Piezo Switches, Keypads & Capacitive Touch Sensors

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1 Piezo Switches, Keypads & Capacitive Touch Sensors Submitted by : PALLAV KOHLI Contents Abstract... 2 Introduction... 2 Literature Overview... 2 Project Description... 3 Conclusion & Recommendation... 10

2 Abstract Piezo touch switches are based on mechanical bending of piezo ceramic, typically constructed directly behind a surface. This solution enables touch interfaces with any kind of material. Another characteristic of piezo is that it can function as actuator as well. Current commercial solutions construct the piezo in such way that touching with approximately 1.5 N is enough, even for stiff materials like stainless steel. Introduction A piezoelectric sensor is a device that uses the piezoelectric effect, to measure changes in pressure, acceleration, temperature, strain, or force by converting them to an electrical charge. The prefix piezo- is Greek for 'press' or 'squeeze'. In both professional and consumer products, switches are the main element by which the operator of a technical apparatus can influence its operational functionality. Hence, switching is the essential element of any User Interface (UI). Contemporary UIs are increasingly often realized by means of touch buttons, touch panels or touch screens based on a piezo switch. These buttons, panels or screens operate in such a way that hardly any mechanical work is needed from the operator in order to realize a switching action, implying that there is a minimization of motions and of the number of moving parts, which results in many advantages. Furthermore, touch panels and screens are one of the most flexible input devices, due to the fact that the interface is designed in software. No problem with different languages, or high initial costs for small changes. Literature Overview The success of touch-operated UIs based on a piezo switch is based on different factors. First, sophisticated manufacturing processes have been established that allow for low cost mass production of switches that are subsequently easily assembled in the containing product. Secondly, touch is being associated with contemporary, young, futuristic electronics and clean design, a factor of growing importance. Already, many of the most highly-desired products like MP3 players, mobile phones, etc. are associated with touch. A piezo switch can offer these advantages very well.

3 Project Description Extreme weather conditions such as ice, snow, extreme heat etc, call for extremely durable technology. A piezo switch is created and designed with the purpose of providing just that, a vandal resistant operation for all kinds of extreme weather conditions, and harsh industrial working environments. A Piezo switch is a solid-state switching technology based on the functional principle of the piezoelectric crystal that utilizes the direct piezoelectric effect. Piezo switch elements typically have a piezoelectric component physically mounted on the back of a thin metal surface, the size of which is based on desired force requirements of the user. The characteristics behind this component are such that when an action force is applied to the piezoelectric element there is a small displacement and subsequent strain on the dielectric material. This strain causes a build up of an electric field that causes a voltage to be induced due to a charge transfer. The voltage generated is converted by the electronic connection into a polarity-neutral, electronic switch contact. One of the main advantages of the Piezo switch is that there are no moving parts such as contacts, springs or actuator systems, resulting in fewer failures, reduced wear, and extending the life cycle. The flat actuation surface is completely sealed, preventing the intrusion of liquids or other contaminants. The electronics within the Piezo switch can also be fully potted protecting the components from the environment and making it impervious to water, oil or dirt. Having this type of design provides a sleek contemporary appearance, protection of control components, seamless, smooth and easily cleaned surface, and an enclosed system not affected by spills. Many solidstate switches are limited by activation with insulated materials such as gloves, but since piezoelectric technology is force related, Piezo switches can be activated by non-conductive materials. There is a growing trend and a number of advantages in moving toward solid state switching devices such as Piezo. How a Piezo Switch Works Finger pressure on the surface of the housing causes the housing to flex very slightly (approx. 2 microns) Pressure activates a piezo crystal which emits low level a signal which is amplified by electronics built into the housing. Advantages of Piezo Switches No moving parts so reliability is typically >20 million cycles Tamper proof housing Impervious to weather, chemicals, beverage spills etc Available in various housing materials

4 Requires no external power source Highly resistant to external impacts Material Options for Piezo Switches Anodized or Powder Coated Aluminum Stainless Steel Brass Plastic In environments requiring high level of hygiene or, in applications where operating a switch manually can sometimes be hazardous, complicated or cut off the work sequence, touch free switches are just the answer. WAvE On WAvE OFF SWITCHES... An infra red sensor suitable for applications demanding flexible installation possibilities. It allows activation of the product by placing a hand within a close distance of the sensor eye. Placing a hand within a close distance of the sensor eye a second time will stop the product operation. A programmable wave on wave off switch that, by including a smart PCB unit, allows for time based and automatic activation functions. Changing some of the programmable wave on wave off settings is possible on site by using the remote control. PROX SWITCHES... An infra red sensor that is based on the proximity of the persons or objects in front of it. It allows activation of the product once the person or object enters its range, and shuts off when the person or object leaves its range. Changing some of the switch settings is possible on site by using the remote control.

