Bouncing
As bouncing a mechanically induced interference effect is at electromechanical switches and buttons called: Instead of immediate electrical contact actuation of the switch causes a short time a multiple closing and opening of the contact forth. The cause is elastic rebound against the suspension . On the other hand, when the switch is turned off or the button is released after the first interruption, repeated renewed contact is made much less frequently.
Effects
This effect of multiple closing and opening of the contact leads to undesired multiple events in fast electronic circuits, the time resolution of which is high enough to detect the bouncing. This affects, for example, digital input devices such as a computer keyboard , input controllers on keypads or electronic circuits that detect a relay or another electromechanical contact. Without debouncing, the bouncing closing operations would erroneously register a keystroke as a multiple keystroke.
But also with electromechanical switches, relays and contactors the bouncing of the contacts occurs and leads to increased contact wear and failure of the contacts. The contact welding, which occurs when overcurrents occur at the same time, is greatly promoted by the bounce phase because the resulting arc repeatedly melts the contact material, which can then weld when it is closed again during the bouncing. If the contact is not designed for an overcurrent, which can arise when loads are switched on, there is a risk of the contact halves welding or sticking. Whether a contact welds therefore strongly depends on the switched load. Melting occurs particularly when switching on at least initially capacitive loads, such as switched-mode power supplies, which cause a switch-on current surge that is 20 to 50 times too high. This leads to severe contact wear after just a few thousand switchings, which then leads to failure of the switch and / or welding.
The picture on the right shows the switch on a socket strip, which was used to switch on two laptop power supplies with only 50 VA each. The nominal current of the two power supply units of only approx. One ampere was not a problem for the 16 A switch. However, in the picture you can see the material that has evaporated from the contacts, which is deposited in the switch housing and is of course missing on the contacts.
When the contacts of relays, contactors or circuit breakers are opened, however, there is usually no bouncing and therefore no welding.
The duration and the number of multiple contacts during the bouncing is determined by the mechanical properties of the switch, the size, actuation force, return spring force, shape and material and mass of the contacts and their fastenings. Typical bounce times for electromechanical switches and buttons are in the time range 100 µs to 10 ms. The bounce time for a 16 A toggle switch for mains voltage, as shown in the picture, is approx. 5 ms. With large contactors for several 100 A, the bounce time can be greater than 100 ms. be. In the case of large contactors with an AC operating coil, the bouncing is also heavily dependent on the switch-on time of the contactor coil, which influences the magnet attraction force and thus the impact speed of the moving contact.
use
In the Hammond organ , resilient wires under the piano keys of the manual are pressed together as push buttons. Their bouncing and non-synchronous closing of the multiple switches is known as Hammond-Click and there are compositions that use and emphasize this property. Some electronic organs have a switchable replica of these noises.
Countermeasures
Since the beginning of electronic signal processing and the associated relevance of this phenomenon in signal switches and relays , various hardware and software processes have been developed to counteract bouncing and its effects. These measures are called debouncing . The debouncing takes place by means of a low-pass filter or a locking logic.
- Debounce by hardware (debounce circuit)
- In the simplest case, an RC element is provided as a low-pass filter and a Schmitt trigger for signal shaping. The low-pass filter suppresses the high-frequency signal components and thus the contact bouncing, the Schmitt trigger ensures the appropriate voltage level for the subsequent digital circuit.
- By means of interlocking logic in the form of a changeover switch . An asynchronous RS flip-flop or an adequate circuit, the "bounce-free button", is used for locking . The mechanical structure of the changeover switch must ensure that the contacts cannot oscillate between the two contact positions. The contact path when switching between the two states must be selected to be sufficiently large.
- Debouncing by activating the AC magnetic coil of a relay or contactor, which reduces the impact energy and makes it even for all closing processes. For this purpose, among other things, the magnetic coil is premagnetized and switched on by phase control with reproducible voltage curves.
- Debounce by software (debounce routine)
- The change of state of the contact is only registered when it has been present for a certain time, the so-called debounce time . This is a form of low-pass filtering and can be implemented like a digital low-pass filter. Usually, because it is simpler, this filter is implemented in the form of a counter. The counter value that triggers the event, together with the counting speed, represents the cutoff frequency of the filter.
- Debouncing can also be carried out in software using interlocking logic. However, since a changeover switch and two digital inputs are required for each key, along with the associated higher circuitry complexity, this type of debouncing is only rarely used.
If the locking logic is simulated by a time control in the form of a monostable multivibrator in software, the pulse is recognized with the first signal edge and all further signal changes are subsequently ignored for a certain time, but this method is sensitive to high-frequency interference pulses. Since it is a form of high-pass filter, it does not represent a reliable debouncing. Also, undersampling does not constitute a reliable debouncing, since it only reduces the probability of detecting short interference pulses, but does not avoid it.
Thanks to a special mechanical structure and contact design, almost bounce-free switches can also be implemented. Liquid contact materials, for example in the form of the mercury switch , are practically bounce-free. However, due to the toxicity of mercury and its chemical compounds, these switches are no longer used to prevent bouncing.
Sensor keys with integrated electronics such as piezo and Hall keys already contain threshold switches internally and usually deliver bounce-free signals.
literature
- Dieter Nührmann: The large work book electronics . 6th edition. tape 3 . Franzis, 1994, ISBN 3-7723-6546-9 , pp. 3191 .
- Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 9th edition. Springer, 1989, ISBN 978-3-662-11942-6 , pp. 256 .
Web links
- (English) Manoj Shenoy: Switch Debouncing , electroSome, January 1, 2018, accessed July 26, 2018
Individual evidence
- ↑ Features: Technics E-33