ITW’s Chameleon range of capacitive sensing products use changes in capacitance to detect the presence of a finger on or near a touch surface, as shown in the illustration below. This touch button example illustrates a capacitive sensor replacing an electro-mechanical pushbutton switch. The sensing function is achieved using a combination of hardware and firmware.
When a finger touches the overlay touch surface, it forms a simple parallel plate capacitor with the sensor pad through the overlay. The result is called finger capacitance and is a simplification of a distributed capacitance that includes the effects of the human body and the return path to the circuit board ground within the ITW sensor. We then interpret this touch using an ADC method. A current source generates a linear voltage ramp on a capacitor. This voltage is input to an analogue comparator circuit. The comparator’s output is monitored and a counter increments whenever it transitions from high to low.
The ITW sensor includes firmware with algorithms which convert the sensor capacitance into a digital count, called raw count. The raw count is interpreted as either a TOUCH or NO TOUCH state for the sensor, within the firmware. The numerical value of the raw count is the digital representation of the sensor capacitance, and increases as the capacitance increases. Sensitivity is a measure of how much the output will change for a given change on the input. The sensitivity of the ITW sensor is optimised to give best overall performance in a broad range of conditions. The illustration below shows a representation of the raw count of a sensor in the touched and untouched states.
ITW’s capacitive sensing method uses a switched capacitor circuit on the front end of the system to convert the sensor capacitance to an equivalent resistor. The current measured through the equivalent resistor is converted into a digital count. When a finger is on the sensor, the capacitance increases and the equivalent resistance decreases. This causes an increase in current through the resistor, resulting in an increase in the digital count, which is interpreted as a touch.
About the ITW Group
A leading international business corporation with over 100 years of history, Illinois Tool Works Inc. (NYSE: ITW) is a diversified manufacturer of advanced industrial technology. Specifically, we design and produce an array of highly engineered components, equipment and consumable systems, and speciality products and equipment for customers around the world. The 825 decentralised business units that comprise our international business corporation are located in 52 countries and employ approximately 60,000 men and women who are focused on creating value-added products and innovative customer solutions. Founded in the 1950’s, ITW Switches, based in Portsmouth UK and Chicago USA, designs and manufactures the world renowned product brands of ITW Switches, ITW McMurdo Connectors and ITW ERG Components.