IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-21, NO. 1, JANUARY 1973 61

New Touch Actuator Based on the Foil-Electret PrincipLe

G. M. SESSLER, J. E. WEST, AND R. L. WALLACE, JR.

Abstruct-A new touch actuator is described that consists of a single metalized foil electret, a number of independent metal backplate sec- tions aligned with the holes of a front cover, and air gaps separating electret and backplate sections. The metal layer of the foil forms a transducer with capacitance C, with each backplate section, The actuator is operated by touching the foil electret through one of the front-cover holes. For a trapezoidal displacement (duration 2T) of the foil electret, the electrical output signal across a terminating resistor R consists of a pair of oppositely poled spikes with duration com- parable to RC,, if RC, is smaller than T. If RC, is larger than T , the electrical output approximates the displacement. For an electret with a charge density of C/cm*, the measured pulse amplitudes due to touching are of the order of 0.3 to over 100 V depending on the applied force, and on the resistive and capacitive loading. The touch actuator is mechanically simple, reliable to use, inexpensive to produce, and can be made with an overall thickness of less than 0.5 cm. Apart from its po- tential use in telephone dials, it may also find applications as a key transducer in digital devices.

T I. INTRODUCTION

HE PRESENT integrated-circuit RC push-button dial used in the Trimline telephone [ l ] , [2] as well as the older

LC dial [3] used in the Touch-Tone telephone employ metallic contacts to switch ac signals. As do all mechanical switches, these contacts occasionally exhibit reliability prob- lems due to unavoidable contamination. The mechanical part of the present Touch-Tone dial is also relatively heavy and bulky, having an overall thickness of about one inch; this is a drawback in Trimline telephone applications. In addition, the mechanical part of the dial (the actuator) contributes signifi- cantly to the total cost of the handset. Hence new techniques for telephone dialing continue to be investigated.

As a result, various alternative solutions to the problem of touch dialing have been brought forward recently [4] -[6]. In this paper, we suggest another approach based on the use of foil-electret transducers [7]. The proposed touch actuator is mechanically simple, reliable in use, inexpensive to produce, and small in size.

U. MECHANICAL DESIGN An exploded 'view of a 12element touch actuator is shown

in Fig. 1. The actuator consists of 12 circular metal back- plates, each 1.6 cm in diameter, embedded in a Plexiglas insu- lator. The backplates are recessed by a distance of 50 pm into the Plexiglas. A single 2-mil (51-pm) Teflon FEP electret is mounted on top of the backplates. The electret has a

Paper approved by the Wire Communications Committee of the IEEE Communications Society for publication without oral presentation. Manuscript received October 1, 197 1.

The authors are with the Bell Laboratories, Murray Hill, N.J. 07974.

8 FRONT COVER '# METAL

/

ETA1 CASE

/

CONNECTORS

Fig. 1. Exploded view of new electret-touch dial.

lOOO-ii metal layer on the side facing out and is held in posi- tion by a thin front cover having holes aligned with the metal backplates in the Plexiglas. This front cover is mounted on the metal case that encloses the Plexiglas insulator. The overall thickness of the actuator, excluding connectors, is 0.5 cm.

The present touch actuator may be modified in avariety of ways. For example, it i s advisable to mount another foil between front cover and foil electret for protection of the electret against environmental, manual, and other hazards. Also, push buttons may be mounted in front of the foil, if this appears desirable. Another possible modification is a sub- division of the backplate elements to derive two or more signals from each section [8]. Finally, the overall thickness and weight of the actuator could be reduced by using photoetching or other film technologies to form metal back electrodes on a dielectric substrate.

111. OPERATION OF TOUCH ACTUATOR One element of the electret actuator is operated by touching

at time t = 0 the exposed foil-electret section causing a dis- placement, which is more or less uniform over the area of the transducer. After release, the foil returns to its quiescent position due to its tension and stiffness. As a result of these operations, a voltage is generated across a terminating imped- ance considered to be a parallel combination of a resistor R and a capacitor C p . An analytical study of the operation of capacitive transducers of this kind will be presented in another paper [9]. Some of the results of this study relating to the present problem are reviewed in the following.

