White Paper. Revision 1.0 B June 23, Densitron Corporation 2330 Pomona Rincon Road Corona, CA Phone: Fax:

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1 DensiTouch TM White Paper Revision 1.0 B June 23, 2010 Densitron Corporation 2330 Pomona Rincon Road Corona, CA Phone: Fax: DensiTouch TM White Paper Page 1 of 16

2 Contents What is Haptics?... 3 Projective Capacitive Touch Screens and Haptic... 4 How are Haptics Generated?... 6 Haptics Control System... 9 Haptic Solutions Extending Haptics Beyond the Display Conclusion References DensiTouch TM White Paper Page 2 of 16

3 What is Haptics? The term haptics in its broadest sense relates to the study of touch and the faculty by which external objects or forces are perceived through contact with the body. The word itself derives from the Greek haptikos, able to touch. Haptics can be found in wide range of devices. In the extreme with respect to precision, surgical simulators use haptics to provide realistic forces that emulate the feel of a real medical procedure. In the extreme with respect to the magnitude of the force feedback, computer gamers can experience added realism to their favorite games with haptic enabled joysticks that allow them to feel every bump, explosion, rumble, burst of gunfire and other vibration-based activity. Haptics is also emerging into cell phone design. The simple act of the phone vibrating when a call is received is the most recognizable and rudimentary form of the technology. Figure 1 Surgical Simulators, gaming joysticks and cell phone As more embedded systems incorporate touch screens, designers can look toward haptics to provide the tactile feedback to compensate for the replacement of mechanical buttons with a glass touch sensor surface. A successful haptics touch screen implementation can simulate the reassuring click of mechanical button or the feel of a potentiometer knob or slider passing over individual detents. This white paper will focus on haptics touch screens in embedded systems similar to those found in medical instrumentation and industrial or commercial environments. DensiTouch TM White Paper Page 3 of 16

4 Projective Capacitive Touch Screens and Haptics In addition to widespread deployment of Projective Capacitive Touch (PCT) in cell phone technology, PCT is accurate enough to control equipment precisely and is ideally suited for harsh environments. These screens can withstand high pressure cleaning with caustic solvents common in industrial, medical, instrumentation environments and harsh environments like ATM machines requiring 7 Mohs hardness 1. The actual touch surface is a protective glass front cover layer that bonded to the PCT sensor. PCT sensing requires a near zero grams activation force. Depending on the PCT technology employed, this touch can be recognized even with gloves ranging from medical gloves to work gloves. Figure 2 PCT Layers While PCT technology offers the highly desirable benefits above, it comes at a price with respect to tactile feedback. Without near zero force feedback reinforcement of a metal membrane dome, push button or detent potentiometer, the user is left with a less than compelling replacement for the legacy mechanical interface they are accustomed to. It is a paradox that the zero force feedback creates the demand for Haptics and also offers the perfect transmission platform for Haptics. PCT is the preferred enabling technology for Haptics solutions over resistive touch because of the low force activation. Resistive touch technology requires a force to deflect the surface. This adds a variable force that the haptics system needs to overcome and compensate for. In addition, it adds latency in the response to the touch and increases complexity to the implementation. (1) DensiTouch TM White Paper Page 4 of 16

5 The ultimate goal for embedded systems designers is to develop high-fidelity tactile feedback verses that of a rudimentary vibration alert found in most cell phones today. At first glance, one might conclude that the design approach in bringing haptics to a cell phone would be the same to as that for a larger embedded design with a PCT. This initial assumption would be wrong and the two design approaches are actually quite different. The only commonality between the two is that they both can use PCT. Cell phones are typically incorporate touch screens and displays from 3.5 to 4.3 in size and typical embedded designs will use larger screens. The larger surface area actually complicates the addition of haptics. Cell phones are also handheld whereas embedded designs are mounted or fixed to surface. This is referred to being a mechanically grounded device (either fixed or through inertia as a heavy instrument setting on a lab bench). Lastly, PCT used in cell phones are typically rigidly mounted to the cell phone case and often bonded to the TFT display, while the PCT in a embedded design can be allowed to vibrate free of the rest of the device or is effectively free floating when the appropriate gaskets and design guidelines are incorporated. These two differences, a grounded device and floating PCT allow embedded designs to achieve a level of haptics beyond what is typically experienced from a cell phone. Since a free floating embedded design vibrates a smaller mass, the floating PCT sensor, more precise and realistic haptics can be generated suing lower force faster actuators. In the case of a cell phone, the PCT is fixed to the mass of the cell phone and the mass of the entire cell phone must be vibrated. Moreover, the effective mass of the cell phone is influenced by how tightly the user holds the phone. Figure 3 Cell Phone with fixed PCT DensiTouch TM White Paper Page 5 of 16

