Abstract:
A circuit is shown which outputs a pulse width having a duty cycle of less than 50% and the circuit having an oscillator providing a desired repetition rate controlled by a changing RC time constant.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to oscillators and in particular magnet-based oscillators that are low duty cycle. 
     2. Description of the Art 
     Magnetostrictive transducers having elongated waveguides that carry torsional strain waves induced in the waveguide when current pulses are applied along the waveguide through a magnetic field are well known in the art. A typical linear distance measuring device using a movable magnet that interacts with the waveguide when current pulses are provided along the waveguide is shown in U.S. Pat. No. 3,898,555. 
     Devices of the prior art of the sort shown in U.S. Pat. No. 3,898,555 also have the sensor element in a housing which also houses the electronics to at least generate the pulse and receive the return signal. The amplitude of the return signal detected from the acoustical strain pulse is, as well known in the art, affected by many parameters. These parameters include the position magnet strength, waveguide quality, temperature, waveguide interrogation current, and assembly tolerances. 
     A low duly cycle oscillator is used to provide a relatively short pulse width at a relatively long period repitition rate. An example would be the waveguide driving circuit for a magnetostrictive position sensor. This type of driving circuit of the prior art supplies a one or two microsecond pulse at a repetition rate with a period of one millisecond to one second. A one microsecond pulse with a repetition rate having a period of one millisecond would be a duty cycle of only 0.1% this function is usually accomplished by driving a one microsecond monostable multivibrator from the output of a one kilohertz oscillator. A similar function is usually required in ultrasonic sensors or other time-of-flight measurement circuits. 
     Several types of magnetic-based sensors are available for measuring linear or rotary position. Magnetic-based sensors have an advantage in that they provide non-contact sensing; so there are no parts to wear out. Examples of magnetic-based sensors are LVDTs, inductive sleeve sensors, and magnetostrictive sensors. 
     A first previous version of low duty cycle oscillators used in, for example, magnetostrictive devices, have used a combination of a 50% duty cycle low frequency oscillator and a monostable multivibrator (one-shot) as shown in FIG.  1 . The oscillator sets the repetition rate and the one-shot outputs the short duration pulse. 
     A second previous version of a low duty cycle oscillator has been based on a standard oscillator using one or more digital gates, where the charge up and charge down times are steered through different paths by using one or more diodes, as shown in FIGS. 2 and 3. This circuit can satisfy some requirements, but has a substantial temperature coefficient because of the approximately 2.2 mV per degree celcius temperature coefficient of the diode forward voltage drop. The forward voltage drop of approximately 0.6V is also an error that must be accounted for in the timing and performance characteristic calculations. 
     A third previous version of a low duty cycle oscillator has used software control of a microcontroller output port to produce the desired dutycycle. This is inexpensive, but requires a high degree of dedication of the microcontroller capability to performing this simple function. 
     It is an object of the present invention to employ a simple circuit to accomplish producing a relatively short pulse at a relatively long period repetition rate. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a simple circuit to realize the function of a low duty cycle oscillator (less than 50% and preferably 10% or less) to provide a relatively short pulse width at a relatively long period repetition rate by using common integrated circuit components. The I.C. components comprise several copies of the same function in one package. Portions of these multi-part components, sufficient to implement the instant circuit, are often left unused in a typical circuit design application. Therefore, this circuit can be implemented with little additional expense and overcomes the inefficiency and inaccuracies of the prior art. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the nature and objects of the present invention, reference should be had to the following figures in which like parts are given like reference numerals and wherein: 
     FIG. 1 is a schematic of a circuit of a prior art for Low Duty Cycle Oscillators; 
     FIG. 2 is a schematic of a circuit of the prior art showing a second Low Duty Cycle Oscillator; 
     FIG. 3 is a third schematic of a circuit of the prior art showing a Low Duty Cycle Oscillator; 
     FIG. 4 is a schematic of a circuit utilizing a Low Duty Cycle Oscillator of the present invention; and 
     FIG. 5 is a schematic of a circuit utilizing a Low Duty Cycle Oscillator of the present invention but using an n channel enhancement mode MOSFET instead of the IC 3  of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention uses an oscillating circuit, the frequency of which is determined by a resistor and a capacitor. Low duty cycle (less than 50% and preferably 10% or less) is achieved by sensing when the on-time occurs and shunting the resistor with a second resistor during that time, thus reducing that portion of the cycle. 
     In the circuit of FIG. 4 for a pulse width having a duty cycle of less than 10%, IC 1  is a CMOS Schmitt trigger, such as a  74 C 14 , normally one of six included in a package. IC 3  is a CMOS bilateral switch, such as an CD4066, normally one of four included in a package. 
     Referring to FIG. 4, IC 1  has an input Pin  1  and an output Pin  2 . Input Pin  1  is connected to a capacitor C 1  which is connected to ground. Typically, capacitor C 1  may be 5.6 nanofarads. Pin  1  is also connected to a resistor R 1  which is connected at its other end to Pin  2  of IC 1 . Typically, R 1  may be a 100,000 ohm resistor. Lead  1  is also connected to a second resistor R 2  whose other end is connected to a switched pin of IC 3 . R 1  is also connected to the control pin of IC 3  in its connection with Pin  2  of IC 1 . Typically, R 2  is 750 ohms. The other switched pin of IC 3  is connected to the five volt reference source. 
     When IC 1  pin  2  is low, R 1  discharges C 1 , taking a time period of approximately R 1 ×C 1  seconds. When IC 1  pin  2  goes high, IC 3  switches to close the connection between +5V and R 2 . This places R 2  in an essentially parallel path to R 1  for charging up C 1 . The faster charge rate results in an on-time for IC 1  pin  2  (the time during which the output is high), of the parallel combination of R 1  and R 2 ×C 1  seconds. An additional advantage of this configuration is that the smaller value resistor, R 2 , is driven by a source other than IC 1  pin  2 . This means that IC 1  pin  2  will not be affected by trying to drive a higher current during the positive half cycle, as would be the case in the circuit of the prior art FIG. 1 or  2 . 
     As shown in FIG. 5, substantially the same circuit is used as in FIG. 4 except that a n-channel enhancement mode MOSFET Q 1  and there is a second circuit which may be identical to IC 1  or could be any inverter. It is connected to Q 1 . 
     Alternatively, any kind of oscillator could be used such as two inverters or any other type of oscillator. There are a myriad number of ways that an oscillator can be made by those skilled in the art. Further, the oscillators can be set for a minority portion of the duty cycle. 
     Because many varying and different embodiments may be made within the scope of the invention concept taught herein which may involve many modifications in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.