62 IEEE TRANSACTIONS ON COMMUNICATIONS, JANUARY 1973

The voltage u(t) generated by the actuator across the parallel resistor-capacitor combination is determined by the electret displacement s( t ) from its quiescent position, or by the actuator capacitance Ca(t). Assuming the initial condition v(t) = 0 at t = 0, one has for arbitrary changes in actuator capacitance [9]

where

UO = D D / E E O . (3)

In (1)-(3), C(t) equals C,(t) t C p , while u is the negative charge density per unit area on the exposed surface of the electret, D the electret thickness, E the dielectric constant of the electret material, and eo the permittivity of free space.

For small displacements s ( t ) << (DIE) t so (where so is the air-gap thickness of the electret actuator) and if

Lr I s ( t ) Idt << Eo AR

where A is the face area of the electret, (1) may be written

r = c. exp [-t/RCo] [ s(t) exp [t/RCo] d t , (4)

uo co (Dl€) + so

where C, = C,(O), and C, = C, + C,. As an example, the voltage response u ( t ) for the case of

negligible parallel capacitance (C, << C,) is evaluated assum- ing a trapezoidal displacement function resembling the dis- placement obtained by manually touching and releasing the foil of the actuator. The displacement function can be ex- pressed as

~ ( t ) = -(Sm/AT) t , O < t < A T (5 a) s ( t ) = -s,, AT < t < 2T- AT (5b)

s ( t ) = 0, t < O , t > 2 T , (5 d) s ( t ) = -(s,/AT) (2T- t) , 2 T - A T < t < 2 T ( 5 ~ )

where AT is the rise time and s, the amplitude of s ( t ) . For this function, v(t) is calculated from (1) for RC, = 0.1T and RC, = 10T assuming s, = so =DIE. The results for v(t)/u,, where us is given by

are plotted in Fig. 2. The' figure shows that for RC, << T the voltage response consists of two spikes and is thus a distorted replica of the displacement, while for RC, >> T the voltage response resembles the displacement function.

S ( t ) / S ,

1 v( t ) /vs

0

-1

RC, -0.1 T *

1 I

I I

I I

R C a = l O T I 1 I

t /T

Fig. 2. Calculated voltage response of electret device to a trapezoidal displacement function. Upper part-trapezoidal displacement func- tion. Lower part-calculated voltage response to the above displace- ment function for RC, = 0.1 T and RC, = 10 T.

Of particular interest is the amplitude u, = v(AT) of the first spike of the voltage response due to the displacement expressed in (sa). While the general result [9] is rather involved and will therefore not be discussed here, two limiting cases of practical interest can be obtained easily. These are the cases AT/RC(AT) << 1 and >> 1, which yield, respectively, the asymptotic values

U h / V s = -C,(AT)/C(AT) (7)

and

u k / V , = - [RC,(AT)/AT] [C,(AT)/C,]. (8)

While (7) represents the effect of purely capacitive loading, (8) gives the generated voltage in a circuit with small time constant. The ratio u k / u h follows as

vk/vL = [RC(AT)/AT] [C,(AT)IC,] . (9)

For an actuator operated in its large-displacement mode with s, = so = D/e = 25 pm, (6) yields v, = 140 V for u = lo-' C/cm2, independent of actuator area. According to (7), this represents the output voltage obtained without capacitive and resistive load.

IV. EXPERIMENTAL RESULTS The electrical output of one unit of the touch actuator as a

result of a displacement similar to the function defined in (5)

SESSLER e t al.: NEW TOUCH ACTUATOR 6 3

TABLE I TERMINATING RESISTANCES, SCALES, AND CHARGE DENSITIES

UTILIZED FOR RESULTS SHOWN IN FIG. 3

Fig. 3. Measured voltage response of one unit of the electret-touch dial due to manual touching. Amplifier of negative unity gain was used. Terminating resistances as well as horizontal and vertical scales are given in Table I .

is shown in Fig. 3(a)-(h). The displacement was obtained by manually deflecting the foil until it touched the backplate. The electret actuator had a capacitance of about 20 pF and was shunted by 120 pF. Terminating resistances between IO5 and 1014 52 (Keithley 600-B electrometer) were used. The resistances, time scales, vertical scales, and electret charges used are given in Table I. In Fig. 3(a)-(d), displacements of about 0.3-s duration were used, while displacements of about 3-s duration were chosen in Fig. 3(e)-(h). These choices make the fall of the voltage spike more visible. A weakly charged electret (a = 0.6 X C/cm*) was utilized for the responses in Fig. 3(e)-(h) since the generated voltage across the large resistances overloads the electrometer if higher charge levels are used (see Table I).

