GB1568811A

GB1568811A - Electric field meters

Info

Publication number: GB1568811A
Application number: GB3480075A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1976-11-15
Filing date: 1976-11-15
Publication date: 1980-06-04

Description

(54) ELECTRIC FIELD METERS (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66-74 Victoria Street, London, S.W.1, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to electric field meters.

It is desirable to be able to measure the strength of an electric field by means of a meter having low drift rate, no moving parts, no radio-active components, and which in general is intrinsically safe. An important requirement arises when it is desired to measure the electric field in fuel tanks during fuelling or tank cleaning. As is well-known electric charge is generated when fuel flows along a pipe and this charge can accumulate to a dangerous level in fuel tanks to cause sparking which can be highly hazardous.

It is an object of the invention to provide an electric field meter with the above features suitable for use in fuel tanks and other appropriate applications.

According to the invention an electric field meter comprises a conducting plate arranged for presentation to an electric field it is desired to measure, an amplifier having a high gain and a high input impedance to an input terminal of which the said plate is connected through a conductive path, and a feedback capacitor connected from the output of the amplifier to the said input terminal, the said feedback capacitor comprising a multi-plate air capacitor and the said path including connections to successive plates of one polarity of the capacitor so that the said path forms a transmission line to slow down any voltage transients, the arrangement being such that in operation the feedback through the said capacitor maintains the said input terminal virtually at earth potential and the voltage at the output terminal of the amplifier is proportional to the normal component of the electric field falling on the plate.

Preferably an earthed guard ring surrounds the conducting plate. Since the conducting plate is held virtually at earth potential it will be at the same potential as the surrounding wall of a fuel tank and hence does not noticeably distort the field pattern or itself increase the risk of ignition.

In embodiments of the invention the feedback capacitor may comprise a multiplate air capacitor and the path from the conducting plate to the said input terminal includes connections between successive plates of one polarity of the capacitor so that a transmission line is formed which slows down any voltage transients.

As a further safety precaution against voltage transients the input terminal of the amplifier may be connected to earth through relay contacts which are held closed except when measurements are being made.

In order that the invention may be more fully understood reference will now be made to the drawings accompanying this specification in which: Figure 1 is a circuit representation of basic features of the invention, and Figure 2 is a circuit diagram of an electric field meter embodying the invention.

Referring now to Figure 1 there is shown therein an electric field meter comprising a conducting plate 1 plate 1 positioned in an electric field E normal to the plate. The plate is surrounded by an earthed guard ring 2. Plate 1 is connected to the inverting input terminal 3 of an operational amplifier 4 the output terminal 5 of which is connected through a feedback capacitor C back to the input terminal 3. The non-inverting input terminal 6 of amplifier 4 is earthed.

Amplifier 4 has a high voltage gain which may be of the order of 105 and the circuit configuration is such that due to the feedback action of capacitor C the voltage V at terminal 3 will be extremely small and accordingly plate 1 can be considered as being virtually at earth potential. If A is the area of the probe then the charge Q induced on the probe by the field E is given by Q = AEE where iS the permittivity of the medium in which the electric field is present. For an output voltage Vo at terminal 5 which causes a charge Q to flow through capacitor C and considering plate 1 as being at earth potential, thus Vo = Q/C and accordingly the field is given by E = VoCiAE It will thus be seen that the output voltage Vo is proportional to the electric field E provided that the capacitor C is completely discharged when the electric field E is zero.

In designing a circuit embodying the principles described above it is important to ensure that voltage drift is as low as possible. Such drift is caused by current leakage across capacitor C and at the input of the amplifier. It is also important to protect the components of the meter from damage by overload, particularly due to transients caused by electric discharges in the fuel tanks and by switching of nearby electric apparatus.

Figure 2 illustrates a circuit diagram of an electric field meter in which like parts have like reference numerals to Figure 1. In Figure 2 the integrated-circuit amplifier 4B is preceded by an inverting input stage comprising a field-effect transistor 4A which is designed to have an extremely low leakage current to the gate electrode. The combination 4A and 4B together perform the function of the amplifier 4 of Figure 1 so far as its non-inverting input 3 is concerned.

