Membrane switches are a type of human-machine interface characterized by being constructed from several layers of plastic films or other flexible materials...
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This article takes an in-depth look at membrane keyboards.
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A membrane keyboard or membrane switch keyboard has thin layers of flexible material that represent the keys for activating signals for a system or device. The individual keys are mechanical switches that control electrical connections, which produce signals to give commands. Each of the switches controls an electrical circuit that is inactive until it is pressed.
The technology for membrane keyboards is found in electronic gadgets and appliances.
They are a means for inputting data into industrial, residential, and commercial devices as well as providing commands for robotics and computer numerical controlled (CNC) machines. Computer keyboards are mainly used for typing and gaming applications.
A membrane keyboard is made up of a few layers. The topmost layer of the keyboard contains all the keys. This is where the users touch and press the keyboard and are exposed to the environment. It can be made of various materials such as plastic or rubber. Below the keys are pressure pads – these pressure pads are made of thin, flexible rubber or plastic membrane with printed conductive traces on its backside. The next layer is called the hole layer; this layer separates the pressure pad next to a key to the bottommost layer such that there will be no mechanical contact when the key is not pressed. Finally, the bottommost layer also contains printed conductive traces connected to the keyboard’s circuit.
When a certain key or button is pressed, the conductive trace underneath the key touches the conductive trace attached to the circuit inside the keyboard. The circuit has an array of switches, and each switch is assigned to a key. This mechanical contact causes the assigned switch to close and allow electrical current to flow. This action registers the pressed alphanumeric character or command by sending the corresponding signal to the processing system of the device. All of the switches in the keyboard circuit are normally open when the key is not in use.
Membrane keyboards are found in a number of applications such as industrial control systems, medical equipment, telephone systems, computers, electrical appliances, retail store machines, and a lot more. The design of these keyboards is not limited to the conventional QWERTY format. Depending on the application, it can be customized flexibly to provide a better user experience. They are lightweight and more portable than mechanical keyboards, the other type, which use a spring-loaded switch. We will be comparing both types and discussing the advantages of membrane keyboards in the succeeding chapters.
Membrane keyboards are characterized by their multiple layers and the design of their membranes, which can be flat switches or dome switches. Flat membrane switches use a pressure pad but do not provide feedback when the switch is actuated. Dome switches have a dome with icons or characters printed on them and offer tactile feedback when the dome is pressed.
When the switches on a membrane keyboard are not being pressed, current cannot flow across the non-conductive gap between the traces and the bottom layer. Contact between the bottom and conductive layer occurs when the symbol on the top layer is pressed to allow the conductive traces to bridge the gap and allow current to flow.
Many users require feedback when pressing keys on a membrane keyboard that works silently if feedback is not included in its design. Depending on the type of keyboard, sounds, lights, beeps, clicks, or other types of indicators are used such that users know that a signal has been sent.
A membrane switch keyboard is made from flexible material that is covered with a transparent flexible membrane. The graphics for the keyboard are printed on the membrane material and operate by temporarily opening and closing an electrical circuit by touching the circuit. When the membrane and key are touched, the circuit closes. When the pressure is removed, the circuit opens. The opening and closing happens instantaneously during keyboard use.
Membrane switch keyboards are the simplest type of keyboard and are used as keypads for household appliances, such as washing machines, air conditioners, refrigerators, and microwave ovens. Commercially, they can be found on vending machines, printers, and photocopy machines.
The five main components of membrane switch keyboards are assembled in layers using heat sealed films or pressure sensitive glue. Included in the layers are graphic overlays, conductive tracks, domes, spacers, and terminals.
The graphic overlay is the outer part of the keyboard and is durable, resistant, and clear, made from flexible polyester material. Graphic inks are printed in reverse and are protected by a plastic film.
Domes provide tactile feedback when the keyboard is pressed and can be made of metal or plastic, but usually plastic. The sizes of the keys for a membrane keyboard are determined by the size of its domes with sizes ranging between 6 mm up to 20 mm and heights of 0.25 mm up to 1.45 mm. The four basic shapes of domes are four legged, triangular, round, and oblong.
The spacer layer serves as a break between the switch and conductors that are on the circuit layer. To allow the switches to touch the circuits on the conductive layer, the spacer has holes where there are keys or domes. The spacer layer can be filled with air or an inert gas to separate the switch and conductor layers.
The circuit layer has the conductive paths that are created by screen printing or photochemical engraving for the keyboard. Screen printing uses a stencil and silver ink to produce conduction while photochemical engraving uses a photographically produced carbon laminate. Circuit layers can have a single or double layer.
