Description
| Requirements for keyboards for weatherproof measuring devices | |
| The choice of a keyboard type depends on the application. You can read here what should be discussed in advance so that the costs remain within limits and the requirements are met. | |
| Keyboards have a wide range of applications. When developing their products, manufacturers must look for solutions that exactly match the use and requirements. At the same time, these solutions should of course also be within a certain cost range in order to be able to sell the product. Nevertheless, the conditions of use are always decisive for the selection of the appropriate technology. Dusty or wet environmental influences, contact with cleaning agents or chemicals, cause the input devices to wear out or break quickly if the technology is unsuitable. The diverse requirements are to be explained on the basis of an application. | |
| The customer application described in this article involved a high-quality antenna measuring device (lead image) that is frequently used outdoors. Therefore, it is absolutely necessary to achieve a certain degree of moisture and dust protection so that the customer can work with this robust device over a long period of time without any problems. Another requirement was illumination for the keypad so that entries can be made even in poor lighting conditions. | |
| The decision for or against a technology ultimately depends largely on the area of application. In addition, the manufacturing process should be defined in advance in the individual steps so that the product becomes calculable for the manufacturer and the customer. | |
| The requirements for backlighting and the "feel" of the keypad finally tipped the scales in favor of the silicone keypad technology for the antenna measuring device application described above. The alternative technology of a membrane keypad was rejected due to the difficulty of illumination and the subjectively better feel of the silicone keypad. | |
| Manufacturing process of a silicone switching mat | |
| To produce silicone keypads, a metallic mold must be made. This process is relatively expensive, but the products made from it are very suitable for mass production. The base material of silicone rubber is introduced into the mold in the precise quantity and then pressed under high pressure and at a high, defined temperature in the mold in such a way that it is distributed throughout the mold and fuses into a uniform workpiece. | |
| Due to the high elasticity of the material, slight undercuts in the design are possible, but the product can still be demolded without any problems. In this manufacturing process, other metal and plastic parts can also be integrated into the mat. The subsequent labeling is usually done by screen printing. To create an abrasion-resistant legend, the ink is baked into the silicone in a thermal process. To further increase resistance, there are additional methods of coating up to the application of plastic or metal key caps. | |
| Depending on the area of use, the keyboard must be able to withstand various environmental conditions such as cold, heat, sunlight, rain, etc. It must not suffer any damage from substances such as cleaning agents, acids or similar aggressive substances and must guarantee a high writing volume (low abrasion). In special cases where hygiene plays a major role, such as in medical technology, there are also antimicrobial coatings that prevent or contain bacterial growth. | |
| In order to realize the illumination, the base mat was made transparent. This ensures that a light source installed under the mat can shine through the mat. However, in order to facilitate usability, areas or keys were to be designed in different colors so that the user does not have to "search" for a required key. Therefore, the mat was color-coated according to the customer's wishes. This additional coating ensures a maximally resistant device. To make the legend or lettering visible, the coating was then removed at the symbol and character positions using a laser. The transparent mat was coated in matching colors on all keys. | |
| The technical design is best illustrated by the principle sketch in Fig. 2. This customer-specific project placed high demands on the design of the mat and the position of the light source, since undesirable shadows can form on the surface under unfavorable conditions. | |
| In this application, the conductive contact pills (2 to 4, depending on the size and shape of the button) are placed at the corner points of each individual button. This ensures light emission in the center of the key, exactly where it is desired. Alternatively, a ring-shaped contact pill would have been possible, but this usually has to be specially made, whereas standard products can very often be used for circular contact pills. The arrangement of the pills is shown in Fig. 3 and allows the light to appear very well and without disturbing shadows on the surface of the key. | |
| Possible applications of silicone safety mats | |
| In general, silicone keypads can be used in various areas. Due to the variety of design options and the resistance of the material, they are suitable for applications from industry, medicine, telecommunications, measurement and control technology, automotive to consumer products (remote controls, etc.). | |
| The silicone switching mat is used to trigger a direct switching function on an underlying printed circuit board. Conductive contact pills made of carbon are attached to the underside of the keys (Fig. 3). When the key is pressed, these press on the meandering conductive paths of the circuit board and thus electrically trigger the closure of the contact and ultimately the keystroke. | |
| Practical implementation with the meter keypad | |
| In order to produce the keypad for the measuring device, a printed circuit board had to be developed in which the design of the meander structure is adapted to the contacts used in the mat. This ensures a reliable switching operation. In this specific case, the traces were adapted to relatively small contact pills placed at the edge. Figs. 5 and 6 show other conceivable designs of the conductor path structure and contact pill shapes, which are only exemplary here for other possible shapes. However, factors such as size or length of the key (e.g. space key), possible backlighting, tactility or stroke of the keys and much more are also important in the geometric design. Figure 4 shows three examples of the design of the traces underneath a key: round, oval, ring-shaped and distributed. Possible designs of the contact pills (underside of the key) are shown in Figure 5: Meander, simple meander and comb. | |
| Depending on the material of the contact pill, the electrical and manual parameters such as contact resistance, maximum current supply and service life can be decisively influenced. The tactile properties of the keypad can also be effectively determined by the geometric design of the keys, especially the bar, and the hardness of the material. The force-displacement behavior can also be very well adapted to the requirements of the application. The distance of the contact pill to the circuit board defines the travel or better stroke, the design of the bar and the material used significantly influence the force component. | |
| Figure 6 shows two examples of very different force-displacement behavior. In Fig. 6a, the switching force is between 20 and 350 g, the switching travel is approx. 0.5 to 3.0 mm and the switching cycles are between 500000 and 2000000. The example in Fig. 6b provides a switching force of 20 to 80 g, a switching travel of approx. 0.2 to 1.0 mm and 500000 to 10000000 switching cycles. | |
| The two illustrations clearly show how the design affects the behavior of the Safety Mat and how the tactility can be influenced by means of the design. Once the mold is finished, such design changes can only be made to a limited extent. The situation is different with the hardness of the material. Here, it is possible to influence the tactility even after the tools have been completed. | |
| During the project phases, several changes were made to the coating to give the surface better tactile feel and transillumination. After final clarification of all technical details and approval by the customer, the product went into series production. |
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