Method for electrically calibrating the output of pressure transducers

In a pressure transducer of the type utilizing movement of a pressure responsive member to move the core of a differential transformer which is excited periodically by an oscillation circuit and produces a.c. voltages converted into a d.c. output voltage by an AC-DC conversion circuit, a calibration circuit which produces a variable calibrating voltage is provided. Calibrating the output voltage of the pressure transducer to a reference value at a reference pressure is attained by two sequential steps. In the first step, relative position between the core and the windings of the differential transformer is determined, with the reference pressure exerting on the pressure responsive member and with the calibration circuit being disconnected from the conversion circuit, so that the d.c. output voltage deviates from the reference value. In the second step, the calibration circuit is connected to the conversion circuit and the calibrating voltage is varied so that the d.c. output voltage is calibrated electrically to the reference value.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for calibrating the output of a 
pressure transducer utilizing a differential transformer and particularly 
to a method in which d.c. voltage of a pressure transducer is forcibly 
deviated from a reference value corresponding to a reference pressure and 
thereafter a calibrating voltage is added to the deviated d.c. voltage so 
that the output voltage of the pressure transducer is calibrated to the 
reference value electrically. 
Pressure transducers utilizing a differential transformer have been known 
well in the field of electrical pressure detection. Such a pressure 
transducer comprises, in general, a housing forming a pressure chamber, a 
pressure responsive member movably supported in the presence chamber and a 
differential transformer associated with the pressure responsive member. 
The transformer has a primary and secondary windings and a movable core 
which are arranged in such a manner that the movable core is moved by the 
pressure responsive member in response to the pressure exerting thereon 
and the secondary windings produce, with the primary winding being excited 
periodically, a.c. voltages corresponding to the position of the movable 
core. An oscillation circuit is connected to the primary winding for 
exciting purpose and an AC-DC conversion circuit is connected to the 
secondary windings for AC-DC voltage converting purpose. According to this 
conventional arrangement, the pressure transducer detects the pressure and 
produces d.c. output voltage indicative of the detected pressure. It is a 
matter of course that each pressure transducer must be zero-calibrated to 
produce correct d.c. output voltage. Zero-calibration for the pressure 
transducer has been attained, as disclosed in the United States Pat. No. 
3,848,180, by mechanically adjusting the position of the pressure 
responsive member fixed to the movable core. Although this conventional 
calibrating method is simple, it is difficult to attain fine 
zero-calibration on the pressure transducer designed to be small in size. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to improve the calibrating method 
for pressure transducers of the type utilizing a differential transformer. 
The calibrating method according to the present invention is improved in 
that the output of a pressure transducer is calibrated electrically. Prior 
to the electrical calibration, relative position between the movable core 
and the windings of a differential transformer associated with a pressure 
responsive member is determined, with the reference pressure exerting on 
the pressure responsive member, such that output value of the pressure 
transducer deviates from a desired reference value corresponding to the 
reference pressure. In the electrical calibration, a calibration circuit 
which produces a calibrating voltage is connected to the pressure 
transducer and the calibrating voltage is varied such that the deviated 
output value is calibrated or compensated to the reference value.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, there is illustrated in cross section the 
mechanical unit of the pressure transducer. This mechanical connection, as 
has been conventional, mainly comprises an evacuated bellows 10 and a 
differential transformer 40 which are supported by respective aluminium 
housings 20 and 22. The evacuated bellows 10 is positioned in a pressure 
chamber 24 formed by the housing 20 into which a pressure to be detected 
is introduced through a port 21. One side of the evacuated bellows 10 is 
fixed to a threaded screw 11 provided through the housing 20 so that the 
evacuated bellows 10 expands and contracts axially in response to the 
pressure exerting thereon. The other side of the evacuated bellows 10 is 
fixed axially to a rod 12 which extends through a hole 25 provided on the 
housing 20. A guide pipe 30 is tightly received in the hole 25 to axially 
surround the rod 12 and communicate with the pressure chamber 24 through 
one open end thereof. The other open end of the guide pipe 30 is sealed by 
a seal member 31. The differential transformer 40 comprises, as has been 
known well, a movable iron core 41, a primary winding 42, secondary 
windings 43 and 44 and a bobbin 45. The movable core 41 is fixed to the 
rod 12 to move axially therewith in the guide pipe 30. The primary and 
secondary windings 42, 43 and 44 are wound axially on the bobbin 45 such 
that the primary winding 42 aligns between the secondary windings 43 and 
44. The bobbin 45, supporting the windings 42, 43 and 44, is supported 
axially by the guide pipe 30. The housing 20 receives the differential 
transformer 40 in a recess 26 provided thereon and the housing 22 is fixed 
to the housing 20 by threaded screws 23 to encase the transformer 40 
therein. A leaf spring 46 is provided in the recess 26 of the housing 20 
to bias the bobbin 45 of the transformer 40 toward the housing 22 axially. 
Electrical leads 47 of the windings 42, 43 and 44 extend outside of the 
housing 22 so that the differential transformer 40 is electrically 
operated. 
