1. Field of the Invention
The invention relates to a proximity switch, especially a capacitive proximity switch.
2. Description of Related Art
Proximity switches, especially inductive, capacitive and optoelectronic proximity switches, are used in a host of quite different applications. They can be proximity switches which operate without contact, i.e., those in which an actuating object simply approaches the sensitive surface of the proximity switch, and also proximity switches which operate with contact, i.e., those in which an actuating object touches the sensitive surface of the proximity switch.
In particular, proximity switches are known which have two sensitive surfaces, also called receiving electrodes, and one active surface, also called a transmitting electrode. In these proximity switches, the approach of an actuating object leads to the impedances changing between the transmitting electrode and the first receiving electrode, on the one hand, and between the transmitting electrode and the second receiving electrode, on the other. Consequently, a first current flows from the transmitting electrode to the first receiving electrode and a second current flows to the second receiving electrode. But, it can also be such that only the voltages on the receiving electrodes change without currents flowing.
In a capacitive electrode, the changing impedances between the transmitting electrode and the first receiving electrode, on the one hand, and between the transmitting electrode and the second receiving electrode, on the other, are the changing impedances.
In the following the invention is described with respect to capacitive proximity switches. The problem underlying the invention, the consequent object and the achievement of this object however are not limited to capacitive proximity switches. In particular everything which is explained below also applies to inductive and also optoelectronic proximity switches.
In the proximity switches underlying the invention, as already mentioned, there are a transmitting electrode and two receiving electrodes; in other words, one signal transmitter and two signal receivers belong to the known proximity switches underlying the invention. Therefore one signal receiver is assigned to each of the two receiving electrodes, in other words a processing channel is connected downstream. This is associated with the system-dictated defect that the processing of the received signal of the first receiving electrode via the first processing channel and in the first signal receiver, and the processing of the received signal of the second receiving electrode via the second processing channel and in the second signal receiver can differ from one another.
The object of the invention is to devise a proximity switch of the type under consideration, especially a capacitive proximity switch, which is not subject to the aforementioned system-induced defect.
The subject matter of the invention is, first of all, essentially a proximity switch, especially a capacitive proximity switch, with two signal transmitters and with one signal receiver, the two signal transmitters, on the one hand, being connected to one another each with their first signal output, and to the first signal input of the signal receiver, and on the other hand each with their second signal output via one signal transmission path at a time is or can be connected to the second signal input of the signal receiver and at least one of the two signal transmission paths can be influenced by an actuating object. In doing so, preferably the xe2x80x9csame signalxe2x80x9d or xe2x80x9csimilar signalsxe2x80x9d are delivered by the signal transmitters. The xe2x80x9csame signalsxe2x80x9d are either dc voltage signals with the same voltage amplitude or ac voltage signals with the same frequency and the same voltage amplitude. xe2x80x9cSimilar signalsxe2x80x9d means either dc voltage signals with a voltage amplitude which is not the same or ac voltage signals with the same frequency and a voltage amplitude which is not the same. AC voltage signals with a frequency which is not the same are therefore neither xe2x80x9csame signalsxe2x80x9d nor xe2x80x9csimilar signalsxe2x80x9d.
If it was assumed above that each of the two signal transmitters has a first signal output and second signal output, this should only indicate that one signal voltage can arise only between two signal outputs or one signal stream can flow only via two signal outputs; with the statement that each of the two signal transmitters has two signal outputs, it should not be indicated that two, optionally different signal voltages arise or signal streams flow. The statement that the signal receiver has two signal inputs is intended to indicate accordingly that one signal voltage can arise only between two signal inputs and one signal stream can flow only between two signal inputs.
It was stated above that at least one of the two signal transmission paths between the two signal transmitters and the signal receiver can be influenced, specifically by one actuating object. That the two signal transmission paths can also be influenced by one actuating object also implicitly belongs to this statement.
It applies both to the proximity switches underlying the invention and also to the proximity switch as claimed in the invention that simply the approach of an actuating object can be detected. But for this purpose actually proximity switches with two signal receivers, as in the prior art, or with two signal transmitters, as provided in accordance with the invention, are not necessary; the simple approach of an actuating object can also be detected with one signal transmitter and one signal receiver. Proximity switches with two signal receivers as in the prior art or with two signal transmitters according to the invention rather also make it possible to detect whether an actuating object has approached nearer one of the two receiving electrodes as provided in the prior art, or one of the two transmitting electrodes as provided as claimed in the invention, than the other receiving electrode or the other transmitting electrode. With proximity switches of the type under consideration here, therefore, with two receiving electrodes, as in the prior art, or with two transmitting electrodes in accordance with the invention, it can also be detected from which direction the actuating object is approaching.
The major difference between the proximity switches underlying the invention and the proximity switches in accordance with the invention can be explained as follows.
In the proximity switches known in the prior art which underlie the invention, the actuating object produces two electrical signals which as such are routed by the two receiving electrodes via the assigned processing channels. Following the processing channels, then, the processed electrical signals are subtracted. Conversely, in the proximity switch in accordance the invention, the two signals which have been influenced in the signal transmission path and which originate from the two signal transmitters are subtracted in front of the single signal receiver or in front of the single receiving electrode. Only one processing channel is connected downstream of the sole receiving electrode so that the system-induced defect explained above, which can occur in the two processing channels, cannot occur as caused by the system according to the invention.
It is especially advantageous if, in the proximity switch in accordance with the invention, the two signal transmitters generate frequency-spread signals. This can be done by the two signal transmitters for generating the frequency-spread signals each having one noise signal source or preferably by one noise signal source being assigned jointly to the two signal transmitters for generating the frequency-spread signals. Regarding what is achieved by frequency-spread signals, reference is made to all the disclosed contents of German patent disclosure document 198 13 013 and corresponding commonly owned, co-pending U.S. patent application Ser. No. 09/275,869, filed Mar. 25, 1999 now U.S. Pat. No. 6,225,710 B1.
It is furthermore especially advantageous if, in the proximity switch in accordance with the invention, the two signal transmitters generate signals which are phase shifted by 180xc2x0. This results in that, when the actuating object influences the two signal transmission paths identically, a zero signal is supplied to the sole receiving electrode or the sole signal receiver, so that any deviation from the zero signal which occurs with identical influencing of the signal transmission paths can be recognized and processed especially well.
It was pointed out above that, when a zero signal is supplied to the sole receiving electrode or the sole signal receiver, any deviation from the zero signal can be recognized and processed especially well. Consequently, one especially preferred embodiment of the proximity switch according to the invention to which special importance is assigned is additionally characterized in that at least one of the two signal transmitters or the output signals of at least one of the two signal transmitters is controlled, preferably such that the signal difference is zero at the first signal input of the signal receiver, therefore a zero signal is also sent to the sole receiving electrode or the signal input of the sole signal receiver when an actuating object does not influence the two signal transmission paths identically.
In particular, there are now a host of possibilities for embodying and developing the proximity switch in accordance with the invention. In this regard, reference is made both to the description of the embodiments of the proximity switch which are shown in the drawings and described below.