GB2286247A

GB2286247A - Capacitive position detection

Info

Publication number: GB2286247A
Application number: GB9501344A
Authority: GB
Inventors: Neil Gershenfeld
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 1994-02-03
Filing date: 1995-01-24
Publication date: 1995-08-09
Also published as: JPH07295735A; GB9501344D0; DE19503203A1; FR2715741A1

Description

DISPLACEMENT CURRENT SENSOR FOR POSITION AND MASS DISTRIBUTION

BACKGROUND OF THE INVENTION

A. Field of the Invention

2286247 This invention relates generally to the sensing of position and the distribution of mass within a spatial reference frame, and in particular to a sensing system that responds to changes in the displacement current in a receiving electrode caused by movements caused by the user's activity.

B. Description of the Related Art

Position sensors are used to provide inputs for a variety of electronic devices. Some of these sensors are electromechanical devices, such as the ubiquitous "mouse', that is used to provide position input signals to digital computers. other sensorsi which are non-mechanical. usually make use of electrostatic or magnetic fields to provide position information. An example of an electrostatic sensor is a capacitive button switch. which is actuated when the user places a finger thereon; in so doing the user effectively increases the capacitance of a capacitor, with the resulting increase in capacitive current being sensed to indicate actuation of the button.

The non-mechanical sensors are advantageous in that they have no moving parts and moreover are, in theory at least, not restricted to operation over a small area such as a mousepad,or the like. Actually, however, because of configuration and sensitivity considerations, these sensors are limited to a small area; indeed, when they are used as "pushbuttons," this is a desirable attribute of capacitive sensors.

Electromechanical sensors are limited by their construction to detection of specific types of user movements. For example, a mouse can detect position along a twodimensional surface and transmit the user's actuation of "click" buttons mounted on the mouse; three-dimensional location and gestures other than the familiar button click, however, are beyond the mouse's capacity to detect. The prior electrostatic and magnetic sensors suffer from the same disabilities.

DESCRIPTION OF THE INVENTION

A. Brief Summary of the Invention

In accordance with the invention, a pair of electrodes are disposed relative to each other so that when an AC signal is applied to them, an object to be sensed intercepts a substantial part of the electric field extending between the electrodes. One of the electrodes. which serves as a "sending" electrode, is connected to an AC source. The other electrode (the "receivingw electrode) is effectively connected to the ground return of the source and the output current from the latter electrode is sensed as an indication of the measured characteristic. If the sensed object is a human hand. introduction of the hand into the electric field of the electrodes reduces the output current. Specifically the human body provides a return path, partly conductive and partly capacitive, that diverts some of the displacement current in the capacitor from the output electrode.

This arrangement provides significant advantages relative to prior capacitive sensors in which the user is associated directly with one of the capacitor electrodes, either by contact or by close capacitive coupling. First, it provides a better signal-to-noise ratio in connection with the introduction of a hand into the electrostatic field, and z 1 movement of the hand within that field. Second, it facilitates the use of multiple electrodes, as discussed below, to substantially reduce or eliminate ambiguity as to the position of the hand.

The present invention is useful in a wide variety of control, monitoring and interface contexts. Using multiple sending and/or receiving electrodes, one can monitor variations in the total mass proximate to the electrodes using combinations of the output signals. One-can also monitor variations in the symmetric distribution of mass relative to the positions of the electrodes by effecting various types of comparisons of the signals. In this context it should be noted that multiple receiving electrodes can share a common sending electrode and that multiple sending electrodes can share a common receiving electrode, arrangements of these types being described herein and depicted in the drawing. While the height parameter is ambiguous in a system using two pairs of electrodes due to the inability to distinguish height from mass distribution, use of one or more additional electrodes can eliminate the ambiguity. Additional electrodes facilitate resolution of more components of the mass distribution (i.e., volumetric shape). Moreover, peripherally located. conductive or resistive electrodes can be driven in a well-known manner to serve as guard electrodes that screen the sensed area from the surrounding fields by imposing suitable boundary conditions. Specific application environments are set forth in greater detail below.

