establishes a voltage drop across itslelf with that voltage drop providin~r the counterpart voltage input to the position amplifier 38.
The position amplifier 38 raises the input signal level to a predetermined level and sends this along line 40 as a set point signal to a difference amplifier 42 compatible with the level of the feedback signal also provided to the difference amplifier 42. The difference amplifier 4~ is the first stage of the motor servo cir-cuit 14. 'Fhe second input to the difference amplifier 42 is the :Eeedback signal provided along line 44 from amplifier circuit 4~ which scales and zeroes the pressure signal provided by pressure transducer 26 which acts as the pneumatic-to-electric converter for ~he 3 to 15 psi output signal established at output line 22.
The difference amplifier 42 senses any deviation of the feedback signal from line 44 to the established set point signal 40 and ~stablishes an error signal along line 48 which is an amplified difference signal so long as such difference between set point and eedback is maintained. This amplified error signal i9 inputed into a proportional and integral controller 50 where it is integrated and scaled up or down with respect to Vref .
i~426 Thu~ bi-directional rotation of the motor 12 iR
achleved by the voltage output of the propor~ional plus integral controller 50 rising or falling below ~he volt-age reference Vref. When the output signal ls abovevoltage reference Vref , the current ~hrough the ~otor 12 will drive the motor 12 in a first rotationsl direc-tion. When the voltage output of the proportional plus - i.ntegral controller 50 is equal to Vref , no current flows through the motor and the motor is stationary. If the output voltage drops ~elow Vref , the ro~:ation of the motor 12 will reverse to a second rotational direc-tion due to the vol~age level applied to it crossing the Vref point.
Turning back now with particular reference to Fig.
1, the amplifier circuit 46 has both a zero adjustmeTIt 60 and a span adjustment 62. The zero and span adjust-ment allows ~he feedback signal to be adjusted to respond over a variety of ranges. The predominant pres-20 sure range and pressure starting or zero pOill~ that t~e feedback will be adjusted for is the 3 ~o 15 psi signal which is the standard for pneumati~ instrumentation as 4 to 20 milliamp is the standard for electrical instru-mentation. Other ranges are also available ~md may be 2S set, including any 50 per cent split range desired ~i.e., 0 per cent is 3 psi, 100 per cent is 9 psi~.
Should an electrical failure occur in t.he system 10, the D.C. motor 12 would stop with the cam 23 remain-ing in its last position to provide the same backpressure restrictiDn from the nozzle 21 to the pneumatic servo assembly 20 and the last conforming pressure output sig-nal would be maineained along outpu~ line 22 by the pneumatic servo assembly 20.
Referring now to Fig. 2, the pneumatic servo mechanism 20 is seen to include a regulator 64 whieh is connected to an air supply of unregulated high pressure 4~GO
air and acts to reduce the ~ir supply pressur~ to a constant clean, low pressure of 22 ~ 2 psi. l~e filtered and regulated air from re~ulator 64 is piped to the backpressure nozzle 21 by way of an orifice 66. AB
is known to those in the pneumatic control art:s area, ~he ~ize of the orifice 66, in conjunction wit:h the opening of the backpressure nozzle 21, act ~o determine - the air consumption as well as response time of the pneumatic servo mechanism 70.
As was previously described, ~he motor servo as-sembly 14 causes the D.C. mo~or 12 to be rota~ed in either a clockwise or counterclockwise direction which direction is dictated by a comparison of the set point and the feedback signals inputed to the control circuit 14 which acts to thus control the D.C. motor 12. The rotation of the motor 12 causes the cam assembly 23, which may be best seen at Fig . 5, to rotate with respect to the backpressure nozzle 21 causing a relative block-age or opening of the backpressure nozzle 21.