The responses in Fig. 3(a)-(e) were measured at resistances < lo9 52, corresponding to RC constants of < 140 ms. These responses were thus obtained for RCo << T and resemble the calculated output voltage in Fig. 2, solid line. For the responses in Fig. 3(g) and (h), the RC constants are 14 and 14 000 s, respectively, and thus RCo >> T. These responses resemble the one in Fig. 2 , dashed line. The similarity of the experimental responses (obtained for C, >> C,) and the theoretical responses in Fig. 2(b) (obtained for C, = 0) is

Terminating

Vertical scale resistance (a) 105 106 107 108 lo9 1010 10" 1014

(V/div) 0.1 0.5 2 5 5 5 5 5 Horizontal scale

(s/div) 0.1 0.1 0.1 0.1 1 1 1 1 Charge density

of electret C/cmZ) 1.2 1.2 1.2 1.2 0.6 0.6 0.6 0.6

I 20 50 I00 200 500

APPLIED FORCE (103dynes)

Fig. 4. Generated voltage of one unit of the electret-touch dial as a function of the applied force for various terminating resistances.

to be expected from (l), which shows that capacitive loading has little effect on the shape of the voltage response for the displacement function under consideration.

Comparing the output voltage at very high R values with that at relatively low R values allows one to determine the rise time AT of the displacement function from (9). I t follows that AT rises from about 1 ms for R = lo5 52 to 10 ms for R 2 lo7 52. The fact that different values of AT are required to explain the experimental results indicates that the leading edge of the displacement function is not a straight line, as assumed previously [see (sa)] .

Fig. 3(a)-(d) also shows that the amplitude of the second (negative) pulse is smaller than the amplitude of the first (positive) pulse. This can be explained by the fact that in manual touching, because of the high impact speed and small release speed of the finger, the rise time of the displacement function is much shorter than the fall time. Equation (8) therefore qualitatively explains the difference in amplitude of the two pulses.

The voltage generated by one unit of the actuator as a function of the applied force is shown in Fig. 4 for terminating resistances of I O 6 , l o 7 , and 1 O8 a. These results were obtained by touching the foil with a rubber plunger (1-cm diameter) mounted on a spring scale. Fig. 4 shows that the generated voltage is relatively independent of force for forces

64 IEEE TRANSACTIONS ON COMMUNICATIONS, JANUARY 1973

TABLE I 1

VOLTAGES FOR CAPACITIVELY LOADED TOUCH ACTUATORS COMPARISON OF CALCULATED AND MEASURED PEAK

Load Ratio

Air-Gap Diaphragm Charge CP

(pm) (10-8C/cm2) C J A T ) (v) (v)

Calculated Measured Thickness so ThicknessD Density u ___ Voltage U, Voltage urn

; Actuator (pm)

1 30 25 6.3 6.2 2

0.70 63

9.1 25 I .04 14.5 7.6 7.6

in excess of 10’ dynes, as encountered in touch-actuator operation. Since the force affects only AT, this is to be expected as long as AT << RC, [see (7)] .

Open-circuit voltages generated by electret actuators can easily exceed 100 V (see Section 111). Since such voltages are too high for most applications, electret actuators are usually loaded with suitable impedances. Table I1 shows a comparison of calculated [from (6) and (7)] and measured peak voltages for a few capacitively loaded touch actuators with high resistive termination (ATIRC, << 1). The experimental results were obtained by deflecting the electret diaphragm with a rubber plunger. The calculated and measured values differ by only a few percent with the deviations due to uncertainties in air-gap thickness and displacement uniformity.

The touch actuators used in these experiments were not adversely affected by frequent touching. The output voltage of the units did not change measurably after 400 000 automatic touch operations performed with amotor-driven rubber plunger by directly contacting the metal layer of the foil. As indicated in Section I1 it is desirable, however, to use an additional foil for protection against hazards of manual use.

The sensitivity of the actuator to air-borne sound is com- paratively very small. A pistol shot fired at a distance of 1 m generated a signal about 70 dB smaller than that due to a touch operation. Dropping of the actuator from an elevation of 1 m produced a signal output that is from 30 to 50 dB lower than that resulting from touching.

The crosstalk signal generated at neighboring units of the touch actuator is typically 40 dB smaller than the signal generated by the touched unit.

