US 3810039 A
The generation of pulses commensurate with random time intervals is achieved by generating a periodic signal at a first frequency and, upon occurrence of an initiating event, translating the signal to a different time base by changing the time constant of the signal generator. The time between the start of each cycle of the first frequency signal and the obtaining of a predetermining magnitude of the signal after translation to the different time base will be commensurate with random time intervals.
Description (OCR text may contain errors)
United States Patent [191 Fein [ METHODS AND APPARATUS FOR GENERATING RANDOM TIME INTERVALS Harry Fein, Indian Cove Rd., Guilford, Conn. 06437 Filed: Feb. 26, 1973 Appl. No.': 335,951
US. Cl 331/78, 307/265, 331/111, 331/179 Int. Cl. H03k 3/82 Field of Search 331/78, 111, 143, 179; 307/265  References Cited UNITED STATES PATENTS 3,205,454 9/1965 Lowe 331/78 STAGE 1.
[451 May 7,1974
3,575,606 4/1971 Bledsoe 331/78 X Primary Examiner-Herman Karl Saalbach Assistant Examiner-Siegfried 1-1. Grimm [5 7] ABSTRACT 11 Claims, 2 Drawing Figures STAGE 2 SAWTOOTH GENERATOR IO .L [4 l PATENTEDMAY 7 m4 160F226 ZEN WMS EP Ill.
m n II t |l b ll I l I r L N mOdFw METHODS AND APPARATUS FOR GENERATING RANDOM TIME INTERVALS BACKGROUND OF THE INVENTION i a subject ata time which is randomly delayed with respect to an initiating event. Devices for electrically generating intervals of time whose length is uniformly random and unpredictable thus have practical application where it is desirable to present an event, in visual or audible form, to a subject whose response to the event is to be measured. By way of example only, means for generating uniformly random time intervals may be employedto test the-response time of individuals under the influence of alcohol. Such a test would presumably assess the ability of the subject to successfully perform tasks such as the operation of a vehicle and, in fact, it has recently been proposed to provide response time I sensitive interlocking mechanisms on automobile ignition systems.
Previous methods for generating random time intervals have includeddigital computer programs for stimulus randomization and special purpose devices such as the apparatus described in the article An Adjustable Random Pulse Generator by L. Rovner and D. O.
Walter which appeared in the January l970issue of IEEE Transactions of Bio-Medical Engineering Vol.
BME-17 pages 76 and 77. All prior art attempts to solve the problems of stimulus randomization have had the common characteristic of complexity and have thus required expensive and generally non-portable apparatus;
It is also to be observed that intervals of time of random duration; i.e., unpredictabletime periods; are extremely useful in many scientific and industrial applicat ions. Thus,.also by way of example, apparatusfor continuously generating random time intervals'may be extremely useful in creating digital noise such as may be used in the shake testing of components for reliability and sturdiness. As observed above, prior art digital noise generators have been characterized by complex-- ity and expense.
SUMMARY'OF THE INVENTION The present invention overcomes the above briefly discussed and other deficiencies and disadvantages of the prior art by generating a periodic signal at a first frequency; the first frequency typically having a rise time whichis sufficiently fast so as to make it impossible for animals or human subjects to be able to interact with the signal in any anticipatory or synchronous manner. The initiation of an event, for example a. switch closure by a test subject, will cause the sampling of the first frequency signal at some unpredictable voltage within its range. As employed herein, the term sampling" encompasses the variation of the time constant of a signal generator, for example a sawtooth voltage oscillator, such that the initiating event will cause a periodic signal to complete its excursion between minimum and maximum voltage levels at a rate or fre-' quency which differs from the initial frequency. The voltage level of first frequency signal at the time of the initiating event is translated to a different time base; i.e., the first frequency signal is replaced by a second periodic signal having a lower frequency which completes the remainder of the excursion to the maximum value of the first frequency signal. An output signal generator responsive to the second frequency signal reaching the maximum or some other selected value of the first frequency signal provides output signals which lag the initiating event by time periods commensurate with the value of the first frequency signal when sampled. As noted, judicious selection of the first frequency will result in a situation wherein the test subject will be unable to interact with the first frequency signal in a predictable fashion and thus output time intervals of unpredictable duration will result.
