Embedded Touch Keyboard for Human-Machine Interface (HMI)

Padless Touch System (PTS) technology may be used in a wide range of applications including process instrumentation, machine tools, auto manufacturing, and consumer goods.

By Control Engineering Staff August 14, 2003

Padless Touch System (PTS) technology may be used in a wide range of applications including process instrumentation, machine tools, auto manufacturing, and consumer goods. Embedded touch keyboards based on PTS provide a cost-effective solution for many human-machine interfaces (HMI). The natural operator interface for industrial applications such as touch keyboards and touch screen operator interfaces can be easily and effectively designed by using PTS technology. Designers may select discrete electronic components or an ASIC (Application Specific Integrated Circuit), depending on the application and control scheme.

The PTS principle of operation is to detect a touch at a touch key location (touch keyboard/panel) by a user coupled to ground. The source model of PTS and discrete circuit implementation is shown in the figure below.

The input section of the circuit (V1, R1, R2 and Q1) produces pulses that are sent to the ground (earth ground or virtual ground). When a user, standing by a touch keyboard/panel, touches the dielectric element (such as a glass window or ceramic plate) on one sensing location, pulses are supplied to a conductive plate located under the dielectric element. A coupling path is thus formed through the body with reference to earth. An equivalent electrical model of the loop is shown by R ground, R body, C body and C glass.

The receiving section of the circuit (V2, R5, R6, Q2 and C3) responds to the predetermined potential variation in the conductive plate and produces an output signal indicative of the presence of a touch. An important advantage of PTS technology is that only the touch of the user, actively coupling the circuit, is activating the sensor—unlike the traditional sensing systems that depend on disturbing an existing capacitive circuit.

After extensive field and laboratory testing, a PTDE (Padless Touch Digital Encoder) ASIC was designed to enhance discrete circuit PTS designs already on the market. The PTDE ASIC is capable of deciphering, synchronizing and transplanting up to 50 padless touch keys, membrane or push button key inputs. The chip converts analog signals from key inputs via a 10-bit ADC (analog-digital converter) controlled by an Atmel AVR 8-bit RISC (reduced instruction set computer) microcontroller. A key matrix block converts the inputs from the keys and outputs a voltage level to the ADC multiplexer input. The encoded key data is directed via a GPIO (general purpose I/O) port or a 4-wire SPI (serial peripheral interface). A flat panel wipeable, rugged keyboard for industrial applications and the back of the panel, the component side of the PCB, are shown below:

Second generation PTS encoders are designed for wide variety of custom-designed operator interfaces with control microcomputer, keyboard modules, etc. microcontrollers are high performance, low power CMOS ICs with full control up to 42 keys. They are ready for operation without any calibration, tuning or other manipulation by the user. The user can set sensitivity according to the design needs. The IC output can be parallel PTS (direct binary code) or serial.

—Information for this article was provided by Dr. Miro Senk, Eng, Pressenk Instruments Inc. ( pressenk@bellnet.ca )and Simon Boutin, Tyco Electronics Corp. ( sboutin@tycoelectronics.com )

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