A sense of touch

Ian Smith, Business Development Manager for Touch Screens at Danielson, reviews the latest developments in HMIs - notably touch screen technologies and membrane switches.

The relationship between man and machine is a profitable one, based on mutual benefit and growth.

For instance, as technical innovations have enabled increased efficiencies and greater profits, so investment in new technologies has given rise to the next generation of machines.

However, in the early days, when man merely flirted with the possibilities offered by technology, there was little interaction between the two.

Machines were simply workhorses, designed and built to complete a specific task.

Consequently, little if any feedback was required or sought.

Yet, as one innovation led to another, the relationship between the two grew.

Knowledge became a defining factor and the human machine interface (HMI) became the tool with which information was obtained and conveyed.

As the Chicago-based International Engineering Consortium (IEC) points out, HMIs have certainly changed, but retain a core purpose based around communication.

"Humans interact more with computer-based technology than with hammers and drills".

"Unlike tools, however, the visible shape and controls of a computer do not communicate its purpose".

"The task of an HMI is to make the function of a technology self-evident".

"Like a well-designed hammer fits the user's hand and makes a physical task easy, a well-designed HMI must fit the user's mental map of the task he or she wishes to carry out".

As more people are interacting with computers, there are fewer machine operators and consequently greater significance is placed on accurate, clear and reliable HMI technologies, as revealed by a study published by Frost and Sullivan.

In it, the market research company reveals that uptake of HMIs in European industrial plants is growing rapidly while operator numbers plummet.

"The number of operators per machine has reduced sharply over the past decade", Gabriela Martinho, Frost and Sullivan Research Analyst, says.

"HMIs facilitate easy monitoring of complex manufacturing systems".

"Easy to install, operate and maintain, these devices allow companies to focus on more value-added operations such as speed to market, reliability and the capability to develop new products".

Today, the HMI market is dominated by cutting-edge technologies and ingenious innovations that enhance the user's understanding of production processes, resting on the basic principle of providing an effective interface where people meet technology.

Through visual displays and inputting devices, HMIs are being used to improve profit margins by enabling greater levels of process appreciation and control.

It is therefore not surprising to learn that HMI products are available as bespoke solutions as well as off-the-shelf components, perhaps the most important of which is touch screens.

Because touch screens are extremely easy to use, they are an ideal medium through which to communicate often-complicated information to a diverse range of users.

For instance, point-of-sale systems and public-information displays are now a common sight on most high streets, and industrial control systems are prevalent in many commercial and industrial settings - an example being process control touch screen terminals on factory floors.

The user friendly touch screen interface can be less intimidating and easier to use than many other input devices, especially for novice users.

As a result, information kiosks and trade-show displays are used regularly by people who have little or no computing experience.

Touch screens help make information more easily accessible by allowing users to navigate through data simply by touching a screen.

This ease-of-use characteristic also lends itself to commercial applications, where touch screens are being used as cash registers and reservation systems in retail outlets and restaurants.

Here, touch screen systems enable employees to work faster and be trained quicker.

Similar self-service touch screen terminals are being introduced to speed-up customer service in airports and cinemas.

Of even greater significance is the impact touch screens have made in industrial process control.

By integrating input devices with the display, monitoring duties and the adjustment of complex operations can be completed simply and valuable workspace saved.

For instance, these systems give assembly line workers access to product drawings and job details that can be viewed and manipulated easily, without relying on large swathes of technical drawings.

In addition, training times, and therefore training expense, can be reduced.

The touch screen interface is being used in a growing number of settings to improve the human-machine experience.

But how do touch screens work?.

Touch screens rely on various technologies to enable navigation, data entry and general interaction with a huge variety of applications.

There are four main types of touch screen: surface acoustic wave (SAW), infra-red, capacitive and resistive systems.

SAW based solutions send acoustic waves across a clear glass panel containing a series of transducers and reflectors.

When a finger touches the screen, the waves are absorbed resulting in contact being detected at that point.

Unfortunately, these systems are only effective if contact is made through a finger and/or where dust and other debris do not present a major problem.

Alternatively, infra-red screens operate by generating a grid of light across the face of the screen and continuously checking for interruptions to that grid.

The drawback with this technology is that the screen may react before the user physically touches it.

Capacitive technologies employ a charge-storing material and a circuit system that detects screen contact by measuring capacitance.

This technology is limited, however, by the fact that when a nonconductive material touches it, the screen is rendered useless.

Resistive touch screens are generally constructed from two layers of material coated on the inner surface with indium tin oxide.

These layers are laminated together with the conductive surfaces separated by nonconductive spacer dots.

The screen is usually placed on glass or a polycarbonate backpanel and mounted in front of an LCD or other type of flat display.

An electrical current is passed through the screen, the value of which changes as force is applied to the surface.

These screens provide high resolution characteristics that can be used in conjunction with a gloved finger or a stylus.

As such, resistive touch screens are suitable for applications where operators require protection, such as in process control, plant maintenance and biomedical systems, or where stylus operation is required.

