Touch Blog General Touch Screens
Touch Screen Article, Blog Aug 2009: Cassandra Backus, Charlie Riegert
Touch screens are user-friendly input devices that are becoming widely accepted in everyday human-machine interaction. From large industrial machines to groceries store check-out lines, all the way down to our cell phones and personal media players, touch screens are quickly becoming the standard interface.
It is important to understand that there is not a single perfect touch technology that fits every design and application. The three primary touch technologies each have different strengths and weaknesses. This chart highlights the main topics for consideration when choosing a touch technology: Cost/Optics/Sensor Life/ Integration Ease/ Multi Touch capability/ and Touch Object (the user input method)
Touch Technology Comparison Chart
Resistive technology easily dominates the touch market, accounting for over 80% of all touch designs, boasting several desirable qualities: low cost, ruggedness, ease of integration, and the ability to use any type of user input method (gloved hand/ stylus/ finger). The only major drawbacks for resistive technology are the optical quality of the sensor and its limited multi touch capability. This article will discuss the basic theory of resistive touch technology and address the crucial role of the resistive touch screen controller within a completed touch system, focusing on the embedded market.
Resistive Touch Technology
Resistive 4, 5, and 8-wire touch sensors consist of two facing conductive layers, held in physical separation from each other. The force of a touch causes the top layer to move and make electrical contact with the bottom layer. Touch position measurements are typically made by applying a linear voltage gradient across a layer or axis of the touch sensor. The touch position voltage for the axis can be measured using the opposing layer.
4 wired Diagram
The basic decoding of an 8-wire sensor is similar to a 4-wire. The difference is that an 8-wire sensor has four additional interconnects used to reference sensor voltage back to the controller. A touch system may experience voltage losses due resistance changes in the bus bars and connection between the controller and sensor. The losses can vary with product use, temperature, and humidity. In a 4-wire sensor, variations in the losses manifest themselves as error or drift in the reported touch location. An 8-wire touch sensor automatically adjusts for the changes, with the additional four reference lines. The reference lines allow the controller to know what the voltage is, at the touch sensor bus bars.
5 wire Diagram
5 wire resistive technology is a bit different and is designed to allow damage to the top layer without interfering with the touch solution. The voltage is not directly applied to the edges of the active layer, as it is for 4-wire and 8-wire sensors. The voltage is applied to the corners of a 5-wire sensor. To measure the X-axis, the left edge of the layer is driven with 0 Volts (ground), using connections to the upper left and lower left sensor corners. The right edge is driven with +5Vdc, using connections to the upper right and lower right sensor corners. To measure the Y-axis, the top edge of the layer is driven with 0 Volts (ground), using connections to the upper left and upper right sensor corners. The bottom edge is driven with +5Vdc, using connections to the lower left and lower right sensor corners.
Resistive Touch Electronics
Most resistive “touch controllers” on the market are basic analog-to-digital converters. They convert the raw electrical data from the sensor to a digital signal. The basic analog-to-digital converters can handle this operation, but still require more development to make them work properly as a touch screen controller.
To measure a touch position, the controller must drive the X axis and then the Y axis, creating a voltage divider where the voltage is sensed off the non-driven axis. While on the surface this is a simple concept, the sensor contains both resistive and capacitive elements that must be taken into account when designing this type of a device. These elements cause an associated RC rise time that varies both with pressure and the aging of the sensor. The analog-to-digital converter logic must be designed with this rise time in mind. In addition, filtering algorithms must be implemented to eliminate any invalid electrical data from the sensor. Calibration routines must also be defined, and then implemented, to map the electrical data to the visual display. Often, the analog-to-digital converter logic is designed around the characteristics, and anomalies, of a particular sensor. This can cause the integrator to pigeon-hole themselves into a single sensor sourced for the system. This can quickly turn sour, should the sensor no longer be sufficient for the system. This could be due to environmental damage to the sensor, manufacturer changes, or trying to implement the controller in a different application that requires a particular sensor construction, optic qualities, or durability. This then forces more engineering time to develop more algorithms for compensating for a different sensor to create a functioning system again.
As you consider touch screen controller options, investigate the details to be certain that you have a complete touch controller solution, rather than just an analog-to-digital convertor. A true turnkey touch screen controller should eliminate the need for reiterative engineering costs by providing filtered, reliable touch coordinates which ultimately provides for lower costs and quicker development cycles for embedded products.