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Controllers Drive Gesture Recognition on Resistive Touch Screens

Tue, 10/04/2011 - 4:58am
Javier Calpe, Italo Medina, Alberto Carbajo, and Maria Jose Martinez of Analog Devices, Inc., www.analog.com

An enhanced, low-cost user interface using touch is a valuable feature for a variety of consumer, medical, automotive and industrial devices. In many consumer applications, designers prefer expensive capacitive touch screens to resistive technologies because they can track a large number of fingers and seem to offer a friendlier interaction with the user. At present, low-cost resistive technologies fill a market niche where only a single touch is required, extremely accurate spatial resolution is paramount, a stylus facilitates specific functionality — such as Asian-language character recognition, or in environments where users must wear gloves.

Although resistive technologies have conventionally been used to detect the position of a single touch on the screen, this article examines a new dual-touch concept that uses a resistive touch screen controller to detect the most common two-finger gestures (zoom, pinch and rotation) using inexpensive resistive touch screens.

The Classical Approach to Resistive Touch Screens

Typically, resistive screens have two parallel indium tin-oxide (ITO) conducting layers separated by a gap (Figure 1). The edge electrodes of the upper layer (Y) are rotated 90° with respect to those of the lower layer (X). A “touch” occurs when the two layers are brought into electrical contact by pressure applied to a small area of the screen. If a DC voltage is applied between the two electrodes of the top layer while the lower layer floats, the touch brings the lower layer to the same voltage as the touch point. The touch coordinate in the direction of the top layer is identified by measuring the voltage on the bottom layer to determine the ratio of the resistance at the touch point to the total resistance. Then, electrical connections for the layers are swapped, and the coordinate of the touch point on the other axis is obtained.

gesture equation 1
  

 

The layer supplied with the DC voltage, which carries a current inversely proportional to its impedance, is called the active layer. The layer the voltage is measured from is called the passive layer, since no relevant current flows through it. When a single touch occurs, a voltage divider is formed in the active layer, and the passive-layer voltage measurement allows an analog-to-controllers gesture recognition 1digital converter to read the voltage proportional to the distance of the touch point from the negative electrode1.

The classical four-wire resistive touch screen is popular for single-touch applications because of its low cost. Resistive approaches for multitouch have employed various techniques that always include a matrix layout screen—but at a daunting increase in screen manufacturing cost. Furthermore, the controller requires many inputs and outputs to measure and drive the various screen strips, increasing controller cost and measurement time. 

 Beyond Single Touch

Nevertheless, more information can be extracted from resistive touch screens by understanding and modeling the physics behind the process. When two touches occur, a segment of resistance from the passive screen, plus the resistance of the touch contacts, is paralleled with the conducting segment of the active screen, so the impedance seen by the supply is reduced and current increases. The classical approach to resistive controllers assumes that the current through the active layer is constant, and the passive layer is equipotential. With two touches, these assumptions no longer hold, so additional measurements are required to extract the desired information.

A model of dual touch sensing in a resistive screen is shown in Figure 2. Rtouch is the contact resistance between layers; in most of the screens currently available, it is typically of the same order as the resistance of both layers. If a constant current, I, flows through the terminals of the active layer, the voltage across the active layer is as follows:gesture recognition fig 2

Gesture Recognition

The idea behind gesture recognition can be better described using a pinch as an example. A pinch gesture starts with touches by two well-separated fingers. This produces a double contact, which reduces the impedance of the screen — and, thus, the voltage difference between the plates of the active layer. As the fingers are brought closer together, the paralleled area decreases, so the impedance of the screen increases, as does the voltage difference between the plates of the active layer.

When tightly pinched, the parallel resistance approaches zero and Ru + Rd increases to the total resistance, so the voltage increases to
gesture equation 2
Figure 3 shows an example where the pinch is executed along the vertical (Y) axis. The voltage between the electrodes of one of the layers is constant while the other layer shows a step decrease when the gesture starts, followed by an increase as the fingers come closer together.

gesture recognition 3Figure 4 shows the voltage measurements when a pinch is executed at a slant. In this case, both voltages show the step decrease and slow recovery. The ratio between the two recovery rates, normalized by the resistances of each layer, can be used to detect the angle of the gesture.

If the gesture is a zoom (fingers moved apart), the behavior can be deduced from the previous discussion. Figure 5 shows the voltage trends measured in both active layers when zoom gestures are executed along each axis and in an oblique direction.

Detecting Gestures with Touch-Screen Controllers

Figure 6 highlights how the AD7879 touch-screen controller is designed to interface with four-wire resistive touch screens. In addition to sensing touch, it gesture recognition 4also measures temperature and the voltage on an auxiliary input. All four touch measurements — along with temperature, battery and auxiliary voltage measurements — can be programmed into its on-chip sequencer.

The controller, accompanied by a pair of low-cost op amps, can perform the above pinch and zoom gesture measurements, as shown in Figure 6.