6 For the embedded design, the goal is to direct the haptic effect to the user s fingertip on the PCT snesor and not to the surrounding mass of the rest of the device. Figure 4 Embedded design with floating PCT How are Haptics Generated? Haptics systems require one or more actuators to create a vibration on the device. Actuators are available in various forms, inertial, piezoelectric and even electrostatic. Each type of actuator will have its own unique response profile with different response times and design tradeoffs. The correct selection of the actuator is crucial in the design. Actuators with slow acceleration, excessive displacement or lack of precise displacement control will yield poor results. The most commonly used actuator is the inertial actuator which is an electromechanical device and typically used in cell phones. Inertial actuators are typically one of two DensiTouch TM White Paper Page 6 of 16

7 types, a linear resonate actuator (LRA) which vibrates a spring loaded mass in a linear motion or an eccentric rotating mass (ERM) actuator which rotates an off-center mass. The LRA is a more specialized actuator of the two, creating vibration in one axis. LRA are driven at the fixed resonate frequency of the mass/spring, typically 175 Hz 1. ERM generates vibrations in all directions of its rotation and has associated with it a spin up and spin down time. Figure 5 ERM Actuator and LR Actuator Piezoelectric actuators are ceramic devices that produce a displacement when a voltage is applied to it. In their favor, they are available in a variety of shapes, have thin form factors, prove fast response time and high bandwidth. The most common shape for touch screen haptics are the beam and disk. Piezoelectric actuators alone do not produce a vibration, but rather a displacement by pushing a sprung mass against a unstrung mass resulting in the haptic effect. The unstrung mass must have a sufficiently higher mass than the sprung mass so that when the actuator deflects, the only the sprung mass moves. This also requires that the two masses be mechanically isolated from each other while allowing the piezoelectric actuator to make contact with both. Additional care in the design must be taken to reduce the risk of breakage during the manufacturing process and usage due their thin form factors. Piezoelectric actuators can require driving voltages between 10 to 100 volts and additional drive and amplification circuitry. Figure 6 Piezo Beam and Piezo Disk Figure 7 Sample Piezo Drive circuit DensiTouch TM White Paper Page 7 of 16

8 Electrostatic actuators use the principle of electrostatic. A charge differential is generated between the touch surface and a sub-surface creating an attractive force, which causes motion of the touch surface. Since attractive forces are used, there are no physical actuator components. Electrostatics provide consistence over the entire surface and scalability as the surface area grows 1. It is much easily to achieve a consistent response profile over the surface since the attractive forces are evenly distributed over the entire surface. The actuation force will scales as the touch surfaces increase while stronger or additional actuators will be needed for inertial and piezoelectric actuators if the touch surface increases. Figure 8 Electrostatic Example In conjunction with the actuator selection, the implementation or placement of the actuator is important. A poor implementation may cause the entire system to vibrate or dampen the effect is beyond detection. The third interrelated design criteria is the degrees and extent of movement the PCT is allowed. Depending the design and size, the PCT may be restricted to just a Z axis movement or in the X and Y axis. DensiTouch TM White Paper Page 8 of 16

9 Haptics Control System With all of the mechanical specifications selected, a control system is required to drive the actuators to create the realistic haptic effect. The design of the control system, hardware and software together should be able to detect a touch and generate a haptic in less than 30ms 2. A typical rule of thumb would be.25 MIPS 1 of spare processing capacity should be allocated on the processor to generate the haptic. Each haptic effect is created by driving the actuator for a specific intensity, sequence and period of time resulting in a haptic waveform for that haptic effect. The required haptic waveform for a haptic effect does not change from design to design, however, the algorithm on how to drive the actuator will. Each algorithm must factor in the characteristics of the actuator used, the mass of the device being vibrated and the dynamic response of the mass being vibrated. In summary, each algorithm must be developed and tuned for the specific mechanical design in order to produce the correct haptic effect. Figure 9 Haptic Waveform DensiTouch TM White Paper Page 9 of 16

