1. Field of the Invention
The invention relates to a control panel for a measuring device with a housing and at least one control and inspection window, and more particularly, to a control panel for a measuring device having at least one finger-operable optical key and at least one finger-operable mechanical key, wherein the at least one finger-operable optical key is operable through the control and inspection window using a finger.
2. Description of Related Art
Control panels for measuring devices have been known for decades in very different configurations. A window provided in the measuring device housing is usually only an inspection window such that measured values can typically be read on the measuring device through the inspection window. Settings of the measuring device e.g., measurement range, parameterization and calibration data, can also be displayed through the inspection window. Keys are provided in the majority of measuring devices for the purpose of control and the keys can be operated without opening the housing of the measuring device. For example, keys can be provided in the form of membrane keys on the outside of the housing.
Additionally, keys can be provided inside the housing of the measuring device. In particular, keys are generally provided inside the housing in instances in which the measuring device is not operated under laboratory conditions, but rather in an industrial process environment under occasionally harsh environmental conditions or in areas subject to explosion hazards. Consequently, in these instances, the measuring device can usually only be controlled when the housing of the measuring device is opened.
However, if it is intended to be possible to control the measuring device when the housing is closed without the control keys penetrating through the housing, then it is possible to use a control panel with at least one finger-operable optical key having a transmitting element and a receiving element. A finger-operable optical key uses the transmitting element to emit electromagnetic radiation, often in the infrared range, and the receiving element for detecting at least electromagnetic radiation of the type emitted by the transmitting element. The method of operation of such an optical key is known and based on the principal that an operating object which approaches the optical key reflects the electromagnetic radiation emitted by the transmitting element to different extents. The result of the reflected radiation is applied to the transmitting element with different intensities depending on the distance between the operating object and the optical key. The reflected radiation detected by the receiving element is then used to detect the state “operate.” In this instance, the optical keys are arranged with the control panel behind the inspection window of the measuring device housing in such a manner that finger operation of the optical key can be detected in a particularly effective manner if the control finger comes to rest on the control and inspection window above the optical key. In particular, there is sufficient reflection of the electromagnetic radiation emitted by the transmitting element at this distance to activate the state “operated.”
However, the disadvantage of the finger-operable optical key is that it is no longer reliably possible to control a measuring device provided with the previously known control panel when the housing of the measuring device is open and the control panel is exposed. For example, this situation occurs when servicing the measuring device. Specifically, this situation occurs when there is an increased setting need under certain circumstances and a comparatively large number of data items have to be input via the keys. In this instance, the distance between the optical key and the control finger, which is otherwise defined by the control and inspection window, is no longer defined. Thus, intentional operation of the optical key can no longer be reliably distinguished from unintentional influencing of the optical key.