B. Brief DescriDtion of the Drawings For a fuller understanding of the nature of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a diagram of a sensor incorporating the invention; and FIG. 2 is a diagram of a multiple-electrode sensor.

C. Detailed Description of the Invention

1. Device Configuration AS shown in Fig. 1, a position sensor 10 embodying the invention is arranged to sense a characteristic of an object 12 by detecting changes in the electric field involving an electrode pair comprising a sending electrode 14 and a receiving electrode 16. For example, the object 12 may be a human hand and the characteristicto be sensed is its position relative to the electrodes 14 and 16.

The sensor 10 includes an alternating-current (AC) source 18 connected between the electrode 14 and a reference point, i.e., ground, with a shielded cable 19 being used for the connection between the source 18 and the electrode 14. The electrode 16 is connected through a cable 19 to the inverting input terminal 20a of an operational amplifier 20. The amplifier is connected in a negative feedback circuit as shown. The terminal 20a is thus essentially at ground potential and the output voltage of the amplifier corresponds to the current from the electrode 16 to ground.

The output of the amplifier 20 is applied to a synchronous detector 22, whose other input is a signal from the source 18. Accordingly. the output of the detector 22 is the component in the output of the amplifier 20 that has the frequency and phase of the source 18. It is thus free of interfering signals and noise that may be picked up by the electrode 16.

The sensor 10 also includes a low pass filter 24 which smooths the output of the detector 22. The signal from the filter 24 is applied to a computer 26. which includes an analog-to-digital converter (not shown) that converts the. voltage from the filter to a digital value. The computer 26 uses the signal from the sensor 10 to drive an output device 28. The output device may. for example, be a meter calibrated v in terms of a characteristic to be sensed; or a two-dimensional display that provides a graphical indication of a sensed characteristic; or any of the computer-related applications described below, which utilize the signal to obtain information from a user.

The frequency fl of the source 18 may be 100 kHz, and the relative spacing of the electrodes 14 and 16 of the order of 1 meter. In any case, the length of the electrode 14 and the spacing between electrodes are substantially less than a wavelength at the frequency fl. Accordingly, there is minimal radiation from the electrode 14 and the coupling between the electrodes 14 and 16 is essentially capacitive.

The introduction of an object such as a human hand into the electric field extending between the electrodes 14 and 16 causes a reduction in the output voltage of the filter 24. This is contrary to what one might anticipate, since the presence of the hand in the field increases the capacitive coupling, between the electrodes 14 and 16, by changing both the effective geometry and the dielectric constant and thereby tending to increase the input current of the amplifier 20 and thus the output voltage of the filter 24. However. since the human body is electrically conductive, the presence of a hand provides a return path to ground through the body of the person. by way of the capacitances between the electrode 14 and the hand and between the rest of the body and ground. This diverts some of the displacement current that would otherwise flow from the electrode 14 to the electrode 16. With the depicted electrode configuration, an object in proximity to the electrodes 14 intercepts a substantial component of the electric field of the electrode pair, thus providing a significant change in the output current from the electrode 16.

The output voltage of the sensor 10 is a function of the frequency f 1 of the source 18 and the, conf iguration and spacing of the electrodes 14 and 16, as well as such object-specific characteristics as position, configuration and composition, It will be apparant that any given output voltage can be the.

result of a number of different combination of characteristics of the object 12. In some applications of the invention, such as use of the output voltage to trigger an event upon proximity of the object 12 to the electrodes, this ambiguity contributes to the usefulness of the sensor. On the other hand, in other applications, it is desirable to provide an output that is indicative of the position of the object 12, for example, when the sensing system is used to position a cursor on a display screen. It may also be desirable to discern the shape of the object, for example, to ascertain the presence of a human hand.

In such situations it is preferable to use a sensor that employs multiple sending and/or receiving electrodes. Such a sensor is illustrated in Fig. 2.