The circuit terminal is a flat ribbon of conductive tracks that connect the keyboard switch to the control unit.
The top layer is a thin strong layer that has the characters or icons to be inputted. Icons and characters are adjusted and configured to the needs of the user and have a conductive metal strip on their backside.
The bottom layer has a conductive metal strip connected to an electric circuit. When a button on the top layer is pressed, the metal strip on its backside touches the metal strip of the bottom layer to register a character or command to the system.
Due to their design, membrane switch keyboards need only light pressure to activate a button, which increases a device’s ergonomics but has low physical tactile feedback. To increase the physical tactile feedback, devices are designed such that lights, vibrations, or sounds are produced when a button is pressed to let the user know that the command has registered to the system. An example of this feature is found in microwave ovens. As buttons on the oven are pressed, a beeping sound is created.
Full travel membrane keyboards are the type of membrane keyboard more frequently seen in computers. Their keys are directly integrated into the keypad and are made of a single piece of plastic or rubber material. Their flexible membrane is placed over a matrix of electrical switches. The keys act as a plunger: as a certain key is pressed down, the key pushes the membrane to the bottom layer of the keyboard which causes the assigned switch to close. The elasticity of the membrane will cause the key to move upward when the finger pressure is released. Most full travel membrane keyboards have a spring underneath each key.
Full travel membrane keyboards are an economical option for computer keyboards. However, their physical tactile feedback is lower than mechanical keyboards.
Dome switch membrane keyboards, or direct-switch keyboards, combine the features of switch panel membrane and mechanical switch keyboards. Their keypads are made of rubber or silicone with a metal or rubber dome underneath each key. The domes contain a conductive trace in each key that closes the corresponding switch when pressed.
Metal domes, known as snap domes or tactile domes, are made of stainless steel and come in circular, four leg, triangle, and oblong shapes. Circular metal domes can only be used in circuit boards of two or more layers or precision switches. Four legged metal domes are designed to be mounted on single or double sided PCBs where traces of negative and positive are used on the same circuit.
Triangle metal domes are used where click feel is required. They are limited to small handheld devices and force control switches to avoid accidental activation. Oblong metal domes are used on double sided PCBs and are ideal for applications that require a low profile on over populated circuit boards or long narrow spaces.
Although stainless steel is the commonly used material for metal domes, they can be plated in gold, silver, or nickel.
Air venting is a necessity for metal domes to ensure that they function properly. The venting process is the removal of air in and out under a metal dome. There are several ways that venting is available including vent channels using a space layer, top venting through polyester fabric, and channels through the board.
Rubber domes are made of silicone rubber and are small and flexible. They are molded with conductive carbon or require conductive printing after being molded. Rubber domes rapidly collapse when pressure is applied, which provides tactile feedback, and spring back after the removal of pressure.
Some of the many advantages of rubber dome switches include tactile feedback, low cost, softer feel, resistance to leaks or corrosion, and noiseless activation.
Dome switch membrane keyboards are commonly found in handheld controllers, consumer electronics, mobile phones, medical devices, and personal computers. The domes of dome membrane keyboards can be too soft or mushy and not supply sufficient feedback, unlike metal dome switches.
Scissor switch keyboards use a rubber dome with a plastic scissor mechanism that links the keycap to a plunger. The rubber dome is depressed by the plunger, which shortens the travel distance of the keypad. Scissor switches are not full travel switches with travel distances of 1 mm to 2.5 mm that are shorter than the 2.5 mm to 4 mm distances for other keyboard types.
The “legs of the scissors” of a scissor switch keyboard connect the keycaps to the keyboard. They shorten the travel distance of the keys, the rubber dome, and the membrane during a keystroke prolonging the service life of the keyboard since it is stretched less when actuated.
Scissor switch membrane keyboards are noisier than other keyboards due to their physical characteristics, which is a reduction in the amount of rubber cushioning. The popularity of scissor switch membrane keyboards is in regard to their durability with a rating of 10 million keystrokes. They are a tightly sealed with smaller gaps between the key cap tops.
Scissor-switch keyboards are quiet during operation, despite the inclusion of the scissor-like components. They are found in laptop keyboards and built-in keyboards. However, they are slightly more expensive than the previous types. They are harder to clean because of the limited movement of the keys and their multiple attachments, but debris is less likely to get trapped as gaps between the keys are often smaller.
Chiclet keyboards, or island-style keyboards, are one of the most popular types of modern keyboard technology found in the latest laptops and desktops. They are known for their distinct sleek and minimalist style, characterized by their square or rectangular keys with rounded corners and relatively short height. The individual keys are separated from each other by a perforated plastic plate that runs across the entire keyboard area.