5 Piezo Switch Sensor Design Based on piezoelectric technology various physical quantities can be measured; the most common are pressure and acceleration. For pressure sensors, a thin membrane and a massive base is used, ensuring that an applied pressure specifically loads the elements in one direction. For accelerometers, a seismic mass is attached to the crystal elements. When the accelerometer experiences a motion, the invariant seismic mass loads the elements according to Newton's second law of motion. The main difference in working principle between these two cases is the way they apply forces to the sensing elements. In a pressure sensor, a thin membrane transfers the force to the elements, while in accelerometers an attached seismic mass applies the forces. Sensors often tend to be sensitive to more than one physical quantity. Pressure sensors show false signal when they are exposed to vibrations. Sophisticated pressure sensors therefore use acceleration compensation elements in addition to the pressure sensing elements. By carefully matching those elements, the acceleration signal (released from the compensation element) is subtracted from the combined signal of pressure and acceleration to derive the true pressure information. Vibration sensors can also harvest otherwise wasted energy from mechanical vibrations. This is accomplished by using piezoelectric materials to convert mechanical strain into usable electrical energy. Capacitive Touch Sensor A Fast Touch capacitive touch system consists of Fast Touch electronics, conductive circuit layer and graphic overlay. At the heart of the Fast Touch electronics is the Fast Touch IC. The various touch pads of the conductive circuit layer are connected to the Fast Touch IC. The pads formed

6 by the conductive circuit layer could be copper pours on a PCB or printed conductive elements on a film. The overlay is non-conductive and placed on top of the circuit layer. The overlay material could be glass, lexan, acrylic or any other plastic of various thicknesses. When the human hand is brought in close proximity to the pads of the circuit layer, the capacitance associated with the pad changes. This causes a change in the voltage measured on the pads. This change in voltage is registered as a button press by the Fast Touch system. Capacitive touch sensing is a very timing sensitive process. This is because voltage measurements happen in the transient state rather than steady state. In fact it is recommended to disable all other interrupts, while voltage is being measured. Good EMI immune software techniques and user perception dictate that, such measurements need to be carried out frequently at fast rates. This leaves very little time left for other tasks. The greater the number of buttons, the more is the I/O pin resource and scan time requirement. This is why you need a separate touch controller IC. There are two main types of keypads : PCB keypad: The PCB keypad has the capacitive touch sensor pads and the capacitive touch driver electronics all in one PCB. The graphic overlay is attached to the PCB with adhesive. Film Keypad:

7 The FILM keypad has the capacitive touch sensor pads printed on a thin flexible film. The film has a ZIF tail which is plugged into the capacitive touch driver electronics. The following process for keypad switch production: 1. Determine type of keypad 2. Confirm functionality and communication requirements 3. Design and fabricate PCB or FILM Keypad and Electronics 4. Print and assemble graphic overlay 5. Configure and Tune keypad 6. Test capacitive touch interface system

8 Piezo Keypad

9 Applications Piezoelectric sensors are versatile tools for the measurement of various processes. They are used for quality assurance, process control, and for research and development in many industries. Pierre Curie discovered the piezoelectric effect in 1880, but only in the 1950s did manufacturers begin to use the piezoelectric effect in industrial sensing applications. Since then, this measuring principle has been increasingly used, and has become a mature technology with excellent inherent reliability. It has been successfully used in various applications, such as in medical, aerospace, nuclear instrumentation, and as a tilt sensor in consumer electronics or a pressure sensor in the touch pads of mobile phones. In the automotive industry, piezoelectric elements are used to monitor combustion when developing internal combustion engines. The sensors are either directly mounted into additional holes into the cylinder head or the spark/glow plug is equipped with a built-in miniature piezoelectric sensor. The rise of piezoelectric technology is directly related to a set of inherent advantages. The high modulus of elasticity of many piezoelectric materials is comparable to that of many metals and goes up to 106 N/m².Even though piezoelectric sensors are electromechanical systems that react to compression, the sensing elements show almost zero deflection. This gives piezoelectric sensors ruggedness, an extremely high natural frequency and an excellent linearity over a wide amplitude range. Additionally, piezoelectric technology is insensitive to electromagnetic fields and radiation, enabling measurements under harsh conditions. Some materials used (especially gallium phosphate or tourmaline) are extremely stable at high temperatures, enabling sensors to have a working range of up to 1000 C. Tourmaline shows pyroelectricity in addition to the

10 piezoelectric effect; this is the ability to generate an electrical signal when the temperature of the crystal changes. This effect is also common to piezoceramic materials. One disadvantage of piezoelectric sensors is that they cannot be used for truly static measurements. A static force results in a fixed amount of charge on the piezoelectric material. In conventional readout electronics, imperfect insulating materials and reduction in internal sensor resistance causes a constant loss of electrons and yields a decreasing signal. Elevated temperatures cause an additional drop in internal resistance and sensitivity. The main effect on the piezoelectric effect is that with increasing pressure loads and temperature, the sensitivity reduces due to twin formation. While quartz sensors must be cooled during measurements at temperatures above 300 C, special types of crystals like GaPO4 gallium phosphate show no twin formation up to the melting point of the material itself. However, it is not true that piezoelectric sensors can only be used for very fast processes or at ambient conditions. In fact, numerous piezoelectric applications produce quasi-static measurements, and other applications work in temperatures higher than 500 C. Piezoelectric sensors can also be used to determine aromas in the air by simultaneously measuring resonance and capacitance. Computer controlled electronics vastly increase the range of potential applications for piezoelectric sensors. Piezoelectric sensors are also seen in nature. The collagen in bone is piezoelectric, and is thought by some to act as a biological force sensor. Conclusion & Recommendation Piezo Application includes : Food Processing Petroleum Dispensing Clean Room Interfaces Internet Kiosks Access Control Fluid Dispensing Mining Equipment Emergency Call Systems Dairy Equipment High-End White Appliances Oceanographic Instruments Elevators Conveyers Car wash Weighing Devices Military applications and many more.