More specifically, as shown in Fig. 2, position sensor 210 has multiple receiving electrodes 16a-16f that share a single sending electrode 14. The electrodes 16a-16f provide the inputs for amplifiers 20a-20f and the outputs of the amplifiers are applied to synchronous detectors 22a-22f. The detector outputs, in turn. pass through filters 24a-24f to the computer 26. The computer 26 compares its inputs from the respective receiving electrodes 22a22f to provide a relatively unambiguous indication of the lateral position of the object 12 (not shown in Fig. 2) andlor provide information about its shape. Also. by combining (e.g., summing) the inputs from the receiving electrodes. the computer can develop information covering the height of the object above the electrodes.

It will be apparent that the invention can be used to provide more three dimensional information by using a three dimensional distribution of electrodes. Moreover. multiple sets of sending and receiving electrodes can be used, with each electrode set operating at one or more frequencies different from those of the other sets.

It should be noted that the diversity furnished by the use of multiple sending and/or receiving electrodes can be provided. in part, by energizing one or more sending electrodes with multiple frequencies. Thus. with reference to Fig. 1, the is electrode 14 can be connected to receive signals from both the source 18 and a second source 182 having a frequency f2. The sources 18 and 182 are coupled to the sensing electrode 14 through isolation filters 30 and 302, tuned to the frequencies fl and f2, respectively. The output of the amplifier 20 is applied to a second synchronous demodulator 222 connected to the sources 182. The output of the detector 222 is passed through a low-pass filter 2421 whose output in turn is fed to the processor 26. Since the output current from the electrode 16 is, in part, a function of frequency, the use of multiple frequency sources provides, in essence, multiple sending and receiving electrodes sharing common physical electrodes..

The use of multiple frequencies, either concurrently, as shown, or in a time-division multiplex arrangement also provides information about the electrical characteristics of the object 12 and thus can be used to distinguish a hand, for example, for an inanimate object. Specifically, measurement of the amplitude and phase of the output current from the receiving electrode as a function of frequency provides information about the composition of the object 12. The phase of the output current can be provided by adding a second phase detector with a quadrature input from the source 18.

2. Device Applications The present invention is amenable to a wide variety of usages involving the detection of user positions and gestures as a means of conveying information. In a computer environment multiple electrode pairs can serve as a position-sensing device. providing output equivalent to that of a mouse or tablet pen without the need for any mechanical assemblies. For example, placing the electrodes on or beneath a desk transforms its surface into an nactive" element of the computer interface. Movement of a user's empty,hand over the desk provides an application program with positional information equivalent to that obtained by rolling a mouse. However. unlike mechanical sensing devices, the present invention can also necover is gestural information derived from height, position and changes in mass distribution.

Thus, for example, the user's sweep of his hand across the desk from left to right generates digital data that can be interpreted as by an application program (such as a text display and/or editing facility) as a page-turning or subject browsing command. The invention can simulate a multi-channel joystick by distinguishing the different patterns of data generated by squeezing motions, hand tilts and button-pushing gestures. The length scale of the invention, even when employed strictly as a position-sensing device, can also be varied considerably to suit different applications. Relatively wide electrode spacing is compatible with monitoring the movement of a user's entire hand or even the position of a person within a room, while smaller (e.g., 1 cm) spacings can be used to facilitate responses to small movements of a finger.

The invention can also be used in conjunction with compliant members having known elasticity characteristics, and which may therefore be used to generate signals indicative not only of position. but of the force being exerted on the resistive member. For example, by interposing an elastic element over a surface containing a set of electrodes, the height of the user's hand reflects the force exerted on the element. thereby further expanding the range of gestural information that may be sensed.

When the sensors are included as part of a unitary device, such as a laptop or notebook computer or a video game.

interactive capabilities expand due to the ability to fix position relative to the screen. A multiple-electrode-pair array mounted at appropriate locations within the computer housing can provide a wcontrol space" above the keyboard, with the invention generating data representing the three dimensional position and orientatign of the user's hand. Thus, by generating an array of on-screen buttons and sensing the position of the user's hand or finger relative to the screen, the computer can interpret the user's gestures as "pushing" the I. 1 Y various buttons even though contact is never made with the screen.

