BEST MODE FOR CARRYING OUT THE PRESENT INVENTION

[0046] Details of a system for detecting tactile information with sensitivity that can be automatically adjusted will now be described, with reference to the drawings.

[0047] FIG. 1 shows an overall configuration of the system for detecting tactile information, and FIG. 2 is a block diagram illustrating a circuit configuration of a touch sensor used in the system. A tactile information detection system 1 has a strain gauge type touch sensor 2 and a controller 3 . The touch sensor 2 has a sensor substrate 4 , a plurality of sensor units 5 i (i=1, 2, 3 . . . ) arranged in a matrix on a surface of the sensor substrate 4 , and a detection surface 6 a formed of a flexible material 6 such as a polymer gel or the like that covers the sensor units 5 i.

[0048] Each sensor unit 5 includes a bridge circuit 53 comprised by a pair of strain gauges 51 and 52 , disposed orthogonally to points corresponding to measurement points of the detection surface 6 a , a differential amplifier 54 that generates a differential signal from an output of the bridge circuit 53 , and a bandpass filter (BPF) 55 for applying an input signal of a specific wavelength to the bridge circuit 53 . The touch sensor 2 also includes an adding circuit 56 that combines outputs of the sensor units 5 (differential amplifier 54 output) and an amplifier 57 that amplifies a composite signal obtained by means of the adding circuit 56 , and supplies an output of the amplifier 57 to the controller 3 , via a single signal output line 58 . The controller 3 includes an A/D converter 7 , an analyzer 8 , an automatic gain control circuit (hereinafter AGC) 9 and a D/A converter 10 . A gain control signal output by the D/A converter 10 is supplied, via a single gain control line 11 , to each of the sensor units 5 of the touch sensor 2 .

[0049] The number of measurement points can be increased by, for example, connecting plural sets of touch sensors 2 in parallel with the controller 3 , as shown in FIG. 3 . The tactile information detection system 1 is able to measure tactile information (touch force, in this example) while at the same time controlling the gain of each of the sensor units 5 of the touch sensor 2 . To control the gain, the sum of sine waves of different frequencies (composite sine wave signal) is output from the D/A converter 10 of the controller 3 as gain control signal y(t). Thereby, via the bandpass filter (BPF) 55 , at each sensor unit 5 disposed at each measurement point, a gain control signal that includes only sine wave components of predetermined frequency is imposed on the bridge circuit 53 . As a result, the gain is adjusted, as described below.

[0050] An amplitude Ai of the signal V _i (t) output from the bridge circuit 53 of the sensor unit 5 i is proportional to a strength of an input to the sensor unit 5 i , and each output is multiplexed by the adding circuit 56 . This makes it possible for detection outputs from a plurality of measurement points to be collectively A/D converted by the A/D converter 7 of the controller 3 . By thus using a one-input, one-output configuration, by means of the D/A converter 10 , the controller 3 can readily measure touch force and other such tactile information at a plurality of measurement points, and adjust the gain.

[0051] Processing operations of the analyzer 8 and AGC 9 of the controller 3 will now be described. To enable force information at the measurement points to be calculated on a real-time basis, the analyzer 8 carries out Fourier transformation and equivalence processing on an i-channel by i-channel basis. At the AGC 9 , continuous gain control is exercised to adjust a signal from the sensor unit 5 i of the touch sensor 2 to a reference (target) value. The function of the AGC 9 is to try and maintain a signal within a fixed range, to thereby avoid an unstable state in which measurement is not possible. If, for example, a very large touch force is imposed on a particular measurement point, the AGC 9 reduces the amplitude A _i of a sine wave that controls the gain of the sensor unit 5 i located at that measurement point. In this way, the signal V _i (t) output from the sensor unit 5 i is kept within a specified range. When the touch force acting on a measurement point is a small one, the AGC 9 increases the amplitude A _i of the sine wave concerned, to thereby also keep the signal V _i (t) from the sensor unit 5 i within the specified range and increase the gain.

[0052] Automatic gain control is used on CCD cameras and microphone amplifiers and the like to maintain signal strength within a fixed range, in cases in which an input signal is above or below a prescribed level. In the case of the present invention, from the perspective of measurement, a new tactile information detection system 1 is realized that incorporates an AGC function.

[0053] FIG. 4 is a block diagram of a signal feedback loop of the ith sensor unit 5 i . This shows a signal flow between the touch sensor 2 and the controller 3 . To enable the gain of the measurement point sensor unit 5 i to be simultaneously controlled from the controller 3 side, the sum of the different-frequency sine waves is output by the D/A converter 10 . The composite sine wave y(t) is expressed by equation (1). 1 $\begin{array}{cc}y\left(t\right)=A_1\sin \left(2\pi f_1t\right)+A_2\sin \left(2\pi f_2t\right)+\cdots +A_i\sin \left(2\pi f_it\right)+\cdots & \left(1\right)\end{array}$