Referring next to FIG. 2, there is illustrated an electric unit which is 
electrically connected to the differential transformer. The electric unit 
comprises a voltage regulation circuit 50 for regulating a voltage across 
the terminals V.sub.B and GND to a constant voltage, an oscillation 
circuit 60 for exciting the primary winding 42 of the transformer 40 
periodically, an AC-DC conversion circuit 70 for converting a.c. output 
voltages of the secondary windings 43 and 44 of the transformer 40 into a 
d.c. output voltage, and a calibration circuit 80 for calibrating the d.c. 
output voltage of the conversion circuit 70. The terminals V.sub.B and GND 
are connected to a positive and negative terminals of a d.c. electric 
power source 200 respectively, and the terminal GND is grounded. A 
terminal Vout is connected to a utilization device. 
The voltage regulation circuit 50 which is conventional in construction 
comprises a constant voltage diode 501, smoothing capacitors 502, 503 and 
504, an oscillation preventive capacitor 505, transistors 506, 507 and 
508, and resistors 509, 510, 511, 512 and 513. The voltage regulation 
circuit 50 regulates an input voltage applied across the terminals V.sub.B 
and GND to a desired constant voltage V.sub.c. The magnitude of the 
constant voltage V.sub.c supplied across positive and negative buses 51 
and 52 is determined by the resistance of the resistor 512 which is 
variable resistance type. The oscillation circuit 60 which is conventional 
in construction comprises a well-known astable multivibrator 601, 
resistors 603, 604 and 605, and transistors 606 and 607. The astable 
multivibrator 601 supplied with the constant voltage V.sub.c oscillates to 
produce pulse signals of a fixed frequency. The periodic pulse signals are 
applied to the transistor 606 through the resistor 603 so that the 
transistor 606 renders the transistor 607 ON and OFF alternately in 
response to the periodic pulses. Since the emitter-collector path of the 
transistor 607 is connected in series with the primary winding 42 of the 
transformer 40 and the resistor 605 between the positive and negative 
buses, the primary winding 42 is excited during the ON condition of the 
transistor 607. 
The differential transformer 40, with the primary winding 42 being excited 
periodically, produces a.c. output voltages across the secondary windings 
43 and 44, respectively. The frequencies of the a.c. output voltages are 
equal to the frequency of the periodic pulse signals of the oscillation 
circuit 60, while the amplitudes of the a.c. output voltages are varied in 
accordance with the position of the movable core 41. The amplitudes of the 
a.c. output voltages of the respective secondary windings 43 and 44 
increase and decrease, in the illustrated embodiment, as the movable core 
41 is moved to the right in FIG. 1, or upward in FIG. 2. The amplitudes of 
the a.c. output voltages become equal to each other only when the movable 
core 41 is held intermediate between the secondary windings 43 and 44. 
The AC-DC conversion circuit 70 comprises full-wave rectifiers 701 and 702, 
capacitors 703, 704 and 705, and resistors 706 and 707. The rectifiers 701 
and 702 are connected to the respective secondary windings 43 and 44 to 
full-wave rectify the a.c. output voltages of the transformer 40. The 
capacitor 703 and the resistor 706 constituting a smoothing circuit are 
connected in parallel with the rectifier 701 to smooth rectified output 
voltage, while the capacitor 704 and the resistor 707 constituting another 
smoothing circuit are connected in parallel with the rectifier 702 to 
smooth rectified output voltage. The capacitor 705 is connected across the 
two smoothing circuits to produce, across the terminal GND and Vout 
connected to one side of the capacitor 705, the d.c. output voltage which 
is equal to the difference between two smoothed output voltages. The other 
side of the capacitor 705 is connected to an electrical lead 803. 
The calibration circuit 80 comprises resistors 801 and 802 connected in 
series betwen the positive and negative buses 51 and 52. The resistors 801 
and 802 provides at the junction therebetween a calibrating voltage which 
is varied in dependence on the resistance of the resistor 801 of the 
variable resistance type. 
According to the mechanical and electrical connections of the pressure 
transducer described hereinabove, method to electrically calibrate the 
output of the pressure transducer is explained with the provision that the 
output of the pressure transducer is desired to be calibrated to a 
reference voltage (0 volt) in response to a reference pressure (200 mmHg). 
Prior to the electric calibration, the mechanical unit is assembled as 
shown in FIG. 1 and the electrical leads 47 extending from the mechanical 
unit are connected to the electrical unit as shown in FIG. 2. In the 
electrical unit, the electrical lead 803 connected to the capacitor 705 of 
the AC-DC conversion circuit 70 is disconnected from the junction between 
the resistors 801 and 802 of the calibration circuit 80 and grounded. The 
mechanical unit is then coupled to a reference pressure source 100 as 
shown in FIG. 1 so that the constant reference pressure is introduced into 
the presence chamber 24. The evacuated bellows 10 moves the movable core 
41 of the transformer 40 to a corresponding position in response to the 
reference pressure and the AC-DC conversion circuit 70 responsively 
produces the d.c. output voltage across the terminals Vout and GND. The 
magnitude of the d.c. output voltage, on this occasion, is dependent only 
on the position of the movable core 41 relative to the windings 42, 43 and 
44, since the electrical lead 803 is grounded. The d.c. output voltage of 
the AC-DC conversion circuit 70 can be recognized by a voltmeter 300 
connected to the terminal Vout. When the d.c. output voltage is lower than 
the reference voltage, the screw 11 is fixed to the housing 20 by an 
adhesive (not shown). When, on the contrary, the d.c. output voltage is 
equal to or higher than the reference voltage, the screw 11 is rotated 
manually to change the position of the movable core 41 until the d.c. 
