Electricity: measuring and testing – Electrostatic field – Using modulation-type electrometer
Reexamination Certificate
1999-07-20
2001-02-13
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Electrostatic field
Using modulation-type electrometer
C324S457000, C307S130000
Reexamination Certificate
active
06188226
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric potential sensor, and more particularly, to a feedback type electric potential sensor which measures the electric potential of an object by causing the electric potential of the sensor to be equal to the electric potential of the object.
2. Description of the Related Art
Recently, many devices (for example a PPC copying machine or a laser printer, an electrostatic dust remover) utilize static electricity. In such devices, a sensor for measuring electric potential at a charged part is essential in order that the electric potential at the charged part can be maintained at a desired value.
FIG. 4
is a schematic diagram showing an example of such an electric potential sensor. This sensor detects an electric potential Vt of an object OBJ in non-contact manner. The sensor includes a pickup device Pu which calculates the difference between the electric potential Vt of the object OBJ and an electric potential Vdet at a detection node DET and controls the base current IB of transistor Tr so that the difference between the electric potential at the detection node DET and the electric potential Vt of the object OBJ becomes 0. The electric potential Vdet at the detection node DET is divided by resistances Ra and Rb (which are situated between the detection node DET and ground GND) and is output at output node OUT. This output is indicative of the electric potential of the measuring object OBJ is to be measured.
There are several drawbacks to the prior art system. Because one side of the power source HV is grounded, only either a positive electric potential or, alternatively, a negative potential can be measured. Further, due to the influence of the leakage current of the transistor Tr, and the like, it is difficult to accurately measure a 0 V electric potential which may be a threshold value in a measuring range of the sensor. For example, in a sensor used to measure a positive electric potential, a 0 V electric potential and negative electric potentials cannot be distinguished, etc.
To overcome these problems, there has been proposed an electric potential sensor shown in
FIG. 5
which can measure both positive and negative electric potentials. In this sensor, two power sources HV1 and HV
2
, and two transistors Tr
1
and Tr
2
, are provided. The mutual connection node of the power sources HV
1
and HV
2
is grounded, and resistances Rc and Rd are provided between grounded GRD and the detection node DET. As a result, both positive and negative electric potentials can be measured, and 0 electric potential can be accurately measured. However, the structure of this sensor is complex and its cost is high.
SUMMARY OF THE INVENTION
The present invention is made in view of the foregoing problems associated with the conventional electric potential sensor and provides an electric potential sensor which is capable of measuring both a positive electric potential and a negative electric potential with a simpler configuration than the prior art of FIG.
5
. The electric potential sensor is also capable of accurately measuring a 0 V electric potential.
An electric potential sensor according to the present invention comprises a power source, a first and a second potential dividing means, a ground node, a first resistance element, a second resistance element, a transistor, a pickup device, and a third resistance element. The power source includes a first output terminal having a first electric potential, a second output terminal having a second electric potential. The first and the second potential dividing means are connected in series between the first output terminal and the second output terminal. The ground node is connected to a mutual connection node of the first potential dividing means and the second potential dividing means. The first resistance element is connected between the ground node and an output node. The second resistance element is connected between the output node and a detection node. The transistor is connected between the first output terminal and the detection node. The pickup device controls a base current or a gate voltage of the transistor such that an electric potential of the detection node is equal to an electric potential of a measuring object. The third resistance element is connected between the detection node and the second output terminal.
In the above mentioned electric potential sensor, the base current of the transistor is controlled by the pickup device, thereby changing the electric potential of the detection node. When the electric potential of the detection node is made equal to the electric potential of the object being measured, the transistor is stabilized. Usually, because the electric potential at the detection node may be high, a voltage divider comprising first and second potential dividing means (typically first and second resistance elements) is provided to generate an output signal indicative of the measured potential but having a suitably lower potential. Further, because the ground node is connected to the mutual connection node of the first potential dividing means and the second potential dividing means, the measurable range of the sensor is:
-
Δ
V
[
R1
R1
+
R2
]
<
Vi
<
Δ
V
R2
R1
+
R2
wherein &Dgr;V is equal to the electric potential across the power source, R
1
is the resistance of the first resistance means and R
2
is the resistance of the second resistance means and Vi is the detected potential of the object being detected. Accordingly, whether the electric potential of the object being measured is either positive or negative, it can be measured. Further, even if the electric potential of the object being measured is a 0 V electric potential, it can be securely measured because the 0 V electric potential is within a measuring range of the sensor.
Preferably, the above mentioned first and second potential dividing means are resistors. One possible alternative is to substitute either one of the resistors with a Zener diode.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
REFERENCES:
patent: 4205267 (1980-05-01), Williams
patent: 4518924 (1985-05-01), Vosteen
Metjahic Safet
Murata Manufacturing Co. Ltd.
Nguyen Vincent Q.
Ostrolenk Faber Gerb & Soffen, LLP
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