INNOVATIONS IN FLAT PANEL SWITCHING

INTRODUCTION

Flat panel switches such as membrane switches are widely used in industrial control panels. They allow a products appearance to be designed consistent with contemporary market tastes, while being durable, functional and environmentally sealed.

In the early days fascias used polycarbonate overlays over the top of discrete switches mounted on PCB’s. While overlays are still widely used, membrane switches gained popularity as new technologies, materials, and designs all contributed to their improved reliability and performance. The one important technical advantage of membrane panels over PCB mounted switches is that the PCB did not have to be positioned near the switch face.

As the buttons are not mounted on the PCB, it is not necessary to locate a PCB adjacent to the switch, or to have multiple cut-outs in the enclosure for PCB mounted switches. As the PCB design is not dependant on the button location, a standardised PCB and microprocessor can be used across a full product range, allowing many variations in visual appearance, size and number of buttons.

Unfortunately, until recently these advantages have only applied to panels incorporating buttons only. Other useful features of a control panel such as encoders, LED’s and backlighting have required PCB mounting, or mounting in the enclosure with a wire loom and associated soldering.

It is pleasing that in recent years, a core theme in flat panel innovation has been integration - with new flat panel switch fascias routinely integrating LED’s, backlighting, rotary encoder, direction control buttons offering. For designers and manufacturers, this offers new opportunities to make the control panels reliable, robust, user friendly.

This article will briefly outline the various characteristics of flat panel (less than 2 mm) interface products and the solutions they offer.

INTEGRATING ENCODERS & DIRECTION CONTROLS

Encoders are used widely and provide a positive control for many functions and data selection. They are however typically hard mounted through rigid backing panels. This introduces opportunity for water and dust ingress. While not necessarily expensive in themselves, encoders can be expensive to install, requiring additional panelwork, wiring looms and soldering.

One solution is Duraswitch’s rotary encoder. This patented design allows an encoder to be mounted securely within a flat panel switch of around 2.0 mm thick. This ensures a robust and environmentally sealed construction, while at the same time simplifying the mounting and installation - a ribbon cable is simply plugged into the PCB in the same way as normal membrane switch. A full range of circuit configurations can be acheived simply by printing the appropriate circuit layout.

Using an encoder in this way offers new opportunities for improving existing panels. For instance, it can be an effective data entry tool for controlling graphic LCD displays. The user simply turns an encoder designed as a switch to select a menu item, selects using a pushbutton, and continues on through the menu system. This means that menu selections can be done without looking at switch panel, while ensuring that the screen is not dirtied by hands as in a touchscreen.

Mouse or directional controls can also be used for simplifying keypads. Using a mouse control - one single directional device can replace a number of buttons or with appropriate firmware, can act as a full mouse or full joystick. Both the encoder and the mouse controller can be included in the one panel without compromising the thickness.

In summary, integrating an encoder, momentary buttons, and direction buttons in the one panel allows a wider range of user experience, while at the same time minimising throughholes, dust and water engress, and costs of soldering. Please take the link to an article on integration and encoders can be found at these links.

SOLUTIONS FOR HOSTILE ENVIRONMENTS

In hostile industrial environments, and in the public domain, there are many situations where a flat membrane switch is desirable, however would be vulnerable to impact through rough use and abuse. In these environments, a conventional membrane switch dome would be flattened and the switch short circuited.

Recent innovations have allowed switches to be more robust. One solution is what Duraswitch call a high impact construction. Here a layer of impact absorbing silicon rubber sits above a Duraswitch Pushgate switch. In conventional membrane switch constructions this would result in a severe loss of tactile feel, however when used with a Pushgate construction, the feel remains crisp. The silicon protects and distributes the impact across the switch face, even allowing the switch to take a hit from a hammer. The construction has found success with the US fuel pump industry where consumers routinely damage switches by using the end of the nozzles to make the choices.

Conventional membrane switches, or pushgate switches can be mounted below hard key caps for additional protection. Refer to the attached artic

A hidden danger in humid environments is silver migration, which affects conductive silver printed circuits such as those used on flex circuits. The silver grows ‘dendrites’ which cause low level shorting between adjacent conductors. A knowledgable manufacturer should be conversant with the critcal issues that effect the formation of dendrites, but you should be aware that increasing the supply voltage through the circuit will strongly correlate with an increase in dendrite growth. Other considerations include track spacing, the use of graphite shields, and the correct design of circuit cross-overs.