With particular reference to Fig. 5, it will beseen that the cam assembly 23, shown as the typical 3 to 15 psi output cam assembly 23, is formed as a sPiral-generated plane 68 having a notched portion 70 with a hub 72 loci~ted in the center of the spiral plane section 68. The spiral is formed to produce a linear function pressure output from output line 22 from the backpressure nozzle 21. By way of example, when the nozzle, which will alway~ seek the edge of the plane 68 aligned there-with, is positioned with point A a 3 psi output will beproduced. Similarly, at point B, a 15 psi output will be produced. Angularly linear outpu~s will be produced between points A and B. Thus, the height of ehe notch 70 is the ran~e of the output signal. The hub 72 is used to mount the cam assembly 23 ~o the shaft of the D.C. motor. Turning next to Figs. ~ through 4, it will o be seen that restricting the b`ackpressure noz~.le 21 causes an increase in backpressure which is piped to the bellow~ spring assembly 74 through line 76. There is a directly-proportional relationship between the pressure in the bellows spring assembly 74 and ~he height to which it will expand. This is dete~ined by the construction of the bellows 78 as well as the spring 80 which ir, mounted in parallel with the bellows 78.
The spring 80 acts to limit the motion of the bellows 78, thereby limiting the output range of the pneumatic output signal along line 22 to a desired ranRe which is determinable by adjusting the spring pressure of the spring 80 by either extending or loosening the spring 80 and setting it in that particular position by way of adjusting nuts 82. Thus, nuts 82 may be used to proYide fine adjustment to the particular output pressure range desired. Should a differen~ pressure range be desired, such as a :3 to 27 psi, different spring 80 having a different spring coefficient may be replaced.
With particular reference to Figs. 3 and 4, it will be seen that the backpressure nozzle 21 is rigidly-mounted to a bracket assembly 84 to which the bellows 78 and the spring 80 are also mounted. The bracket assembly 84 is then mounted to a stationary frame member 86 through a hinge 88 to thus allow rotational motion of the backpressure nozzle 21 and the bellows 7~ and spring 80 around the pivot point 88.
This mounting of the backpressure nozzle 21, bellows 78, and spring 80 as a gingle unit makes the pneumatic servo assembly impervious to ~ibration in-duced errors by allowing the entire assembly to move as a single unit in response to any vibration induced by external sources into the pneumatic servo asse~bly 20.
In operation, it will be seen that as the motor ~ ~ 4Z 6 ~
12 and cam assembly 23 rotate to variably restrict the backpressure nozzle 21, the bellows 78 will expand causing a pivoting of the previously-mentionecl assembly aro~nd pivot point 88 until the backpressure nozzle 21 reaches a position along the edge of the cam plane surface 6 producing a backpressure feedback si.gnal which will balance the set point signal and stop the rotation of the mo~r 12 and cam assembly 23. This will result in an outpu~ pressure signal along line 22 which 1~ pro-portional to the electrical input signal which originally had caused the D.C. ~otor 12 to rotate.
As may be seen, there is no mechanical c:ontact be-tween the backpressure noz~le 21 and the cam assembly 23. Therefore, the D.C. mo~or 12 need only overcome its own internal friction to rotate along with a small amount of drag on the cam assembly 23 which may be caused by the nozzle clamp 86. The nozzle clamp 86 rides on the cam assembly 23 loading it away from the motor 12 to thus take up any end play in the shaft of the motor 12.
This force is minimal and there are no forces developed which will turn the motor 12 off if the motor 12 and the cam assembly 23 do not turn clearly the backpressure output along line 22 will not change.
Certain modifica~ions and improvements Will QCCUr to those skilled in the art upon reading the foregoing Specificat:ion. It will be understood that all such improvements and modifications are deleted he~-ein for the sake oE conciseness and readability but are properly intended to be within the scope of the following claims.