10 Haptic Solutions For most embedded system designers, the details of the mechanical designs, the development of the control system and the complexity of the haptic algorithm would place haptics beyond their reach. However with advances in haptic technology, the focus of haptics specifically toward embedded systems and key technology partnerships, the inclusion of haptic is close to an off the shelf implementation. PCT screens are available that are haptic ready with mechanical design details for preengineered haptics. Haptic control systems are available as integrated circuits. The haptic algorithms required to produce the haptic effect are stored on the control system and can be controlled by an API from the embedded processor. The development and prototype cycle has been reduced with Haptic PCT screens that can be driven by a Windows system and USB ports. Figure 10 Embedded System Densitron 5.7 Haptic Reference Design Consider the Densitron 5.7 Haptic Reference Design (Reference " Haptic Reference Design Application Paper) as an example of a haptic ready design. This reference design demonstrates the full integration of haptics onto a 5.7 PCT and LCD. It includes the mechanical design to create a free floating PCT, the selection and DensiTouch TM White Paper Page 10 of 16

11 placement of the actuator, NEMA 2 sealing, off the shelf control system, the main housing design and the host demonstration software. The main housing serves as the mechanically grounded device that the rest of the layers of the design are built upon. First, a NEMA rated gasket is attached the cover bezel. Then an aluminum carrier with the actuator is attached to the grommets. Next the touch PCT screen is attached to the front of the bezel by securing it to the gasket. Next the bezel assembly is attached to the housing that constrains the TFT display. In this design the PCT assembly is comprised of the PCT, an aluminum carrier and the actuator. The PCT is attached to the carrier by double sided adhesive and the actuator is mounted directly on to the carrier. The PCT assembly is also attached to the main housing. But unlike the LCD, the PCT assembly is the free floating mass of just the sensor and carrier for the haptic effect to manage. The PCT assembly is attached to the main housing using specialized suspension grommets. These grommets not only hold the PCT assembly to the main housing but at the same time allows the PCT assembly to remain a free floating mass. In addition, the grommets are designed to promote movement in the Z axis and reduce the unwanted vibration in the X and Y axes. Since the motion is predominantly in the Z axis, a common and inexpensive but very reliable ERM actuator can be used. Without this control from the grommets, the ERM actuator would vibrate in all directions of its rotation making the development of the haptic effects more complicated. For the mechanical design, less clearance between the PCT assembly and the main housing in the X and Y plane is required since the movement is predominantly in the Z direction which is desirable in sealing the units that are concerned about the debris gap. To achieve a NEMA 2 sealing, a Poron dust seal is placed between the PCT sensor and the Bezel ledge and again since movement is only in the Z axis, a secondary dust seal will not interfere with the haptics effect. The mechanical design provides the control system a known identity for it to manage, that is, the fixed mass of the PCT assembly, motion predominantly into the Z axis and the known dynamic response of the actuator. Just as importantly, the mechanical design also isolates the rest of the design from the control system. LCDs of different sizes and form factors can be used without affecting the control system. The control system used in the reference design is an Immersion Corporation Aries Haptics controller licensed for use by Densitron. This Aries haptic controller is specifically tuned for this mechanical design, stores all the haptic waveforms required to generate an effect and directly drives the actuator. In most embedded system, the embedded CPU would interface directly to the Aries controller and the embedded DensiTouch TM White Paper Page 11 of 16

12 software would issue high level API commands to instruct the control system to perform a haptic effect. This distributed processing architecture allows for reduced load on the main embedded processor and greatly insures that latency is held to a minimum. For a Windows based demonstration system, two USB interfaces and a VGA port were added to the reference design system. In this case, the control system interface is a USB interface instead of the typical embedded I2C interface. The API commands used for the embedded CPU and the Windows system would be the same. The PCT screen also interfaces to the Windows systems using a PCT/USB interface board and uses a second USB port. The 5.7 VGA TFT is driven from a VGA port interface. The Windows based demonstration application will place the Haptics Menus on the reference design display, any touches on the PCT touch screen on the reference design will be exported to the windows based computer as mouse actions via USB and the windows application will then provide the necessary API calls and directly drive the Aires IC via USB. The Aries IC on the embedded reference design will then send the appropriate waveforms to the actuator and the user will feel the actual result... The demonstration source code will provide code samples on how to issue API commands to the controller. Moreover, the developer can use this source code and reference design as a windows based test bed to prototype and test a complete user interface before an embedded system is available. The design effort for the embedded system can utilize and leverage key components of the engineered solutions contained in the reference design. For the embedded system, the development task can be reduced to: 1. Replicating the PCT assembly per the reference design 2. Suspending the PCT assembly to your housing per the reference design 3. Interfacing the Aries controller chip to the embedded CPU board 4. Developing the application software DensiTouch TM White Paper Page 12 of 16

