CURRENT ISSUE
Contests
FROM THE BENCH
|
|
Issue #213 April 2008
Digital Touch
A Potentiometer with No Moving Parts
by Jeff Bachiochi
Start | Touch Slider IC | Digital Potentiometers | Control and Feedback | Value to Digits | Linear Segmentation | Worth It | Sources & PDF
TOUCH SLIDER IC
Quantum Research Group has packaged three capacitive sensors in a single IC. The sensors can perform 128-bit resolution position translation by comparing the inputs. Each of the three sensor circuits requires two pins: a drive pin and a receive pin. The receive pin is the common connection of an RC network. The free end of the sampling capacitor goes to the drive output and the free end of the resistor becomes the sensing detector. When a grounded object (i.e., a finger) approaches the sensor, it places added capacitance to ground. When it approaches the resistor toward the sensor input, there is minimal resistance. At the opposite end of the resistor, there is maximum resistance. Not only is there an RC timing difference at opposite ends of the resistor, there is an RC timing difference at all points between the two extremes. If the sensing resistor elements are connected between sensor inputs, then a comparison of inputs can be used to mathematically determine the position of a grounded object approaching the sensor between the two sensing inputs. Because there are three inputs, there are three exclusive sensing elements. Detection takes place when an element (between two inputs) senses an object. Large objects that approach multiple elements are rejected.
The QT411 has a simple SPI. Few commands are necessary to establish a working sensor. Figure 1 shows that the interface uses two additional outputs to the normal SDI, SDO, SCLK, and *SS signals. A low DRDY output indicates a busy state. This is when the QT411 is making its acquisition bursts. When a burst recognizes an input at up to two of the sensors, the DETECT input is driven high. This optional output can be used in Free Run mode where the IC makes periodic acquisitions without operator intervention. Otherwise, a detect status bit holds the same information and is returned with each command transfer.
The lowest power can be implemented using the Polled mode because an acquisition is performed only after the device has been selected and deselected (*SS). There are three modes: Forced Acquisition (when *SS goes high), Free Run (approximately 30 ms after *SS is held high), and Sleep (*SS is held low). If you are familiar with SPI communications, you know that data can be clocked in both directions at the same time (see Figure 2). This means that if you are asking for information, you must wait until the next command to receive a reply. The QT411 has only five commands and three are special and used for initialization. Because the device initializes on powerup with default values, unless you want to change these, you may never use these special commands. Once an error-free calibration has occurred, the NULL and DRIFT commands are the only ones necessary. The commands return a status byte that indicates whether a touch has occurred (MSBit = 1) and, if so, what the value is (0–127 LS6–bits). The DRIFT command is used periodically to enable the device to “self adjust” to environmental changes.
To produce a linear element that you can use for detection purposes, you don’t need to use a continuous resistive element, such as iridium-tin-oxide (ITO) ink. The element can be made up of segments connected together with discreet resistors (see Photo 2). The capacitive nature of an object (e.g., a fingertip) is linearly divided by adjacent segments when each segment is sized appropriately to the object doing the touching. The total resistive value of the element should be approximately 400 kW. The three sampling capacitors’ (mentioned earlier) values can be varied slightly to attain proper sensitivity based on insulator material and thickness. In this case, the material is .063 FR4 PCB, possibly with a thin acrylic label on top.
 |
| Photo 2 — Segments on this prototype are about 0.25² wide. They are 0603 SMT resistors hand-soldered between segments. |
|