An alternative method is to use a capacitive switch. With appropriate decoders, a capacitive switch panel can take the form of either a flex or PCB circuit, and can be mounted protected from harm behind glass or polycarbonate. With a circuit construction similar to a conventional membrane, the capacitive switch has the benefit of allowing a much larger key contact area.

INTEGRATING SWITCH AND PCB

While the paragraphs above have focuses on the benefits of flex circuit, there are also many situations where integrating the switch and the PCB can have a number of benefits. In complex flat panel switches, it can be difficult to take to the tail out of an area densely populated by buttons. This is because the tail is made of the same layer of material as the bottom circuit.

In these switches it can be practical to replace the bottom conductor (usually a flex circuit) with a thin PCB (say 0.8 mm) This also gives the opportunity for other components to be mounted on the reverse side of the switch. This could for instance, include the microprocessor, resistors, and LED’s and other components.

Replacing the bottom flex conductor with a PCB also allows the construction of complex and tight circuit matrix without the risk of silver migration. A further advantage is that the PCB can act as the rigid mounting panel with standoffs for LCD mounting etc.

LIGHTING

Backlighting has always been an issue in flat panel switching panels as the domes or silver pads have obstructed lighting from the rear. Electroluminescent lighting has been well accepted as a form of backlighting in switches as it is thin, and cold, and can be mounted directly below the overlay. Now that EL has been well accepted, users are more concerned with how the backlighting layer affects the tactile feel of the switches. Adding a layer to the switch ‘deadens’ the tactile feel. The Duraswitch Pushgate does offer a better alternative in these situations as the tactile feel is not as affected by heavy overlays.

It is worth noting that EL does have a definite life (typically half-life is quoted at 3-5000 hours) and may not be suitable for high humidity, or intrinsically safe environments. EL operates on an AC voltage so will require an inverter (such as sipex chipset)

Indicator lighting is routinely undertaken using LED’s surface mounted on the flex circuit. Given that the typical thickness of a membrane switch is 1.00, and a Duraswitch at 2.00 mm, LED’s have had to be embossed at additional cost for both tooling and manufacture. With LED packages now being as thin as 0.6 mm, embedding LED’s often be undertaken without embossing. LED’s would typically be positioned to the side of the button, although the Pushgate allows an LED to be mounted in the centre of the button. Resistors can be mounted if required to circuit.


RFI & ESD SHIELDING

RFI and ESD shielding layers can be easily integrated within flat panel switches. In its simplest form, a printed silver conductive RFI grid can be included on the upper side of the circuit layer of a tactile switch, while it could also be include as a separate layer. Earthing of the RFI can be acheived using a separate pin on the connector, or terminated using a screw contact on a flap extending beyond the outside dimension of the switch.

If a printed silver grid does not provide the required amount of protection, specialist material layers are available to integrate between the upper layers of the switch. It is worth noting that the window area of a control panel is often vulnerable to rfi, so a transparent conductive window shield is required.

When allowing for electrostatic discharge, the discharge typically extends across the surface of the switch and to any conductive surfaces on the edge. It is therefore important that the designer ensure that there are no conductive tracks running close to the edge of the switch.

In summary, recent innovations in technology have allowed flat panel switches to far exceed their initial capability. In all situations, but giving your manufacturer the fullest information regarding the environment where the switch will be used, as well as the intended use, will allow the manufacture to make informed decisions as to an appropriate design. The purpose of this article was to illuminate other options that are available to improve the overall function and application of the switch



RESOURCES

"Haptic feedback in button technologies - a Guide for Industrial designers", Timothy Griffin

A good introduction to switching technologies from a designers perspective (linked to with permission)

"Harnessing the power of the membrane switch", Nigel Davies, Whats New in Electronics, 1999 (950 kb)

A guide to membrane switch design

"Duraswitch Technology provides solutions for the Appliance Industry, Anthony J van Zeeland, International Appliance Manufacturing, June 01

An example of solutions using integrated encoders, direction controls for appliance control panels

"Industrial Keyboard Design Concept" - Duraswitch Industries. JPG

 

SPECIFICATION SHEETS

Rotary Encoder

Pushgate Switch