The conducting plate 1 is connected to the gate 14 of transistor 4A while the source thereof is connected to earth and the drain to the non-inverting input terminal 11 of operational amplifier 4B. The feedback from the output terminal 5 of the operational amplifier is connected through a resistor R1 to a multi-plate air spaced capacitor C which constitutes the feedback capacitance.

Capacitor C is designed for minimum charge leakage and has an interleaved series of plates, alternate ones being connected to opposite lines 12 and 13. Thus in effect capacitor C has four terminals two at opposite ends of line 12 and two at opposite ends of line 13. Line 12 is connected in the lead from plate 1 to input transistor 4A and in combination with a very high impedance resistor R2 in that lead it acts as a transmission line for any transient voltages which may occur as a result of exposure of plate 1 to an electric field. The rise times of such transients will be slowed by the line allowing time for the circuit to react. Further slowing of transients may be effected by threading a ferrite bead around the lead to the gate of the transistor 4A. The capacitors C1 and C2 are connected from opposite ends of line 13 to earth, to provide current paths for the transients. As a further safety precaution a spark gap S3 is connected between plate 1 and the guard ring 2, to limit the amplitude of transients.

Reed switch S1, connected between the gate 14 of transistor 4A and the earth terminal 2, protects the transistor from excessive gate voltage. Relay S1 has contacts which are opened in normal operation by energising the coil CS1. A protective circuit (not shown) connected to the Reset line de-energises the coil if the output voltage at terminal 5 passes outside the present normal operating range.

The field-effect transistor S2 acts as a switch which is closed to discharge capacitor C when zeroing the instrument. Resistors R3 and R4 form a potential divider which supplies a fraction of the output from the amplifier 4B to the gate of transistor 4A during zeroing. This feedback reduces the effective gain of the amplifier and enables the offset adjustment to be made more easily. A time delay circuit, R5 and C4, ensures that after zeroing the reed switch Si opens before the transistor S2 switches off.

This prevents spurious charging of capacitor C.

Transistor 4C, which is similar to transistor 4A, provides compensation for the variation in characteristics of transistor 4A with temperature.

Resistor R1 is chosen to provide a good frequency performance of the meter, and is shunted by C3 to prevent high-frequency oscillations.

In use of the meter described above it will be noted that plate 1 can be mounted flush with the wall of a tank in which the field is being measured and does not need to project into the tank space.

WHAT WE CLAIM IS: 1. An electric field meter comprising a conducting plate arranged for presentation to an electric field it is desired to measure, an amplifier having a high gain and a high input impedance to an input terminal of which the said plate is connected through a conductive path, and a feedback capacitor connected from the output of the amplifier to the said input terminal, the said feedback capacitor comprising a multi-plate air capa