Chiclet keyboards first appeared in the late 1970s to the mid-1980s in the personal computers TRS-80 Color Computer, TRS-80 MC-10, and Timex Sinclair 2068. These keyboards are named after the chewing gum brand “Chiclet”, as the keys resemble their manufactured gum. Chiclet keyboards had a lot of negative reviews after the first few releases due to the poor quality experienced by the users. However, in the 1990s, the design of the chiclet keyboards was reinvented and gained the approval of millions of users worldwide. Up to this time, chiclet remains one of the widely used keyboard designs adapted by many PC manufacturers.
Chiclet keyboards have the same operating mechanism as the conventional membrane keyboards but with minor differences. To close a specific key switch and record the keystroke, the mechanical contact of the top and bottom conductive traces can be accomplished by either of the two methods:
Chiclet keyboards operate more silently than mechanical keyboards and provide a good typing experience. They are considered “business keyboards”. Yet, the feedback of the users regarding these keyboards is split. Since the gap between each key is larger, the occurrences of typographical errors are minimized. Due to their short height, the keystroke during each click is also shorter and flatter, which some typists perceive as comfortable. However, the wider spaces can be a problem to some as they need to overextend their fingers when typing. The tactile feedback is not comparable to mechanical keyboards. Chiclet keyboards are also not the best for hardcore gaming, as they are prone to “ghosting.” Hence, manufacturers innovate the design of their chiclet keyboard in order to accommodate more extensive gaming applications.
Unlike membrane switches and mechanical push buttons, capacitive technology works by detecting the change in capacitance, the ratio of electric charge stored in a conductor to electrical potential, on a surface that is monitored by a microcontroller that has a base capacitance value. When a change is detected in the capacitance, the base capacitance triggers output.
The touch surface or graphic overlay of capacitive keyboards can be made of any nonconductive material with the output customized to fit the needs of an application. Capacitive touch screens are more responsive to human touch and is the reason they are used for high tech smartphones. The surface of a capacitive screen is smooth and requires only the presence of a finger to be activated without the need for the application of force.
The two forms of capacitive touch keyboards are PCB and film, which are selected in accordance with the needs of a customer and the application.
PCB type capacitive keyboards have touch buttons as part of a circuit board with the overlay applied directly to the circuit board. This type of capacitive keyboard is very passive and measures the amount of charge or energy a capacitor can carry.
The film type capacitive keyboard is used when a PCB type cannot be used and has a conductive ink layer under a graphic overlay that terminates to a ribbon that is connected to electronics. The film layer is made of polypropylene, Teflon, or polyester and comes in different colors, sizes, styles, and shapes.
With the film type capacitive keyboard, there is no dielectric loss and includes high insulation resistance. The various types of film layers have excellent resistance to changes in the ambient temperature.
The following are the terms used in describing the properties of keyboards. Though subjective, they can be great criteria in selecting and assessing keyboards. We may be subconsciously aware of these properties, though they significantly affect our overall typing experience.
The key travel refers to the distance traveled by the key from rest until it reaches the actuation point, the point at which the input through the keyboard is registered. Key travel is typically measured in millimeters. A shorter key travel may provide a lighter touch or feeling while typing.
The actuation force refers to the force required to reach the actuation point of the keys. This tells how hard a user must press the keys for the input to be recorded by the keyboard. It is usually measured in centi-Newton (cN) or gram-force (gf).
Do you feel a bump on your fingers as you press a key on your keyboard? The feeling we get as we type on our keyboards is attributed to a property called tactility. Tactility refers to the sensory feedback that a user receives when pressing the keys of a keyboard. This feedback “tells” the user that his/her input has been recorded by the keyboard. It is associated with the position of the key travel.
Tactility can affect the finger pressure exerted by the user on keys. The finger pressure of the user on the keys can exceed the actuation force if the device gives little feedback which signals him/her that his/her input is recorded.
As introduced in the previous chapter, the forms of tactile feedback are audible, tactile, and visual feedback. A combination of those forms exists in most keyboards.
Audible feedback is the sound that a key makes during a keystroke. A clicking sound is oftentimes naturally caused by the key itself. In electronic devices such as cell phones and telephones, an artificial keypad tone may be activated. When the keypad tone is activated, a tone will be heard as the user types or dials a number on the keypad.
Another property that arises from audible feedback is the noise level, which is determined by the switching mechanism. As the audible feedback produced by a keyboard increases, the noise level also increases.
