Conventional pushbuttons, which are operated mechanically, no longer have much in common with today's capacitive pushbuttons. While the former require mechanical pressure for operation, the capacitive, electronic sensor switches with touch function are depressurized by touch. The button builds up an electric field and continuously measures the capacitance between the surface and the sensor environment. As the human body consists mainly of water, it has a relatively high conductivity. If a hand approaches the sensor, the capacitance increases and a switching operation is carried out. This principle alone ensures greater safety in the workplace - because even a glove, which may be used to protect against cold and dirt and to prevent injuries to the hands, does not impair the function of the button.

As touch alone is sufficient with capacitive switches, the force required to operate a system is reduced - a fundamental, albeit often underestimated, factor for safe working: After all, those who have to use little physical energy to operate a pushbutton do not tire as quickly, avoid mistakes and are less likely to injure themselves as a result. The simpler and more effortless operating procedures are, the safer they are.

Compared to mechanical pushbuttons, capacitive sensor switches are more robust, as touch has a much lower impact than pressure. Sensor switches also have no mechanical components that can fail. On the one hand, this reduces the risk of failure and, on the other, makes the switches completely maintenance-free. Fail-safety can be further increased, for example, by using waterproof electronics encapsulated with special resin and a robust polycarbonate housing material, as offered by Captron. Their safe capacitive switches meet the IP69K protection standard with regard to the effects of water and oil and the IK08 requirements for impact resistance and robustness - additional protection against vandalism. This means that even lighter flames and targeted blows to the touch surface cannot harm the push-buttons.

The capacitive sensor buttons from Captron enable several 100 million switching cycles. Of course, high reliability in the processes is not just a matter of the push-button material used. Equally important are built-in automatic mechanisms that make the operation of sensors - and therefore the machines they control - safe for the user.

Use as a two-hand control

Employees working with presses and punches, for example, are exposed to a high risk of injury. For this reason, DIN standard EN ISO 13849-1 prescribes a two-hand control system in certain cases, with which a machine can only be started by pressing two buttons simultaneously. Capacitive systems are particularly suitable here, as they enable pressureless and safe operation of heavy machines and systems in production automation. To activate the machine, the user presses two capacitive buttons simultaneously. The high level of safety is achieved, among other things, by the fact that the two buttons are connected to each other by a functional safety cable. In this way, the operating system detects incorrect or inattentive switching with just one hand or another part of the body; in this case, the machine is not triggered. Dangerous operating errors can be avoided.

Such two-hand control solutions in combination with a safety relay - for example the 'safeCAP SC4' series from Captron - are now approved by the German Social Accident Insurance (DGUV).
Other solutions for two-hand control - such as the 'safeCAP SC3' series certified by TÜV Süd - require two identical capacitive buttons in combination with an evaluation unit. With such an operating system for a capacitive two-hand control with evaluation unit, the functional safety cable between the two push-buttons can be omitted, as the safety switching devices meet the high requirements of IEC 61508 / IEC 62061 (SIL 3) and EN ISO 13849-1 (PL e).

Ergonomics as a safety factor

Visual display elements are another factor for the safety of work processes. After all, the operator of a press needs to know whether his signals have been interpreted correctly by the machine.
Capacitive sensor buttons that provide clear feedback via an LED display, for example, increase error-free and safe processes over the duration of a production shift. Intuitive color symbols can be used to signal the operating status via a green LED and a switching process via a yellow LED. With the Captron solutions, users of technically sophisticated systems receive feedback via 16 bright LEDs per button.

At the same time, a capacitive push-button in a safety-critical application must not only function reliably at all times, but must not trigger an unintentional switching operation in the event of a failure, for example. Unintentional switching, for example triggered by dirt, must be prevented in the electronic push-buttons with touch function. This is made possible by the automatic calibration of the push-button in order to avoid interference factors. As a result, the touch function remains active even with a thick layer of dirt, and the capacitive sensor switches can also be installed and operated behind glass.

Author: Bettina Greiffer, freelance journalist from Munich.