Capacitive sensing devices detect changes in a stray capacitance through modification of a sensed field and can be used for a variety of purposes including touch screens, liquid level sensing, and short range precision measurements. These devices are useful because a small change in capacitance can be sensed and used to determine changes in the environment such as temperature changes, pressure changes, the presence or absence of an object, or changes in the mass of a fluid or object.
Capacitive measuring systems generally lack flexibility to adapt to changes in the environment or to have multiple modes of operation. Systems that are too sensitive are subject to frequent false alarms caused by minor variations in the sensor's environment while systems that are not sensitive enough are liable to fail to respond when necessary. In addition, previous devices often used fixed thresholds which were not dependent on the position of the unit or which were subject to the influence of large objects that were always near that unit. These devices are not suitable for use in systems that contain moving parts, such as a moving gate or door, nor are they flexible enough to use in systems that require multiple modes of operation, such as security systems for private homes or public buildings.
Automatic gates and doors have many uses including limiting access to parking lots or private communities, automatic garage door openers, automatic doors for public buildings and security gates that close to secure a store front. Many automatic gates either have no object detection system or have only a rudimentary system to enable an exit gate to open based upon detecting an automobile through the use of a safety loop buried in the ground. These systems suffer because a pedestrian will not activate the safety loop and person on a bicycle or motorcycle may also not have enough mass to activate it, thereby allowing it to possibly cause bodily injury to an individual Many automatic garage door systems have optical transmitters that create a path of light and detectors that detect when an object blocks the path of light, and will prevent the garage door from closing when an object is present. Furthermore, these optical detection systems are unable to detect an object in the path of the garage door that does not block the path of light and are not used when the door is opening. This enables a garage door to close on objects that are in the path of the doorway but do not block the light path, such as an overhanging automobile bumper or a person or an object that is straddling the path of light. Conventional garage door openers are designed not to react to a reversing signal when they are opening, but instead only react when the door is closing, causing an object or person caught on a garage door to be carried upwards while the door opens.
Security systems often use multiple sensors throughout an area or location including sensors that detect if a window or door is opened and motion detection devices that detect movement within a room or an area. Sensors that detect a window or door has been opened often consist of an electrical connection that is “broken” by movement of the door or window. These devices are readily defeated by breaking a window, or glass in a door, and entering through the resultant hole. Security systems using motion detection devices lack flexibility in that any small amount of motion will cause the alarm to trigger or a person “creeping” slowly may not activate the sensor. This prevents home owners with pets, or buildings that have security guards, from using these motion detection devices. Motion detectors also cover large areas and do not allow the option of protecting a specific small area such as a painting or display in a museum.
For the foregoing reasons, there is a need for a capacitive sensing device for safety and security applications which has the flexibility to work under a large number of circumstances and be flexibly programmed.