[0054] A _i and f _i are the ith voltage amplitude and frequency. As expressed in the above equation, the composite sine wave signal y(t) is imposed on the bridge circuit 53 at each of the measurement points, but because of the presence of the bandpass filter (BPF) 55 , only sine wave components of a predetermined single frequency fi are imposed on the bridge circuit 53 . Thus, only A _i sin(2πf _i t) is imposed on the ith bridge circuit 53 ; when the strain gauges 51 and 52 are subjected to a force, the output voltage V _i (t) from the ith sensor unit 5 i is as shown by equation (2). 2 $\begin{array}{cc}{V}_i\left(t\right)=G_i\times \frac{\Delta R_i}{2R}A_i\sin \left(2\pi f_it+{\phi }_i\right)& \left(2\right)\end{array}$

[0055] Here, G _i is a gain of the differential amplifier 54 , φ _i is a phase deviation from an applied frequency, ΔR _i is a change in resistance of the strain gauges 51 and 52 produced by the touch force, and R is balance resistance of the bridge circuit 53 . As shown by this equation, since the gain G _i of the differential amplifier 54 is fixed, the gain of the sensor unit 5 i can be changed from the controller 3 side by changing the voltage amplitude A _i applied to the bridge circuit 53 . The signals V _i (t) output from the measurement points are multiplexed by the adding circuit 56 and at the same time can be measured by the controller 3 . Equation (3) expresses the signal V _input (t) input to the controller 3 . 3 $\begin{array}{cc}{V}_{\mathrm{input}}\left(t\right)=\sum _{i=1}^nV_i\left(t\right)\left|V_{\mathrm{input}}\left(t\right)\right|<V_{\mathrm{input}}{}_{\max }& \left(3\right)\end{array}$

[0056] Here, V _input|max is a maximum input voltage to the A/D converter 7 or the like. The force imposed on each of the measurement points is calculated by the analyzer 8 , but a force that exceeds the maximum input voltage cannot be thus calculated. Therefore, the AGC 9 is used to appropriately control the gain to keep the V _i (t) signal strength within a fixed range. Forming a gain control feedback makes it possible to prevent signal saturation when a force input to the sensor unit 5 i exceeds the prescribed value, and when an input force is small, resolution can be raised by increasing the gain until the input is within the prescribed range. That is, the signal level can be maintained within a fixed range, preventing the system falling into an unstable state in which measurement is not possible. The analyzer 8 and AGC 9 will now be described in further detail.

[0057] (Analyzer)

[0058] As shown in FIGS. 4 and 5 A, the output V _i (t) from the sensor unit 5 i , as shown in equations (2) and (3), includes frequency components that are amplitude-modulated in accordance with the force acting on the measurement points. Therefore, the force acting on the measurement points can be obtained by demodulation, as follows. A frequency (carrier wave) applied to the each of the measurement points is know beforehand, so it is only necessary to obtain a relationship between a requisite frequency and amplitude. Equations (4) and (5) show how a correlation between the sine wave and cosine wave is obtained with respect to the output V _sum (t) obtained by the A/D conversion of the output V _input .

V _x ( t )= V _sum ( t )×sin(2 πf _i t ) (4) 4 $\begin{array}{cc}{V}_y\left(t\right)=V_{\mathrm{sum}}\left(t\right)\times \sin \left(2\pi f_it+\frac{\pi }{2}\right)& \left(5\right)\end{array}$

[0059] If respective low-pass filter (LPF) is applied to the V _x (t) and V _y (t) thus obtained to give X _i (t) and Y _i (t), an amplitude of the frequency concerned, that is, a touch force F _i (t) can be written as follows.

F _i ( t )= d _i {square root}{square root over (X _i ² ( t )+ Y _i ² ( t ))} (6) 5 $\begin{array}{cc}{\mathrm{Phase}}_i\left(t\right)=\arctan \frac{X_i\left(t\right)}{Y_i\left(t\right)}& \left(7\right)\end{array}$

[0060] Here, d _i is a constant determined by calibration, and phase information Phase _i (t) shows a direction of the touch, such as whether the touch is from above or below the sensor unit 5 i , for example. For example, a positive Phase _i (t), as in the case of FIG. 5 B, indicates the direction of the touch is downward, while a negative Phase _i (t), as in the case of FIG. 5 C, indicates an upward touch direction. The LPF utilizes trigonometric function orthogonality to cut frequencies other than the cutoff frequency F _cut Hz out of components other than frequency f _i . The cutoff frequency f _cut is determined according to the scale of the input to the touch sensor 2 , and must satisfy the following condition.

2 f _out <f _i , f _n =πf _i (8)

[0061] The value of the frequency f _i can be increased if it is required to detect amplitude and high frequencies.

[0062] (Automatic Gain Control)

[0063] The object of the AGC 9 is to automatically prevent saturation of the A/D converter 7 and the like, and to adjust the resolution of the tactile information. To explain this with reference to the FIGS. 4 and 6 , the AGC 9 performs the following operation to adjust the voltage amplitude A _i (t) applied to the sensor unit 5 i to an appropriate value.

