Driven shielding capacitive proximity sensor

Abstract

A capacitive proximity sensing element, backed by a reflector driven at the same voltage as and in phase with the sensor, is used to reflect the field lines away from a grounded robot arm towards an intruding object, thus dramatically increasing the sensor's range and sensitivity.

Description

ORIGIN OF THE INVENTION

This invention was made by employees of the U.S. Government and accordingly may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

accompanyand spaced apart in substantially overlapping parallel fashion from the sensor 18 and will be considered subsequently since it goes to the heart of this invention.

Typical prior art proximity sensing of an object 20 is known to involve the use of a capacitive sensor 18 which is mounted externally of the robot arm 16 as shown in FIG. 2A. The stand-off distance for example, is in the order of 1 in. The electric field associated with this type of proximity sensor is furthermore shown by the arrows emanating from the sensor 18 to ground via the skin of the robot arm 16 as well as back to the object 20 being sensed. Also, there is an electric field from the object 20 to the grounded arm 16. Such a configuration has resulted in a sensitivity which is unacceptable for space robotic applications where detection ranges in excess of 1 foot is required so that the robot 10, for example, can be deactivated or commanded to take some type of evasive action in order to prevent a collision with the object 20.

The sensor 18 is essentially a thin sheet of conductive material driven by an electronic circuit and acts as one electrode of a capacitor 19, with the second electrode being provided by the intruding object 20 as shown in FIG. 1, for example. The capacitor 19 controls the frequency of an oscillator, not shown, so that when an object for some reason or another intrudes, the output frequency of the oscillator changes. The grounded objects in the vicinity of the conductor sheet 18 and the lead wires, not shown, between the circuit and the sheet 18 create a large fixed parasitic capacitance which reduces sensor sensitivity to the intruding object 20.

To increase the sensitivity of the proximity sensor by reducing the parasitic capacitance exhibited by the electric field shown in FIG. 2A, the present invention provides for the insertion of a second relatively thin sheet 22 of conductive material, which is substantially wider than the sensor 18, spaced apart in substantially overlapping parallel fashion between the sensor 18 and the grounded robot arm 16. This generates an electric field distribution as shown in FIG. 2B where it can be seen that the member 22 acts as a shield for the capacitive sensor 18 wherein the field concentration is centered between it and the object 20, with little, if any, of the field returning directly to ground.

In operation, the shield member 22 is driven at the same instantaneous voltage as the capacitive sensor 18, but is not frequency sensitive to nearby objects as is the sensor. Thus the sensor 18 is shielded from nearby ground, i.e. the robot arm 16, such that the capacitance between it and the arm is substantially reduced if not eliminated.

Referring now to FIG. 3, a relatively low frequency oscillator circuit operating at, for example, 20 kHz is coupled to a circuit node 26 to which the capacitive type proximity sensor 18 is connected for the configuration shown in FIG. 2B. The total capacitance 19 between the sensor 18 and the intruding object 20 is shown comprised of the capacitance C_sg of the sensor 18 to ground, i.e. the arm 16, the series combination of the capacitance C_so from the sensor 18 to the object 20 and the capacitance C_og of the object 20 to ground. The composite of these three capacitances comprise a tuning capacitance for an oscillator 24 which is configured from an operational amplifier (op amp) 28, the sensor 18, a voltage divider comprised of two fixed resistors 30 and 32, and the series resistors 34 and 36 which act as feedback resistors between the output of the op amp 28 and negative (-) and positive (+) inputs, respectively. A voltage follower circuit comprised of an operational amplifier 38 couples the instantaneous voltage at the circuit node 26 to the shield 22.