Multifoil systems can be utilized to provide a train of pulses of either positive or negative polarity. Such pulse trains can be generated if two or more metalized foil electrets are placed across a single backplate with both metal layers up. Some air gap has to be provided between the foil layers. If the outputs, taken between adjacent metal layers, are delayed and added, multiple pulses can be obtained. For example, a system with two layers of foil electret and one electrical delay can be utilized to give four different pulse trains. Such pulse trains can be used to address logic circuits of many devices.

V. ELECTRONIC CIRCUITRY Circuitry has been developed [lo] for converting a voltage

pulse to signals that will actuate the integrated RC oscillators of the present Touch-Tone dial. This circuit has an input

impedance of 200 kf2 and operates with input signals of any duration in excess of 20 ps at levels of 5.5 to 6.5 V. Field tests on this device, which was developed for automatic dialers, will be conducted in the near future. The output of the electret actuator could be coupled into this circuit after amplification with an FET or bipolar amplifier.

Presently, a circuit for directly converting the voltage pulses of the electret actuator to the signals required in the Touch- Tone dial is being designed. This circuit consists of a JFET amplifier for impedance transformation, an inverter stage, and a one-shot multivibrator. The circuit generates a 50-ms pulse each time an element of the actuator is touched. These pulses control the low and high Touch-Tone frequencies. Details of this circuit will be given in a forthcoming publication [ 111 .

VI. CONCLUSIONS Presently, foil-electret transducers are under extensive inves-

tigation. As a result, technologies are being developed which are directly applicable to the electret touch actuator. Like foil-electret transducers, the touch actuator is simple to implement. Its greatest advantages are that it is reliable in use, small in size, and inexpensive to produce.

ACKNOWLEDGMENT The authors are grateful to J. B. Allen, J. L. Flanagan, T. R.

Meeker, J. R. Nelson, and A. Nigam for valuable discussions on various aspects of this paper. The authors also appreciate aid in mechanical design by J. H. Kronmeyer.

REFERENCES [ 11 R. W. Berry, P. Miller, and R. M. Rickert, Bell Lab. Rec. , vol. 44,

[2] W. H. Orr, R. M. Rickert, and D. M. Hill, “An integrated RC p. 319, 1966.

oscillator for TOUCH-TONE dialing,” ZEEE Trans. Commun. Technol., vol. COM-16, pp. 624-628, Aug. 1968.

[3] a. M. L. Benson, F. L. Crutchfield, and H. F. Hopkins, “Application of touch-tone calling in the Bell system,” IEEE

b. D. F. Gagne and C. J. Schulz, “Central office and PBX Trans, Commun. Electron., vol. 82, pp. 1-5, Mar. 1963.

Electron., vol. 82, pp. 5-8, Mar. 1963. arrangements for touch-tone calling,” IEEE Trans. Commun.

c. R. N. Battista, C. G. Morrison, and D. H. Nash, “Signaling system and receiver for touch-tone calling,” IEEE Trans. Commun. Electron,, vol. 82, pp. 9-11, Mar. 1963.

IEEE Trans. Commun. Electron., vol. 82, pp. 11-24, Mar. 1963. d. J . H. Ham and F. West, “A touch-tone caller for station sets,”

[4] L. R. Learner, U.S. Patent 3 281 541, Oct. 25, 1966. [5] B. S. Stevens, Bell Telephone Lab., Holmdel, N.J., private

[6] D. H. Nash, T. P. Nenninger, and R. E. Prescott, Bell Telephone communication.

Lab., Holmdel, N.J., private communication.

IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM-21, NO. 1, JANUARY 1973 65

[7] G. M. Sessler and J. E. West, J. Acoust. SOC. Amer., vol. 40, p. 1433,1966.

[ 81 J . H. Condon and D. W. Hagelbarger, Bell Lab., Murray Hill, N.J., private communication.

[9] G. M. Sessler, to be published. [ l o ] C. E. Riehm, Bell Telephone Lab., Indianapolis, Ind., private

[ 11 ] J. R. Nelson, Bell Lab., Murray Hill, N.J., to be published. communication.

Gerhard M. Sessler (SM’69) was born in Rosen- feld, Germany, on February 15, 1931. He received the Vordiplom degree from the Uni- versity of Munich, Munich, Germany, and the Diplom and Dr. Rer. Nat. degrees from the University of Gottingen, Gottingen, Germany, in 1957 and 1959, respectively.