E _i ( t )= A _ri ( t )− A _io ( t ) (9) 6 $\begin{array}{cc}\Delta W_i\left(t\right)=\alpha {\int }_0^TE_i\left(t\right)t& \left(10\right)\end{array}$ A _i ( t )= A _ri ( t )−Δ W _i ( t ) ( 11 )

[0064] Here, A _ri (t) is voltage amplitude measured at the ith measurement point, A _io (t) is a target value at the ith measurement point, E _i (t) is an error (difference) relative to the ith target voltage, ΔW _i (t) is an applied voltage correction amount, and α is a small constant. With reference to equation (10), integration is used to impart a high-frequency cutoff effect for smooth, continuous adjustment of errors that change frequently.

[0065] Memory of the D/A converter 10 is rewritten to update the voltage amplitude A _i (t) in accordance with the applied voltage correction amount ΔW _i (t). The D/A converter 10 can be associated with memory for high-speed sine wave generation, but memory rewrite time cannot be ignored. For example, when it is desired to output an ideal output waveform (b) shown in FIG. 7 , the waveform actually output from the D/A converter 10 is a waveform (a) obtained by multiplying the ideal output waveform (b) by a rectangular wave (c). In FIG. 7, T _i is the memory rewrite time of the D/A converter 10 and T is a rewrite update period.

EXAMPLES

[0066] As shown in FIG. 8, a touch sensor 2 was fabricated by forming 1 mm cuts in a steel plate 21 to form a plurality of regions, in each of which a sensor unit 5 is located. Each sensor unit 5 included a bridge circuit 53 comprised of two strain gauges 51 and 52 adhered to a measurement point. An output of the bridge circuit 53 was temperature-compensated. A software-generated composite sine wave y(t) from a D/A converter 10 of a controller 3 was updated by a 30 kHz analogue output. Via analogue BPF 55 , a single sine wave was applied to the bridge circuit 53 at each measurement point, and a secondary bi-cut type BPF 55 was used that was able to increase a quality factor to apply only a single sine wave to the bridge circuit 53 . Regarding the BPF 55 , due to device element variation, it is difficult to accurately align a center frequency with a set value, so a frequency of the sine wave output by the D/A converter 10 was aligned with the center frequency of the BPF 55 . Each analogue BPF 55 was designed so that mutual interference portions were attenuated by at least 100 dB. The output from each measurement point was amplified approximately 1000-fold by an instrumentation amplifier 54 . To enable signal outputs from the touch sensor 2 to be handled by just one line, the outputs from the sensor units 5 are added by an adding circuit 56 to generate an amplitude-modulated, frequency-multiplexed signal. The signal output by the touch sensor 2 is sampled at 5 kHz by an A/D converter 7 . Digital LPF with a cutoff frequency of 50 Hz used by an analyzer 8 that obtains a correlation between sine wave and cosine wave was designed with third-order Butterworth characteristics. In the test, to make τ/T approximately 1.0, a gain update period T of 250 ms was used; the test was conducted at τ/T=0.80. Actual measurement was carried out in a stable period following memory update.

[0067] FIG. 9 shows the results of the test conducted in respect of two measurement points, without automatic gain control. The D/A converter 10 applied sine waves of 313 Hz and 604 Hz to the sensor units (strain gauges). For the measurement, first a measurement point to which the 604 Hz sine wave was applied was touched, after which both measurement points were touched at the same time. FIG. 9A shows the waveform of the output from the touch sensor 2 , and FIGS. 9B and 9C show data obtained after processing by the analyzer 8 . These figures show data obtained when the sensor unit to which the 604 Hz sine wave was applied were touched with t=0.9 to 1.3 sec, and when both sensor units were touched with t=1.5 to 1.9 sec. FIG. 9B shows a slight output, even though there was no signal input from 0.9 to 1.2 sec. This was probably due to the fact that in the BPF 55 with the center frequency of 604 Hz, there was insufficient attenuation at frequency of 313 Hz when there was a high applied voltage or input displacement.

[0068] FIGS. 10A and 10B show when automatic gain control was not used and the fixed-gain touch sensor 2 became saturated. FIG. 10A shows a signal output from a single measurement point, and FIG. 10B shows the applied voltage from the D/A converter 10 supplied to a single measurement point. FIG. 10A shows that the A/D converter 7 became saturated after t=3900 ms.

[0069] When automatic gain control was used, FIGS. 11A to 11 D show the results when the gain of the touch sensor 2 was reduced as the touch force increased gradually. FIG. 11D shows the signal output from a single measurement point, and FIGS. 11A and 11B are partially enlarged views of FIGS. 11C and 11D , respectively. In FIG. 11A, T is the gain update period of the AGC 9 , T _d is an effective measurement period of the sensor, and T _i is the memory rewrite time of the D/A converter 10 . Taking into consideration function lag introduced by the BPF and LPF, a measurement period T _d was provided after T _i to allow the system to stabilize. From FIGS. 11C and 11D , it can be seen that the gain of the sensor unit 5 decreased in accordance with the touch pressure.