In accordance with one embodiment of the invention the first frequency signal was provided by a sawtooth voltage generator which operates under the control of a bistable device. The setting of the bistable device will cause the output frequency of the sawtooth generator to change frequency instantaneously. At the end of the rise time of the sawtooth voltage the bistable device will be reset thereby'generating an output event randomly delayed with respect to the initiating event.
BRIEF DESCRIPTION or THE DRAWING DESCRIPTION OF THE P EFERRED EMBODIMENT Before describing the embodiment of the invention depicted by FIGS. 1 and 2, it is believed that it may be advantageous to discuss briefly some underlying statistical theory. In order for a method of generating random time intervals to be acceptable it is necessary that the occurrence of any time interval length within given range of time intervals be equally likely to occur upon the initiation of an input event. This requirement, in turn, demands that the length of time after each initiating event, a switch closure for example, must fall within the appropriate range but each specific trial must have an unpredictable resulting duration. Thus, a plot of the probability of occurrence versus interval length should desirably be rectangular. A time function which has the requisite probability density is a sawtooth function as shown in FIG. 2. Although the sawtooth function is periodic, its value at any arbitrary time is unpredictable. Thus, when a sawtooth function is sampled at will, the resulting sampled voltage amplitudes will be random. In order for these random voltage amplitudes to be used to generate proportionate time delays that are also random it is necessary to insure that the time interval resulting is proportional to the value of the function at any arbitrary sampling time and that it be impossible for an operator or test subject to anticipate the sawtooth function because of its periodicity.
In accordance with the preferred embodiment of the present invention, a sawtooth oscillator, which for convenience may be called the fast sawtooth," runs freely at a rate which is arbitrary but fast with respect to human reaction times. Thus, considering the environment of a reaction time testor for humans, the fast sawtooth will be at a frequency of cycles per second or faster. In the manner to be described below in the discussion of FIG. 1, when an event is initiated, for example by a switch closure, the fast sawtooth will be sampled at some unpredictable voltage amplitude within its range. The voltage level at which the fast sawtooth" is sampled will then be translated to a different time base; i.e., the fast sawtooth" is replaced by a lower frequency signal which completes the remainder of the excursion of the voltage range of the fast sawtooth. In the most basic form of the present invention, the lower frequency signal will be a slow sawtooth.
As may be seen from FIG. 2, the above described action is commensurate with part of a selected fast sawtooth wave being instantaneously stretched in time with the remainder of the cycle I, being proportional to the amplitude of the fast sawtooth at the instant of sampling. The statistical density of time intervals so generated by repeated trials is ideally rectangular.
With reference now to FIG. 1, apparatus for generating random time intervals in accordance with the present invention is depicted in block diagram form. While FIG. 1 depicts a two stage random time interval generator, it is to be understood that a single stage of the device may be employed alone. Each stage of the apparatus of FIG. 1 includes an input flip-flop circuit 10 which is responsive to an initiating event. Thus, by way of example, the closing of a switch by a subject may apply a signal momentarily to input terminal 13 which will cause the setting of flip-flop circuit 10. In' the manner to be described below, the flip-flop circuits 10 and 10, respectively of stages 1 and 2, control respective sawtooth voltage generators '12 and 12'. The output of amplifier of the ramp voltage generator portion of the sawtooth voltage generator 12, and a similar amplifier in sawtooth voltage generator 12, is applied to respective voltage sensitive triggering circuits l4 and 14.