output voltage becomes lower than the reference voltage. When the d.c. 
output voltage is forcibly deviated from the reference voltage, rotating 
the screw 11 is stopped and the screw 11 is fixed to the housing 20 by 
adhesive. It should be noticed in this step that, since the d.c. output 
voltage of the AC-DC conversion circuit 70 is required not to become equal 
to but to deviate negatively from the reference voltage, adjusting the 
position of the movable core 41 relative to the windings 42, 43 and 44 can 
be attained roughly. The deviation of the d.c. output voltage from the 
reference voltage should be necessarily smaller than the constant voltage 
Vc supplied across the buses 51 and 52. 
The pressure transducer, the d.c. output voltage having been deviated 
negatively from the reference value, is subjected to the electrical 
calibration. In this step the port 21 is still kept coupled to the 
reference pressure source, while the lead 803 connected to the capacitor 
705 of the AC-DC conversion circuit 70 is connected to the junction 
between the resistors 801 and 802 of the calibration circuit 80. With the 
reference pressure being applied to exert on the evacuated bellows 10 and 
the positive calibrating voltage being added to the deviated d.c. output 
voltage of the AC-DC conversion circuit 70, the resistance of the resistor 
801 is varied manually so that the positive calibrating voltage is varied. 
When the d.c. output voltage produced across the terminals Vout and GND 
becomes equal to the reference value owing to the calibrating voltage 
equal to the absolute deviation of the deviated d.c. output voltage, 
varying the resistance of the resistor 801 is stopped and the resistor 801 
is fixed by the adhesive (not shown) to fix the resistance thereof. The 
pressure source 100, the power source 200 and the voltmeter 300 must be 
disconnected, when zerocalibration is completed. It should be noticed in 
this electrical calibration that, since the change in the calibrating 
voltage can be made small relative to a large amount of adjustment on the 
resistor 801, adjusting the d.c. output voltage of the AC-DC conversion 
circuit 70 can be attained precisely with ease. The electrical calibration 
is very advantageous for the pressure transducer designed to be small in 
size. 
Provided that the pressure transducer, having been zero-calibrated as 
described hereinabove, is coupled to a variable pressure source such as 
the intake manifold of an internal combustion engine, the d.c. output 
voltage of the pressure transducer varies in proportion to the pressure 
applied thereto. The transducer output relative to the pressure is shown 
by a solid characteristic line A in FIG. 3. The phantom characteristic 
line B in FIG. 3 shows the transducer output relative to the pressure with 
the assumption that the abovedescribed electrical calibration has not been 
attained on the pressure transducer. The sensitivity of the transducer 
output relative to the pressure can be adjusted, as shown by the other 
solid characteristic line A' in FIG. 3, by varying the resistance of the 
resistor 512 in the voltage regulation circuit 50. It should be noticed 
that adjusting the sensitivity of the pressure transducer, having been 
zero-calibrated, does not cause any changes in the reference output value 
of the pressure transducer relative to the reference pressure. 
The abovedescribed embodiment can be modified as follows. 
The evacuated bellows 10 may be fixed directly to the housing 20 without 
the threaded screw 11. Provided that the screw 11 is eliminated, the 
length of the rod 12 connecting the movable core 41 and the evacuated 
bellows 10 must be designed long or short enough so that the pressure 
transducer, having not been zero-calibrated, necessarily produce the d.c. 
output voltage deviated negatively from the reference voltage in response 
to the reference pressure. 
The resistor 801 connected to the electrical lead 803 may be disconnected 
from the positive bus 51 so that the electrical lead 803 is grounded 
through the resistor 802. 
The resistor 801 may not be necessarily the variable resistance type. 
Provided that the variable resistor is not used in the calibration circuit 
80, resistors having respective constant resistances must be connected one 
by one in the calibration circuit 80 so that the calibrating voltage is 
varied to become equal to the deviated d.c. output voltage in the absolute 
value. 
The electrical unit may be designed to provide a negative calibrating 
voltage from the calibration circuit 80. Provided that the calibrating 
voltage is negative, the mechanical unit must be designed or adjusted 
prior to the electrical calibration so that the d.c. output voltage of the 
pressure transducer deviates positively from the reference voltage in 
response to the reference pressure. 
The present invention described with reference to the illustrated and 
modified embodiments can be applied to other pressure transducers in which 
other pressure responsive members such as a pressure responsive diaphragm 
are utilized.