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Capacitive sensing

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Overview

Capacitive keyboards have an interesting characteristic that the capacitive sensing takes place directly under the plunger. This is where the return spring would be in many other switch types. As a consequence, capacitive keyboards tend to have the return spring or buckling rubber sleeve on the outside, underneath the keycap. Topre’s conical spring method is unusual in allowing the rubber dome to be placed safely inside each key.

Capacitive sensing is a technique that requires the keyboard to be designed as a complete assembly. Individual switches don’t operate on their own: a capacitive keyboard takes the form of a special PCB layout, open-bottom switches that allow the capacitive element to approach the PCB, and special sensing circuitry or firmware. While many designs have discrete plunger assembly units for each key station, these units do nothing on their own when removed from the keyboard, as half of the variable capacitor that makes up the capacitive sensor is formed of PCB pads.

The majority of capacitive keyboards are matrix scanned, with centralised electronics that query each key in sequence. However, there were also designs where each key had its own dedicated capacitance sensor. The various implementations are listed where possible in the approximate order in which they were invented.

Selective shielding

Around 1967, IKOR introduced an AC coupling arrangement between a bank of emitter pads and corresponding PCB tracks (or so the patent appears to indicate). Selective shielding between the emitter and receiver lines allowed for each output bit to be defined separately, making this a self-encoding technique. Unlike other capacitive sensing techniques, there is no variable capacitor involved: the capacitance emitter and receiver are permanently fixed in position and operate at a fixed capacitance. Every output bit has its own separate AC emitter and receiver, with each receiver coupled capacitively to its emitter. A metal shield within the key bears apertures that allow the electromagnetic field to pass for each output bit that should be enabled. In its inactive state, the apertures in the shield do not line up with emitter–receiver pairs, and no output bits are active. As the key is pressed, the shield is lowered, allowing the AC signal to pass through as the apertures in the shield align with the emitter–receiver pairs. In IKOR’s design, there are two facing sensing arrays to provide the required number of output bits.

This implementation was vertically bulky, even with the apertures and holes placed into two columns, and the design did not seem to last for many years. It may be possible to achieve a non-encoding version where each key has a small shield that only enables a single bit. This would reduce the keyboard’s profile significantly.

Capacitance-controlled transistor

Around 1970, Control Devices invented a method for detecting capacitance by connecting the switch’s variable capacitor to the base of a transistor. Unlike the movable shield method, this approach uses each key as a variable capacitor just as with the matrix-scanned types. Each key has its own dedicated capacitance sensor, comprising a transistor, a capacitor and two resistors. Control Devices arranged for each key to be connected to a diode matrix for output encoding. The per-key portion of this circuit is shown below, taken from the Control Devices patent:

The fixed capacitor is held charged, and this keeps the diode matrix columns high. When the key is pressed, an AC signal passes from the oscillator into the key’s transistor, and this drains the key’s capacitor, grounding the diode matrix columns. There is no matrix and no scanner, and thus no way to determine the co-ordinates of an individual key. Each key must therefore be connected to a means of providing its identity. One option is two-of-N coding. Another option is a diode matrix, as shown above.

In reality, the circuit was more complex than the diagram shown above. The diode matrix output required amplification, and to deal with the individual columns triggering at different times, synchronisation was required to withhold the output code until all the columns had activated and settled. This method was considered low-cost at the time, with a per-keyboard price of under $100, or under $600 in February 2021 prices.

A similar arrangement can be found in the operator’s console for the Monarch 120 Call Collect System; this panel was manufactured by Pye Electro-Devices. Here, only a single resistor is used per key. The technical description handbook for the Monarch 120B notes:

The operator’s keypad is driven by a 100 kHz oscillator. When the operator’s finger is inserted in the key depression, a capacitive coupling to earth of about 1 pF is applied (about 1 pF at the point of application of the finger in series with, say, 100 pF to 150 pF body capacity to earth).

This is sufficient to cause a shift in DC output level from a transistor circuit driven by the oscillator and apply a DC marking condition on a lead corresponding to the key. The circuit elements and the way in which the keypad is connected into the system are shown in Figure 55.

The per-key circuit per the Monarch 120B Compact Call Collect System Technical Description Part 2 (page 106) is as follows:

The encoding process is again diode-based, but instead of producing output codes directly, it produces a form of two-of-N code that is fed to a separate circuit. (There is some disagreement within the documentation as to whether the oscillator ran at 100 or 110 kHz.)

The indication is that PED only produced touch-contact panels and keyboards with this technology, i.e. they were not full-travel switches: each key station responds purely to the operator’s fingers, with no moving parts. Creation of full-travel switches using this circuit appears to be possible however.