To briefly describe the operation of one of the random time interval generators of FIG. 1, the sawtooth oscillator 12 is normally free running at its fast rate. Each time the sawtooth voltage reaches its maximum level, or some other value established by the voltage sensitive triggering circuit 14, the trigger circuit generates a reset pulse which causes the output voltage of the sawtooth oscillator to be returned to zero. The retriggering pulses from circuit 14 are also applied to the reset terminal of flip-flop 10. An initiating or starting event, in theform of a voltage applied to input terminal 13, will cause the setting of flip-flop 10. When flip-flop 10 is set it will change state and, in the manner to be described below, the output frequency of the sawtooth generator 12 will instantaneously change; usually to a lower frequency. Since the starting event will occur when the level of the sawtooth voltage is somewhere between the minimum and maximum levels ofits range,
the switching of flip-flop 10 has the effect of instantaneously changing the slope of the sawtooth ramp typi cally as depicted in FIG. 2. At the end of the excursion of the sawtooth voltage at its new frequency, the triggering device 14 will generate an output pulse which resets both the sawtooth generator 12 and flip-flop 10.
The resetting of flip-flop 10 will provide an output signal which may be employed to trigger a successive stage of the random time interval generator, as shown in FIG. 1, or may be applied to a suitable output device. By way of example, the pulse resulting from the resetting of flip-flop 10 may, via a buffer circuit 16, be applied to an output terminal 18 which is connected to a signal lamp and timer. The test subject will, upon the energization (flashing) of this signal lamp, be required to perform some function and, should that function not be completed within the prescribed time, a condition such as an ignition system disabling function may be automatically established. Buffer 16 may be an emitter follower or isolation amplifier.
To discuss the apparatus of FIG. 1 in more detail, and starting with flip-flop 10 in the untriggered state, the 0 output of the flip-flop 10 is high resulting in current flow through resistor R1 of sawtooth generator 12. Resistor R1, as well as the similar but different value resistor R2, forms part of an integrator or ramp circuit which includes capacitor C1; the capacitor being connected in parallel with an operational amplifier 20. The output of the ramp voltage generator portion of sawtooth generator 12, at a frequency determined by the time constant established by R1 and C1 with flip-flop 10 in its untriggered state, is applied to a Schmitt trigger circuit 14. Schmitt trigger circuit 14 generates a sharp output pulse when the output of the integrator, which falls linearly, as shown in FIG. 2, reaches a preselected negative level. The output pulse of Schmitt trigger 14 causes conduction ofa switching device 22 connected in parallel with capacitor C1 and thereby causes the capacitor to be discharged to essentially ground potential in substantially instantaneous fashion. The output of the Schmitt trigger 14 is also applied to the reset terminal of flip-flop 10. This reset pulse will not result in any change in circuit operation since Q-high" is flip-flops 10 normal or reset state. Thus, in the absence of an input trigger the circuit will function as a sawtooth generator which runs continuously at an arbitrary rate determined by the time constant RlxCl.
When flip-flop 10 receives an input triggering pulse, as applied to input terminal 13, the multivibrator will be set and the high output voltage will appear at the Q terminal. Accordingly, immediately upon the setting of flip-flop 10, the amount of current flowing into the summing junction of the integrator will be dictated by resistor R2 rather than resistor R1. Since the input trigger occurs at an unpredictable time, the amount of time left in which the sawtooth can run to completion and trigger the Schmitt trigger 14 is unpredictable. The time to triggering Schmitt trigger 14 is a function of the product R2XC1, the new time constant of the integrator, and the initial voltage existing across capacitor C1 at the instant the input trigger is applied to flip-flop 10. When the preselected voltage is again reached, the Schmitt trigger 14 will generate another reset pulse which returns flip-flop 10 to its normal Q high state while simultaneously discharging capacitor C1. The resetting of flip-flop 10 will, of course, return the sawtooth oscillator to its original output frequency as determined by the time constant RlxCl. The 6 output of flip-flop 10 may be used as a circuit output terminal to yield a rectangular wave whose dwell is the duration of the random time interval or, as briefly described above, buffer circuit 16 may be employed to provide output pulses of duration commensurate with the length of each random time interval.
