Automatic sensor identification for ultrasonic proximity switches
BACKGROUND OF THE INVENTION 
     The present invention relates generally to ultrasonic proximity switches for use with different types of sensors, and more particularly to an ultrasonic proximity switch with an oscillator which oscillates with an oscillator frequency. 
     In known ultrasonic proximity switches, the received echo is usually evaluated in separate evaluation electronics. To enable ultrasonic proximity switches to operate with different sensor designs, the evaluation parameters in the evaluation electronics must be adapted to the type of sensor being used. To avoid errors, the system is advantageously switched over automatically when a new sensor type is connected. 
     German Patent 34 28 773 discloses a method of sensor identification which is realized by coupling different voltage levels to a sensor output signal during exactly defined time spans. Other methods use manual coding. However, as more sensors are used and consequently have to be differentiated, these types of sensor identification become all the more costly and interference-prone. 
     The present invention is directed to the problem of developing an ultrasonic proximity switch, which is simple and reliable regarding the sensor recognition. 
     SUMMARY OF THE INVENTION 
     The present invention solves this problem by providing a counter which continuously emits a first signal with a first pulse having a duration that corresponds to the product of the preset counter content and the pulse length of a second pulse of the oscillator frequency supplied to the counter. Since the internal oscillator frequency conforms with the transmitting frequency of the sensor, the sensor being used can be inferred from the knowledge of the transmitting frequency. Accordingly, the first signal bears the information concerning the oscillator frenquency in a corresponding pulse-width modulation. 
     It proves to be advantageous when the oscillator frequency is able to be applied periodically by means of a switch operated at the transmitting clock pulse to a transmitting stage, and the first pulse is able to be synchronized with the transmitting clock pulse. It is also advantageous when a device is provided, which starts the counter and consequently the first pulse when the transmitting clock pulse has a state change, and when a monoflop which is connected to the counter in outgoing circuit is able to be triggered by the end of the first pulse, whereby at its output the monoflop generates a second signal with a second pulse of a duration corresponding to a physical measured variable. To compensate for the effect of the ambient temperature on the transmission times, it is expendient for the monoflop to be wired to a temperature-sensing device and for the duration of the second pulse to represent the measured temperature of the surrounding medium. 
     A particularly simple refinement consists in making the counter a component of an integrated circuit, which includes the oscillator. If the monoflop with the temperature-sensing device is a component of the integrated circuit, then additional components and costs can be reduced. 
     To avoid having unnecessary signal-transmission lines from the ultrasonic proximity switch to the evaluation electronics, it is also advantageous, when a third signal produces both the first pulse with the first pulse duration and also the third pulse with the second pulse duration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an embodiment of the present invention showing improved sensor identification. 
     FIG. 2 is a timing diagram of the signals of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, an oscillator 1 is shown, whose oscillator frequency 2 is applied to a transmitting stage 7 in accordance with a specified transmitting clock pulse 6. The oscillator frequency 2 from the freely oscillating oscillator 1 is fed to a counter 3. The transmitting clock pulse 6 starts the counter 3 via an external device (not shown). The output of the counter 3 is connected to a monoflop 8, which is wired to a temperature-sensing device, in this case to an Negative Temperature Coefficient (NTC) thermistor 11. 
     For clarification of this circuit, reference is made to the pulse diagram according to FIG. 2. Here, the oscillator frequency 2 is portrayed. The circuit delivers a transmitting pulse 12, which is clocked by means of the transmitting clock pulse 6 and is fed to the transmitting stage 7. The transmitting pulse 12 covers a transmitting time 13, which repeats itself periodically after a cycle time. As a result of the state change at the end of the transmitting time 13, the counter 3 is started by means of the device which is not depicted. This causes the first pulse to begin to run with the pulse duration 5. In accordance with the preset counter content, the counter 3 counts the pulses of the oscillator frequency 2 fed to it. When the counter content is reached, that is at the end of the first pulse duration 5, the monoflop 8 is triggered as a result of the state change of the first pulse. A signal 4 in accordance with FIG. 2 is applied to the output of the counter 3. By means of the trigger pulse, a third pulse 9 with the pulse duration 10 is introduced, and corresponds to the air temperature measured by the temperature-sensing device with the help of the NTC thermistor 11. This signal is applied to the output of the monoflop 8. By way of the two diodes 14 and 15, the transmitting clock pulse 6 is gated by the output of the monoflop 8, in accordance with signal 9, to third signal 16. This third signal 16 contains information both about the oscillator frequency during the first pulse duration 5 as well as the air temperature during the third pulse with the second pulse duration 10. In application cases in which the oscillator 1 and the circuit elements for the air-temperature sensing 8, 11 are already integrated in a circuit, a simple and cost-effective design approach for additional sensor identification is provided by incorporating an additional counter in the integrated circuit in accordance with the above described circuit arrangement.