Since a relatively low frequency is generated, a quasi-static case obtains exists. Assuming that a momentary positive potential exists at circuit node 26, it can be seen that electric field lines (FIG. 2B) emanating from the sensor 18 towards the object 20 induce negative charges on the object's surface nearest the sensor. Thus that surface can be considered one plate of a capacitor 19 and the sensor 18 the other. An ungrounded conductive object, on the other hand, is charged neutral so that an equal amount of positive charge will form on the surface away from the sensor so as to ensure that there is no net electric charge on the conductor. Accordingly, the tuning capacitance for the oscillator 24 at node 26 is:

(C_so ·C_og /(C_so +C_og))+C_sg =C₁9. (1)

Since the object 20 shown in FIG. 1 is grounded, and C_og is therefore shorted, equation (1) reduces to:

C₁9 =C_sg +C_so (2)

In examining equations (1) and (2), since detection of small changes in C₁9 are desired, it becomes evident that the capacitance from the sensor to ground C_sg must be relatively small. Therefore, the shield or reflector element 22 operates to force the field lines from the sensor 18 towards the object 20 as much as possible as shown in FIG. 2B.

Considering the case where the object 20 is not grounded, it is known that:

C=Q/V (3)

It is also known that a good conductor must have the same potential everywhere on its surface. Therefore, the potential on the object 20 will be that of its farthest point from the sensor 18. If the potential on the sensor 18 is defined as V and the potential on the object as V₀, then the following relationships are obtained:

Q_i /V-V₀ =C_so and (4)

Q_i /V₀ =C_og (5)

Where Q_i is the charge induced on each side of the object. It is apparent that an object with any dimension more than a few inches in any direction forces the potential on the entire surface of the object to be very low and as experimental evidence indicates, all objects are substantially grounded.

Thus where a capacitive sensing element 18 is backed by a reflector 22 driven at the same voltage as and in phase with the sensor, the field lines will be reflected away from the grounded robot arm 16 towards the intruding object 20. This dramatically increases the range by a magnitude of at least 10, i.e. from 1 in. to 12 in., for example. Furthermore, with this technique, capacitive sensor(s) 18 can be mounted closer to grounded surfaces without any penalty in performance and modern circuit techniques employing flexible printed circuit boards can be utilized to great advantage.

Having thus shown and described what is at present considered to be the preferred embodiment of the invention, it should be noted that the same has been made by way of illustration and not limitation. Accordingly, all modifications, alterations and changes coming within the spirit and scope of the invention are herein meant to be included.

Claims

1. A capacitive type proximity sensor having improved range and sensitivity between a machine and an intruding object in the immediate vicinity of the machine, comprising:

an outer electrical conductor on said machine forming one electrode of a sensor capacitor, the an other electrode comprising said intruding object, said outer electrical conductor comprising a first thin sheet of conductive material;

an intermediate electrical conductor located between said outer electrical conductor and said machine and being of a size substantially larger than said outer electrical conductor to act as a shield for reducing the parasitic capacitance between said outer electrical conductor and said machine, said intermediate electrical conductor comprising a second thin sheet of conductive material substantially wider than said first outer electrical conductor;

said outer and intermediate conductors attached to a surface on said machine with no gap between the conductors wherein said conductors are spaced apart in substantially overlapping parallel fashion, and no gap between said surface and said intermediate conductor and said outer and intermediate conductors coplanar with each other and said surface of said machine, said surface of said machine acting as a ground plane, with said intermediate electrical conductor and said surface of said machine spaced apart in substantially overlapping parallel fashion;

first circuit means for coupling in phase the instantaneous voltage at said outer electrical conductor to said intermediate electrical conductor;

second circuit means coupled to said outer electrical conductor and being responsive to the capacitance of said sensor capacitor for generating a control signal to said machine.

2. The sensor of claim 1 wherein said first circuit means comprises a voltage follower circuit comprised of an operational amplifier.

3. The sensor of claim 2 wherein said second circuit means comprises a signal generator having an output signal indicative of the capacitance of said sensor capacitor.

4. The sensor of claim 3 wherein said signal generator comprises a variable frequency oscillator.

5. The sensor of claim 1 wherein said machine comprises a robotic apparatus.

6. The sensor of claim 5 wherein said robotic apparatus comprises a movable member of said robotic apparatus.

7. The sensor of claim 6 wherein said movable member comprises an articulated member.

8. The sensor of claim 7 wherein said articulated member comprises a robotic arm.