13 Extending Haptics Beyond the Display Designers should not limit haptics to just menu and buttons on the display. In many instruments without a touch screen, mechanical user interface devices like buttons, keys, sliders and dials (potentiometers) are placed around or near the display screen. As designs move to touch screen display to eliminated mechanical interfaces, the initial design trend was to position these touch screen buttons, keys and sliders on the active area of the display.. In this approach, the display area now has to share its display area content with the presentation of the user interface. To preserve the original size of information display area, the designer could increase the size of the display to include the user interface or multiplex the display data with the presentation of buttons and keys by implementing a series of menus and multiple screens. Increasing the size of the display adds unnecessary cost to the design. Implementing menus and multiple screens, adds complexity to the design and could make the user interface cumbersome and difficult by requiring too much navigation thru multiple menus and screens to reach specific high traffic buttons or keys. Figure 11 Example interfaces Traditional Mechanical instrument and DensiTouch Haptics enabled Versapanel Solution To take advantage of the benefits of both approaches and avoid the downside of each, consider combining a PCT that is larger than the display, for example a 7 PCT on top of 4.3 TFT display screen. The entire 7 surface of the PCT is an active sense area with haptics. The designer can utilize the portions of the PCT that are not above the TFT display for high traffic or always present buttons and keys. Icons for these fixed and always present buttons, keys, sliders and potentiometers can be label bonded directly to the front glass cover layer of the PCT sensor. The area directly on top of the TFT display can be used in combination for display content as well as implementation of soft menus and funcitons. DensiTouch TM White Paper Page 13 of 16

14 The Haptic PCT is free floating. The haptics are not directly affected by the size of TFT display used or its position beneath the Haptic PCT. The designer has another degree of design freedom in the size selection and position of the TFT to be used typically producing a superior interface with a reduced Bill of Materials Cost. Figure 12 VersaPanel To take further advantage of the PCT design, the front glass layer can be modified such that button surfaces are raised or lowered above the plane of the glass to enhance the user interface. Circular groves for dials and straight groves can be added to guide the movement of a finger on a dial or a slider. Thru holes for cable connections can also be accommodated as long as the location is of the hole is not directly above the PCT sensor. (Custom Instruments Versapanel Touch Designs) DensiTouch TM White Paper Page 14 of 16

15 Conclusion As more embedded systems incorporate touch screens, designers can look to haptics to provide the tactile feedback to compensate for the disappearance of mechanical buttons. A superior haptics touch screen design and implementation can simulate the reassuring click of mechanical button or the feel of a knob or slider passing over individual detents The design of a haptics system requires achieving specific mechanical specifications and the development of a control system as well as the development of haptic algorithms. Haptic Ready PCT screens with the mechanical design details pre-engineered are available thanks to advances in haptic technology, market focus on the needs of embedded system and key technology partnerships. The design complexity of the control system for haptics has been simplified with the availability of control systems on integrated circuits that include haptic algorithms. The modularity of PCT, haptics and TFT display frees designers to be creative in the interface design by allowing the different sizing and positioning of TFT inside a PCT area. All of the above make the inclusion of haptics come very close to becoming an off the shelf design that is also flexible to accommodate a wide range of implementations. DensiTouch TM White Paper Page 15 of 16

16 References 1 Michael Levin and Alfred Woo Tactile-Feeback Solutions for an Enhanced User Experience Information Display Oct Steve Kingsley-Jones Install haptic technology to touchscreens EE Times-India Feb 2008 Picture credits Surgical Simulator Picture Hospital Clinic of Barcelona laboratori de simulacio clinica de la facultat de medicina/ Joystick Picture Wikipedia.org Haptic Cell Phone picture WordPress.com PCT picture Touch Screen Magazine.nl touchtechniques/projected capacitive All actuator pictures Tactile Feedback Solutions for an Enhanced User Experience, Information Display Oct Piezo drive circuit Driving Piezeoelectric Actuators by Sam Robinson, Power Electronics Technology Haptic Waveform Touch Sense Haptic Performance Kit data sheet Haptic Performance Kit_may09 v1.pdf Oscilloscope Picture Gao Tek Inc Menu Picture Hometheaterhifi.com media server part 1.html DensiTouch TM White Paper Page 16 of 16