Tactile or haptic feedback is the physical sensation felt by the user when a key is pressed. It provides a physical “texture” during typing. Bumps and increase in resistance during key travel are the common forms of tactile feedback caused by the key. Vibrations are produced as the keys are pressed in some devices to increase tactile feedback.
Visual feedback is the visual cue generated during a keystroke. It can be as simple as seeing your keys move downward while typing. Another example of this feedback is when you press a button of a device (e.g., power button, caps lock or num lock key), a bright flash of light will be displayed on the keyboard or screen.
Light or soft touch is the desirable feature that both ergonomists and users sought. It is affected by both actuation force and key travel, and it is achieved when the finger pressure exerted in pressing the keyboard is little. Lighter and easy-to-press keys reduce stress on the finger joints while typing; however, these keys reduce the accuracy during typing, as some keys can be inadvertently pressed since they are easily actuated.
Mechanical keyboards are one of the main categories of keyboards based on mechanism. Mechanical keyboards have a key switch, consisting of a spring, a plunger, and a pair of metal contacts, underneath each key. When a certain key is pressed, the plunger moves down to push the spring, forcing the pair of metal contacts together. This closes the switch and records the input on the keyboard. When the finger pressure is released, the spring and the plunger will return to their original position which gives a bumping sensation while typing. The different types of key switches are linear, tactile, and clicky switches, characterized by their tactility, actuation force, and noise level.
Linear switches produce the least noise and give a smooth keystroke. The keys move up and down with little resistance. Hence, they can be actuated rapidly, but they do not provide tactile feedback. Tactile and clicky switches give a bumping sensation near or at the actuation point. Tactile switches produce moderate noise, while clicky switches are the loudest among the key switches.
The following are the comparison of both types of keyboards, based on different criteria:
Membrane keyboards produce little noise during typing because the membrane absorbs the audible feedback naturally created during a keypress. These keyboards are generally quieter than mechanical keyboards. Mechanical keyboards, on the other hand, produce a loud click sound at the actuation point. Membrane keyboards are ideal if you intend to work in a public space or wish not to disturb anyone nearby.
Membrane keyboards are much cheaper compared to mechanical keyboards. You can get a high-quality membrane keyboard at an affordable price.
Membrane keyboards are more lightweight, portable, and compact compared to mechanical keyboards. They have a relatively simple design. Hence, they are suitable for individuals who travel often with their computers. This is because there are fewer moving parts, and the switching mechanism is primarily composed of silicone, rubber, or plastic membrane. Membrane keyboards have a more minimalistic and classic appearance.
Membrane keyboards have a wider range of applications compared to mechanical keyboards. The application of membrane keyboards extends outside the computer realm; membrane keyboards are found in household appliances, telephones, and industrial, laboratory, and medical equipment.
All membrane keyboards and mechanical keyboards utilizing linear key switches require the keys to “bottom out” (i.e., the lowest position of the keys) to reach the actuation point and record the input. In tactile and clicky switches, the actuation point is reached before bottoming out, allowing the user to release his/her fingers in the middle of the key travel and still register a keystroke.
Anti-ghosting is a keyboard feature that allows the user to press several keys simultaneously. This feature is critical in hardcore and competitive gaming and high-speed typing applications. N-key rollover (NKRO) is a rating of keyboards that tells how many keys can be pressed simultaneously without missing any keystroke, denoted by the letter N in the acronym. For instance, a 6-KRO keyboard means that a user can make six registered keystrokes simultaneously. In full NKRO keyboards, a user can press all the keys and register all the keystrokes at the same time. Full NKRO is more common in mechanical keyboards, as their construction is insusceptible to ghosting.
Due to the singular construction of their membrane, membrane keyboards generally have low NKRO. Full NKRO is found in high-end membrane keyboards.
Membrane keyboards give a lighter touch but with a “mushy” feeling while typing. These keyboards provide less feedback during a keypress, but high-end membrane keyboards nowadays incorporate an improvised tactile speed bump and audible feedback to increase responsiveness. On the other hand, mechanical keyboards naturally give more solid and tactile feedback due to the presence of mechanical switches.
Membrane keyboards are harder to clean and replace keycaps. If one key or component fails, you will have to replace the entire keyboard. In mechanical keyboards, the keycaps can be easily dismantled for cleaning and replacement.
Mechanical keyboards last for 20 million to 100 million keypresses, while membrane keyboards only serve 5 million to 10 million keypresses during their lifetime. Nonetheless, membrane keyboards are a cost-effective option for low to medium-duty applications.
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