Although stylus operation has been until relatively recently confined to courier companies and PDAs this requirement is growing within the industrial and medical markets.

For instance, instead of using rolls of papers for sign-off QC purposes, chart recorders use digital signatures via touch screens.

Resistive touch screens are ideal for this purpose, although a specific material needs to be used for the actuation layer in order to prevent the ITO from cracking when high forces are applied by the tip of a stylus.

Similarly, the size of the spacer dots is important as a smooth and continuous operation is required to prevent signature break up.

There are two types of resistive touch screens, both of which offer low-cost and reliability.

Matrix resistive screens are etched to form switch areas arranged in rows and columns.

When contact is made, the circuit is closed and the row and column selected is identified.

This design is used where defined switch positions are required within the display area.

The display can be used via software to highlight different icons or text in each switch area, this allows each switch to have as many functions as the software designer wishes.

Alternatively, analogue resistive touch screens offer the same advantages as their matrix cousins, but with the added bonus of greater flexibility in terms of the number of switch positions.

Analogue systems are created when two opposing planes of material are supplied with a constant DC voltage.

When contact is made, the opposing plane registers a drop in voltage, thereby determining the x and y co-ordinates of the connection.

This allows an infinite number of switch positions within the view area of the touch screen however it is important to use the correct interface electronics to fully optimise this, especially if signature capture is required.

Of course, resistive touch screens are constantly evolving as new technologies become available.

For instance, a new breed of resistive touch screens have recently been introduced which employs a chemically tough glass layer on the front of the touch screen.

With the introduction of this feature resistive touch screens can now be specified for use in market sectors where previously other touch technologies were specified due to the need for a hard scratch-resistant front surface.

As this new technology is resistive based unlike capacitive it can be actuated by gloved hands, stylus or other implements that operators my choose.

Touch screen solutions are one of a number of methods by which HMIs can be enhanced.

Membrane switches, for instance, are used to tailor the appearance and functionality of applications in sectors as varied as medical technology, white goods, process control and machine tools.

Officially described as momentary contact devices, membrane switches are typically constructed of three layers, the top of which is a flexible film with movable contacts screened to the underside.

Stationary contacts are deposited on the topside of the bottom layer and the two layers are separated by an insulator through which openings are cut at the aligned contact points.

When force is applied, the normally open, or separated position, is flexed.

Most membrane switches also incorporate an additional layer laminated to the top circuit, which contains a graphic nomenclature that describes the switch's function.

Membrane switches are normally delivered with a self-adhesive layer on the back, which means they are easy to mount and cost effective.

Often measuring less than a millimetre in thickness, membrane switches may be thin but they are extremely versatile.

For instance, miniaturised, surface-mounted components, such as light diodes, can be incorporated to provide added functionality.

Recent developments in membrane-switch technology include low-voltage backlighting solutions that can illuminate almost every type of membrane switch and keyboard.

By ensuring consistent and even light distribution without affecting the tactile response of the keyboard, these cutting-edge solutions have exceptionally long operating lives and can be easily manufactured in both high and low volumes.

Unlike conventional backlighting solutions, which suffer from hot spots or relatively short operating lives, these latest backlighting solutions include a custom-designed layer inserted directly beneath the graphics and, depending on requirements, may also include switch and tactile layers found on a standard membrane keyboard.

LEDs are positioned within machined pockets in the base of the custom layer to act as a light source.

Innovative switch actuators are also incorporated to allow the light path and degree of diffusion to be controlled carefully, effectively eliminating bright spots.

Although these new methods of illumination break new ground in terms of longevity, more generally, membrane-switch technology is being increasingly incorporated with touch screens to create entirely integrated solutions.

Apart from the obvious advantages of reduced manufacturing costs and times, a single panel consisting of a touch screen and membrane switches which can include traditional features such as tactile response, embossing and high-quality graphics can be sealed to IP65 standards providing it is mounted correctly within its housing.

This process also reduces the number of circuit components and connections since the panel can be designed around a single circuit tail.

Innovations in HMI technology have followed closely the growing influence of computers and are inextricably linked to the increasing emphasis placed on process control and, more importantly, profits.

As a result, prices will inevitably fall due to the increased integration of touch screen and membrane switch technologies, and the growing influence of single-source suppliers, such as Danielson.

However, as the role of HMI solutions becomes more apparent, the possibilities surrounding this technology are also unfolding.

For instance, bespoke manufacturing solutions lift any constraints placed on design as the interface is no longer constrained by the size of the keyboard or screen.

With dedicated manufacturing and technical support, the custom-build route can be as cost effective as buying an off-the-shelf product, especially if application volumes are high.

Nevertheless, high-volume demand for standard size touch screens remains, many of which come from the Far East.

That said, achieving the best performance from a HMI solution could depend on a dedicated supplier capable of offering in-depth local support and technical advice.

After all, HMI products are not only consumable products, they are also technical components with an ever-increasing range of possibilities associated with them.

More importantly, they represent the very important point at which people meet technology.

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