Metal dome

Metal dome capacitive keyboards were introduced by Colorado Instruments in 1970. These keyboards used a snap-action metal dome as the movable portion of the variable capacitor, providing a click sound and feel in direct conjunction with actuation. Example keyboards are depicted in magazine articles, but these are yet to be seen. No model numbers or product names are yet discovered. US patent 3653038 “Capacitive electric signal device and keyboard using said device” was filed in February 1970 for this design.

Matrix scan

In matrix scan capacitive keyboards, each key contains a variable capacitor. Pressing a key typically increases its capacitance, although in some designs (e.g. IBM beam spring, Digitran KD) the capacitance is reduced upon actuation. Centralised electronics are used to query to each key in turn, detecting its current capacitance, for example by measuring the time required to charge the key to full capacitance. The vast majority of such designs use a metallic disc as the variable element, affixed to a foam pad to provide overtravel; such keyboards are widely described as “foam and foil”. However, there are numerous other approaches, as detailed below.

Conductive plastic

Some capacitive designs use a conductive plastic block as the variable element. IBM’s beam spring and Model F designs both take this form. The US patent application for IBM’s beam spring keyboard appears to have never been granted, but the overseas patents reference the 1971 application.

Foam pad

Foam pad—or “foam and foil”—switches have a foam pad fitted to the bottom of the plunger, affixed onto which is a thin layer of metallic foil. The distance between foil layer and the PCB affects the capacitance registered at that key position. Keystrokes are detected when the foil layer is resting on top of the PCB; the solder mask on the PCB and (where present) the plastic coating on the foil presumably prevent a short circuit. The compressible foam pad allows the plunger to continue being pressed past the point that the foil layer reaches the PCB, to provide overtravel. As such, the complete assembly will not operate without the use of an extra moving part (the foam pad) beyond the plunger itself. Worse, the foam pads are a weakness in the design: as foam can stiffen with age and lose its flexibility, some older keyboards fail due to the foam pad failing to re-expand after a key is struck.

US patent 3965399 “Pushbutton capacitive transducer” filed in March 1974 indicates that the reason for the flexible foil coating on a foam pad is so that the foil coating can be pressed uniformly against the PCB:

A substantial change in capacitance between the members is effected when the thin dielectric material is disposed across a gap between the capacitance forming member. In order to provide a consistent change in capacitance, a gap of uniform width is preferably maintained. This is provided by the use of a resilient backing which causes the flexible metallic and dielectric films to conform to surface irregularities of the capacitance forming member and its two elements.

The foam pad can also be referred to as an “overtravel” pad: it permits the key to be pressed further after the foil layer makes contact with the PCB. The foam pad design, just like the conductive plastic approaches, appear to require the movable object to rest against the PCB for detection to occur. Topre’s conical spring method is a full analogue sensing method that allows for firmware-configurable pretravel and electronically-defined hysteresis.

Foam pad keyboard manufacturers include:

Digitran amusingly referred to foam pad keyboards as “sponge-on-a-stick” in an advertisment in Computer Design in January 1979; this was a slight against foam pad designs, as compared to their Golden Touch leaf spring design.

Leaf spring

Leaf spring capacitive sensing uses a metal leaf spring that is pushed down onto the PCB. One side of the leaf spring is attached to the PCB, and the plate hinges down towards the PCB under pressure from the plunger. Overtravel is achieved by a special prong on the leaf spring. The best-known leaf spring capacitive design is that of Digitran’s Golden Touch. Cortron filed a patent for their own design, but in both the one known CP-4550 keyboard and the CP-4550 advertisements, the design is the same as Digitran’s.

The exact date of introduction of leaf spring capacitive keyboards is unclear. CP-4550 appeared around 1981. Golden Touch keyboards have been found as far back as 1976, and advertised only as far back as 1978.

Membrane

Although the vast majority of membrane keyboards use conductive sensing, some manufacturers produced conductive membrane keyboards. Manufacturers of capacitive membrane keyboards include Micro Switch with SC and ST Series. Micro Switch’s approach appears to date back to around 1981.

Conical spring

Conical spring switching uses a small, low-force conical spring placed under a rubber dome. As the key is pressed, the rubber dome compresses the conical spring. Topre have made such keyboards since 1983, and continue to produce them to this day. They introduced APC—Actuation Point Changer—to permit the pretravel of the keyboard to be set to any of 1.5 mm, 2.2 mm or 3 mm; this is made possible by the inherently analogue nature of capacitive sensing. See the Realforce website for details and illustrations of Topre keyboards.