In a similar way, the invention can be applied to devices other than computers (e.g., appliances, televisions, furniture, etc.) to facilitate user interaction. By associating the invention with an appliance containing various manually operated buttons, switches or the like, a user's proximity to these devices can be sensed and the consequences of impending actions evaluated before they are completed. Thus, for example. as the user's hand approaches the ignition key of an automobile, an audible tone and/or visible display can alert him to shift into parking gear before turning off the engine. The invention can also be used to remotely operate appliances such as televisions or recording systems without the need for the traditional hand-held device. And because the electrodes need not be exposed to the atmosphere, the invention is especially useful in controlling sealed (e.g., waterproof) devices, potentially replacing expensive isolation switches and broadening user control over such devices.

on a larger scale, the invention can be used to sense proximity to a reference object for security purposes, to warn of danger, or to conserve energy by withholding power until a potential user approaches the object. Distribution of a series of sensing capacitors about a room enables the invention to provide output indicative of a user's position within the room, the number of people in the room and their relative positions, etc. The accuracy of this information,.of course, depends on the resolution necessary to the application and the number of sensors employed. For example, a security system that provides a trigger signal upon detection of a single person entering a reference space requires less resolution than an application that monitors the positions of multiple individuals.

1

Claims

1 2 3 4 6 7 11 12 13 14 is 16 17 1 2 3 4 5 6 7 2 3 4 5 1 2 3 4 1. A sensor for sensing a characteristic of a sensed object, said sensor comprising:

A. an electrode pair comprising a sending electrode and a receiving electrode, said electrodes having fixed positions relative to each other, B. an AC source connected between said sending electrode and a reference point, C. means connecting the receiving electrode to the potential of said reference point, and D. output means sensing the displacement current from said sending electrode to said receiving electrode, said object being positioned in the electric field of said electrodes and diverting displacement current from said sending electrode in accordance with its position in the field, whereby the displacement current sensed by said output means is a function -of the position of said object.

2. The sensor defined in claim 1 in which said connecting means is an operational amplifier having an input terminal connected to said second electrode and including a feedback circuit connected to force the potential at said input terminal to said potential of said reference point, whereby the output voltage of said operational amplifier corresponds to the current from said receiving electrode.

3. The sensor defined in claim 1 including a detector connected to rectify a signal corresponding to said current in synchronism with the output of said source, said indicating means responding to the signal from said detector.

4. The sensor definec in claim 2 including a detector connected to rectify the output of said amplifier in synchronism with the output of said AC source. said indicating means being responsive to the output of said detector.

l 1 2 3 4

5 6 7 11 12 13 14 is 16 17 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 5. A position sensor for sensing a position of an object, said sensor comprising:

A. a plurality of electrodes including multiple sending andlor receiving electrodes, B. means for applying alternating current signals to each of said sending electrodes, each of said receiving electrodes thereby receiving displacement current from one or more of said sending electrodes, said electrodes being configured so that id object intercepts the electric field of at least three of said electrodes, and C. output means responsive to the displacement currents through the respective receiving electrodes, said object diverting displacement currents of said sending electrodes in accordance with its position in the electric fields thereof, whereby the output means responds to the position of said object.

6. The sensor defined in claim 5 in which a plurality of receiving electrodes receive displacement currents from a common sending electrode.

7. The sensor defined in claim 5 wherein the output means is a computer. and the electrodes are spaced to facilitate response to movement of a user within a room.

8. The sensor defined in claim 5 wherein the output means is a computer. and the electrodes are spaced to facilitate response to movement of a user's hand.

9. The sensor defined in claim 5 wherein the output means is a computer. and the electrodes are spaced to facilitate response to movement of a user's finger.

10. The sensor defined in claim 5 wherein the output means is a computerr and the electrodes are spaced to facilitate response to a user's manual gesture based on changes in sensed mass distribution.

12 1

11. The sensor defined in claim 1 further comprising an elastic member positioned 2 to be deformed by movement of the sensed object, whereby the displacement 3 current sensed by the output means is a function of the force exerted by the 4 object on the elastic member.

6

12. 7 8 9 10 A sensor for sensing a characteristic of a sensed object, said sensor being substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

11 12 13 14 J z