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Amplifier 4 has a high voltage gain which may be of the order of 105 and the circuit configuration is such that due to the feedback action of capacitor C the voltage V at terminal 3 will be extremely small and accordingly plate 1 can be considered as being virtually at earth potential. If A is the area of the probe then the charge Q induced on the probe by the field E is given by Q = AEE where iS the permittivity of the medium in which the electric field is present. For an output voltage Vo at terminal 5 which causes a charge Q to flow through capacitor C and considering plate 1 as being at earth potential, thus Vo = Q/C and accordingly the field is given by E = VoCiAE It will thus be seen that the output voltage Vo is proportional to the electric field E provided that the capacitor C is completely discharged when the electric field E is zero. In designing a circuit embodying the principles described above it is important to ensure that voltage drift is as low as possible. Such drift is caused by current leakage across capacitor C and at the input of the amplifier. It is also important to protect the components of the meter from damage by overload, particularly due to transients caused by electric discharges in the fuel tanks and by switching of nearby electric apparatus. Figure 2 illustrates a circuit diagram of an electric field meter in which like parts have like reference numerals to Figure 1. In Figure 2 the integrated-circuit amplifier 4B is preceded by an inverting input stage comprising a field-effect transistor 4A which is designed to have an extremely low leakage current to the gate electrode. The combination 4A and 4B together perform the function of the amplifier 4 of Figure 1 so far as its non-inverting input 3 is concerned. The conducting plate 1 is connected to the gate 14 of transistor 4A while the source thereof is connected to earth and the drain to the non-inverting input terminal 11 of operational amplifier 4B. The feedback from the output terminal 5 of the operational amplifier is connected through a resistor R1 to a multi-plate air spaced capacitor C which constitutes the feedback capacitance. Capacitor C is designed for minimum charge leakage and has an interleaved series of plates, alternate ones being connected to opposite lines 12 and 13. Thus in effect capacitor C has four terminals two at opposite ends of line 12 and two at opposite ends of line 13. Line 12 is connected in the lead from plate 1 to input transistor 4A and in combination with a very high impedance resistor R2 in that lead it acts as a transmission line for any transient voltages which may occur as a result of exposure of plate 1 to an electric field. The rise times of such transients will be slowed by the line allowing time for the circuit to react. Further slowing of transients may be effected by threading a ferrite bead around the lead to the gate of the transistor 4A. The capacitors C1 and C2 are connected from opposite ends of line 13 to earth, to provide current paths for the transients. As a further safety precaution a spark gap S3 is connected between plate 1 and the guard ring 2, to limit the amplitude of transients. Reed switch S1, connected between the gate 14 of transistor 4A and the earth terminal 2, protects the transistor from excessive gate voltage. Relay S1 has contacts which are opened in normal operation by energising the coil CS1. A protective circuit (not shown) connected to the Reset line de-energises the coil if the output voltage at terminal 5 passes outside the present normal operating range. The field-effect transistor S2 acts as a switch which is closed to discharge capacitor C when zeroing the instrument. Resistors R3 and R4 form a potential divider which supplies a fraction of the output from the amplifier 4B to the gate of transistor 4A during zeroing. This feedback reduces the effective gain of the amplifier and enables the offset adjustment to be made more easily. A time delay circuit, R5 and C4, ensures that after zeroing the reed switch Si opens before the transistor S2 switches off. This prevents spurious charging of capacitor C. Transistor 4C, which is similar to transistor 4A, provides compensation for the variation in characteristics of transistor 4A with temperature. Resistor R1 is chosen to provide a good frequency performance of the meter, and is shunted by C3 to prevent high-frequency oscillations. In use of the meter described above it will be noted that plate 1 can be mounted flush with the wall of a tank in which the field is being measured and does not need to project into the tank space. WHAT WE CLAIM IS:

1. An electric field meter comprising a conducting plate arranged for presentation to an electric field it is desired to measure, an amplifier having a high gain and a high input impedance to an input terminal of which the said plate is connected through a conductive path, and a feedback capacitor connected from the output of the amplifier to the said input terminal, the said feedback capacitor comprising a multi-plate air capa

citor and the said path including connections to successive plates of one polarity of the capacitor so that the said path forms a transmission line to slow down any voltage transients, the arrangement being such that in operation the feedback through the said capacitor maintains the said input terminal virtually at earth potential and the voltage at the output terminal of the amplifier is proportional to the normal component of the electric field falling on the plate.

2. The meter as claimed in Claim 1 in which a guard ring surrounds the conducting plate which guard ring is connected to a ground terminal.

3. The meter as claimed in either one of the preceding claims in which the input terminal of the amplifier is connected to a ground terminal through relay contacts which are held closed except when measurements are being made.

4. The meter as claimed in any one of the preceding claims in which the conducting plate is connected to a ground terminal through a spark gap which limits the voltage thereon.

5. The meter as claimed in any one of the preceding claims in which the conducting plate is connected to the said feedback capacitor through a high value resistor.

6. The meter as claimed in any one of the preceding claims in which the said path is surrounded by a ferrite head to further slow down voltage transients.

7. An electric field meter substantially as described herein with reference to Figure 1 of the accompanying drawings.

8. An electric field meter substantially as described herein with reference to Figure 1 and Figure 2 of the accompanying drawings.