After joining the Bell Telephon’e Laboratories, Murray Hill, N.J., in October 1959, he began working on acoustic wave propagation in gases and plasmas and on the analysis of acoustical

properties of auditoriums. More recently, he has worked on electret transducers and on problems in psychoacoustics. Presently, he is Supervisor of the Acoustic Analysis and Electroacoustics Group of the Acoustics Research Department. He is responsible for research in electroacoustic transducers and in physical and architectural acoustics.

Dr. Sessler is a fellow of the Acoustical Society of America, and a member of the American Physical Society, the Electrochemical Society, the Acoustical Society of America, and the German Physical Society.

J. E. West (SM’72) was born in Prince Edward County, Va., on February 10, 1931. He received a degree from Temple University, Philadelphia, Pa., in 1956. He also attended Hampton Institute, Hampton, Va., and Rutgers University, New Brunswick, N.J.

He has been a member of the Acoustics Research Department since joining the Bell Telephone Laboratories, Murray Hill, N.J., in 1957. He has done research in electroacoustics, physical acoustics, and room acoustics.

Mr. West is a member of the Acoustical Society of America.

R. L. Wallace, Jr. (SM’53-F’56) was born in Texas in 1916. He received the B.A. and M.A. degrees from the University of Texas, Austin, in 1936 and the M.A. degree in physics and communication engineering from Harvard Uni- versity, Cambridge, Mass.

He served as an Instructor and as a Special Research Associate at Harvard doing work for the NDRC related to acoustics, communications systems, quieting of aircraft, and the use of radar. In 1946 he joined the Research Depart-

ment of the Bell Telephone Laboratories where he has had responsibil- ities for work on transmission systems, solid-state electronics, and video and audio conference systems. At present he is Head of the Special Communications Research Department, Murray Hill, N.J.

Mr. Wallace is a member of Phi Beta Kappa, Sigma Xi, the Acoustical. Society, and the Audio Engineering Society.

Concise Papers

Visual Displays Using Pseudorandom Dot Scan

SID DEUTSCH

Abstract-Pseudorandom dot scan yields a visually tolerable small- area flicker for low frame rate television as compared to the large-area flicker of a conventional scanning sequence. With a short-persistence phosphor, one can use 12 frames/s. With a long persistence (such as a P38), one can display 3 frames/s. The “drift illusion factors” of various scanning systems are calculated. If n square-wave generators are used to cover a rectangle, n! different scanning sequences are possible; this is applied to the scrambling of pictures. Hybrid systems, which avoid some of the disadvantages of a square-wave scan, are described. The re- sults of an audience-reaction experiment in determining effective reso- lution are given.

of the IEEE Communications Society for publication without oral Paper approved by the Communications System Discipline Committee

random dot scan,” in Proc. Soc. Inform. Display, vol. 12, no. 3, presentation. This paper appeared as “Visual displays using pseudo-

pp. 131-146, 1971. The author is with the Department of Surgery, College of Medicine

and Dentistry of New Jersey, Rutgers Medical School, New Brunswick, N.J. 08903.

I. INTRODUCTION

Our most commonplace visual display -a conventional tele- vision picture-requires a video bandwidth of 4 MHz (U.S. standards). The only simple way to drastically reduce this bandwidth without sacrificing resolution is to r educe the num- ber of complete pictures per second, the f rame ra te [ 1 1 , [ 2 ] . This paper concerns pseudorandom dot scan [ 31, a me thod . that yields a visually tolerable small-area flicker for low frame rates as compared to the large-area flicker of conventional scanning sequences.

Research in pseudorandom dot scan has been conducted at the Polytechnic Insti tute of Brooklyn with the support of the U.S. Air Force Cambridge Research Laborator ies [4] , [5] , the Nat ional Science Foundat ion [6] , and the Battelle De- velopment Corporat ion. Some work has a lso been done a t the Massachusetts Institute of Technology [ 7 1 , [ 81 .

The general conclusions are ‘that one can decrease the frame frequency unti l there is an appreciable loss in effective resolu- tion because of the sm,all-area flicker itself. The visual effect of small-area flicker is similar to tha t o f no ise or “snow” added to the video signal. If a short-persistence tube is used, audience-reaction tests (described near the end of the paper)