The output of stage 1 may, simultaneously with or alternatively to application to output terminal 18, be applied via buffer 16 to the set input of flip-flop 10 of stage 2 thereby functioning as the initiating event for the second stage. The second stage of the random time interval generator will function in the same manner as the first stage with, however, the exception that the ap paratus can be employed in a tandem-feedback manner to continuously generate random intervals. Should this be desired, the output of Schmitt trigger 14 of stage 2 can be applied to the set input of flip-flop 10 of stage 1 via a normally open switch 24. Alternatively, the output of stage 2 as measured at output terminal 26 may be applied to a further random time interval generator stage identical to stages 1 and 2. Also, output pulses commensurate with the triggering of Schmitt trigger circuit 14' may be provided by applying, via a monostable circuit 28, the retriggering pulses from Schmitt trigget 14 to output terminal 30 of stage 2.
While a preferred embodiment has been shown and described, various modifications and substitutions may be made thereto without departing from the spiritand scope of the invention. Thus, by way of example, while the invention has beendescribed in terms of using sawtooth voltage generators, exponential or other nonlinear function generators may be employed in place of the sawtooth generators if the probability density of the output is desirably other than rectangular. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.
What is claimed is:
1. Apparatus for generating electrical output pulses commensurate with time intervals of random duration comprising: i
signal, generator means, said signal generator means selectively producing a periodic signal at either of two different frequencies, said signal generator means further providing control signals commensurate with the periodic signal reaching a preselected magnitude; v switch means having a pair of input terminals and a pair of output terminals, a first of said switch means input terminals being connected to receive an input signal commensurate with an initiating event and the other of said switch means input terminals being connected to receive control signals from said signal generator means, said switch means output terminals being connected to the input of said signal generator means, said control signals causing said switch means to assume a first state whereby the output of said switch means causes said signal generator means to produce periodic signals at a first of said two different frequencies, an input signal commensurate with an initiating event causing said switch means to assume a second state whereby the output of said switch means causes said signal generator means to produce periodic signals at the other of said two different frequencies; and
means connected to said switch means for providing an output signal commensurate with the state of said switch means.
2. The apparatus of claim 1 wherein said signal generator means comprises:
ramp voltage generator means for providing an out put signal having a magnitude which varies linearly with time; and g voltage level sensitive means connected to said ramp voltage generator means for providing control signals commensurate with a preselected magnitude of the output of said ramp voltage generator means.
3. The apparatus of claim 2 wherein the rate of change of the output voltage of said ramp voltage generator means is a function of an RC time constant.
4. The apparatus of claim 2 wherein said switch means comprises:
means for selectively varying the time constant of said ramp voltage generator means.
5. The apparatus of claim 4 wherein said ramp voltage generator means comprises:
a capacitive storage element;
a first resistive element;
a second resistive element;
means connecting first terminals of each of said resistive elements to said capacitive storage element;
means connecting second terminals of each of said resistive elements to said switch means; and
means responsive to the generation of said control signals for discharging said capacitive storage element.
6. The apparatus of claim 5 wherein said means for selectively varying the time constant comprises:
a bistable circuit having a pair of output terminals, second terminals of said resistive elements being connected to respective of the output terminals of said bistable circuit.
7. The apparatus of claim 4 wherein said means for selectively varying the time constant comprises:
a bistable circuit having a pair of output terminals connected to said ramp voltage generator means.
8. The apparatus of claim 7 wherein sensitive means comprises:
a trigger circuit, said trigger circuit being responsive to said linearly varying output signals from said ramp voltage generator means for generating said control signals.
9. The apparatus of claim 4 wherein said voltage level sensitive means comprises:
a trigger circuit, said trigger circuit being responsive to said linearly varying output signals from said ramp voltage generator means for generating said control signals.
10. A method of generating random timeintervals comprising:
generating a periodic signal at a first frequency;
intercepting the first frequency signal at some unpredictable voltage within its range in response to an initiating event;
translating the first frequency signal to a different time base immediately upon intercepting; and
providing an indication of the time required for the translated signal to reach a preselected level.
11. The method of claim 10 wherein the step of translating comprises:
varying the RC time constant of a circuit which generates the periodic signal at a first frequency.
said voltage level