The following steps describe the procedure to recognize gestures:

1. In the first semi-cycle, a DC voltage is applied to the top (active) layer, and the voltage at the X+ pin (corresponding to VY+ – VY–) is measured. This provides information related to motion (together or apart) in the Y direction.
2. In the second semi-cycle, a DC voltage is applied to the bottom (active) layer, and the voltage at the Y+ pin (corresponding to VX+ – VX– ) is measured. This gesture recognition 5provides information related to motion (together or apart) in the X direction.

The circuit in figure 6 below requires the differential amplifiers to be protected against shorts to VDD. During the first semi-cycle, the output of the lower amplifier is shorted to VDD. During the second semi-cycle, the output of the upper amplifier is shorted to VDD. To avoid this, two external analog switches can be controlled by the controller’s GPIO, as shown in Figure 7.

In this case, the controller is programmed in slave conversion mode, and only one semi-cycle is measured. When the controller completes the conversion, an interrupt is generated. The host processor reprograms the controller to measure the second semi-cycle and changes the value of the controller GPIO. At the end of the second conversion, results for both layers are stored in the device.

A rotation can be modeled as a simultaneous zoom in one direction and an orthogonal pinch, so detecting one is not difficult. The challenge is discriminating clockwise (CW) and counterclockwise (CCW) gestures; this cannot be achieved by the process described above. Detecting both a rotation and its direction requires measurements on both layers, active and passive, as shown in Figure 8. Since the circuit in Figure 7 cannot meet this requirement, a new topology is proposed in Figure 9.

The topology proposed in Figure 9 allows the following:
* Semi-Cycle 1: Voltage is applied to the Y layer while (VY+ – VY–), VX–, and VX+ are measured. The controller generates an interrupt after each measurement is completed, allowing the processor to change the GPIO configuration.
* Semi-Cycle 2: Voltage is applied to the X layer while (VX+ – VX–), VY–, and VY+ are measured.

gesture recognition 6

The circuit of Figure 9 permits all the voltages required to achieve full performance to be measured, namely, a) single touch location, b) zoom, pinch and rotation gesture detection and quantification, and c) CW vs. CCW rotation discrimination. Single-touch operation when performing a dual-touch gesture provides an estimation of the gesture centroid. 

  gesture recognition 8gesture recognition 7

Practical Hints

The variations in voltage associated with soft gestures are quite subtle. The robustness of the system can be improved by increasing these variations by means such as adding a small resistance between the electrodes of the screen and the pins of the controller; this will increase the voltage drop in the active layer, with some loss of accuracy of single-touch positioning.

gesture recognitionAn alternative is to add a resistor to only the low-side connection, sensing just the X– and Y– electrodes when they are active layers. By doing this, some gain can be applied, since the DC value is pretty low.

 A variety of amplifiers and multiplexers, such as those from Analog Devices, can fulfill the needs of the applications shown in Figure 6, Figure 7, and Figure 9. For these examples, the AD8506 dual op amp and ADG16xx family of analog  multiplexers, which offers low on resistance with a single 3.3-V supply, were used to test the circuits.

Conclusion

Zooms, pinches and rotations can be detected using the controller with minimum ancillary circuitry. These gestures can be identified with measurements in the active layer only. Rotation direction discrimination can be achieved by measuring the voltage in the passive layer, which can be achieved by using two GPIOs from the host processor. Fairly simple algorithms executed in this processor can identify zooms, pinches, and rotations, estimating their range, angle, and direction.

References

“New Touch-Screen Controllers Offer Robust Sensing for Portable Displays,” Gareth Finn, Analog Dialogue, Vol. 44, No. 2. February 2010. 

 “New Touch-Screen Controllers Offer Robust Sensing for Portable Displays,” Gareth Finn, Analog Dialogue, Vol. 44, No. 2. February 2010.
About the authors:

 

JavierJavier Calpe is a design center manager of Analog Devices, Inc. in the Valencia Development Center. He received his bachelor’s degree in Electronic Engineering in 1989 and his PhD in physics in 1993, both from the Universitat de Valencia (Spain).  

 

ItaloItalo Medina is an analog designer in the Precision DAC Group for Analog Devices, Inc. (Limerick, Ireland). He received his bachelor’s degree in Electronic Engineering from Universidad Politécnica de Valencia, Spain. 

 

 

 

 

AlbertoAlberto Carbajo is a senior design engineer and works for the test and design departments at Analog Devices, Inc. His work focuses on IC-based sensing products, including signal processing and integration with microcontroller-based designs. Alberto received a Bachelor’s degree in Electronic Engineering from Universidad Politécnica de Valencia, Spain and obtained his MEngSc from University College Cork (UCC), Ireland.  

 

 

 

MariaMaría José Martínez is an applications engineer in touch-screen products at Analog Devices, Inc. She is responsible for CapTouch and touch-screen controller and lens driver products and works for the portable segment. Maria received a bachelor’s degree in Telecommunications Engineering from the Universidad Politécnica de Valencia in 2005. Maria is currently based in Valencia working for the portable segment.  

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