Claims (11)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A pneumatic servo assembly for an electro-pneumatic closed loop control system providing a pneumatic output signal in response to a corresponding input signal comprising:
variable restriction means for producing different pneumatic output signals from the pneumatic servo assembly;
motor means for moving said variable restriction means to vary the pneumatic output from the pneumatic servo assembly;
means for actuating said motor means in response to a control signal;
means for establishing a set point signal in response to the electrical input signal indicative of a desired pneumatic output signal;
means for establishing a feedback signal indica-tive of the pneumatic output signal;
combining means for comparing the set point signal with the feedback signal to establish a control signal to said actuating means.
variable restriction means for producing different pneumatic output signals from the pneumatic servo assembly;
motor means for moving said variable restriction means to vary the pneumatic output from the pneumatic servo assembly;
means for actuating said motor means in response to a control signal;
means for establishing a set point signal in response to the electrical input signal indicative of a desired pneumatic output signal;
means for establishing a feedback signal indica-tive of the pneumatic output signal;
combining means for comparing the set point signal with the feedback signal to establish a control signal to said actuating means.
2. A pneumatic servo assembly as set forth in claim 1 wherein said feedback establishing means in-cludes a pressure transducer connected to the pneumatic servo assembly to measure the pneumatic output therefrom and establish an electrical signal representative thereof.
3. A pneumatic servo assembly as set forth in claim 2 wherein said feedback establishing means further includes an amplifier circuit having a span and zero adjustment to allow the pneumatic output from said pneumatic servo assembly to be scaled to a desired range of output pressures from a desired starting pressure.
4. A pneumatic servo assembly as set forth in claim 1 wherein said motor means is a D.C. motor having a cam assembly connected thereto for variably restrict-ing the pneumatic servo assembly depending upon the position of said cam.
5. A pneumatic servo assembly as set forth in claim 4 including a backpressure nozzle mounted proxi-mately to said cam assembly to be variably restricted by said cam assembly.
6. A pneumatic servo assembly as set forth in claim 5 wherein said cam assembly is a flat, substan-tially spiral-generated plane having a generating center mounted hub for mounting the cam assembly to said motor means.
7. A pneumatic servo assembly as set forth in claim 6 wherein said backpressure nozzle is connected to a bellows assembly to pressurize said bellows as-sembly in response to the pressure from said back-pressure nozzle.
8. A pneumatic servo assembly as set forth in claim 7 wherein said bellows assembly has an output connected to said feedback signal establishing means.
9. A pneumatic servo assembly as set forth in claim 8 wherein said bellows assembly includes an adjustable spring connected in parallel with the bellows to adjust the output from the bellows assembly.
10. A pneumatic servo assembly as set forth in claim 9 wherein said backpressure nozzle is rigidly attached to said bellows assembly to move simultaneously with said bellows assembly.
11. A pneumatic servo assembly as set forth in claim 10 wherein said backpressure nozzle and bellows assembly is attached as a unit to a single pivot point allowing the rotation of both as a unit around said pivot point in response to bellows expansion or contraction.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000521404A CA1224260A (en) | 1983-02-24 | 1986-10-24 | Pneumatic servo assembly for an electro-pneumatic converter |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US46920083A | 1983-02-24 | 1983-02-24 | |
| US469,200 | 1983-02-24 | ||
| CA000447995A CA1225450A (en) | 1983-02-24 | 1984-02-22 | Pneumatic servo assembly for an electro-pneumatic converter |
| CA000521404A CA1224260A (en) | 1983-02-24 | 1986-10-24 | Pneumatic servo assembly for an electro-pneumatic converter |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000447995A Division CA1225450A (en) | 1983-02-24 | 1984-02-22 | Pneumatic servo assembly for an electro-pneumatic converter |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1224260A true CA1224260A (en) | 1987-07-14 |
Family
ID=25670301
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000521404A Expired CA1224260A (en) | 1983-02-24 | 1986-10-24 | Pneumatic servo assembly for an electro-pneumatic converter |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1224260A (en) |
-
1986
- 1986-10-24 CA CA000521404A patent/CA1224260A/en not_active Expired
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| MKEX | Expiry |