CA1098596A - Amplifier malfunction detector - Google Patents

Abstract

ABSTRACT

A detection device for sensing malfunctions in a wide band power amplifier driving the take-up spool motor of a reel-to-reel tape transport system, is disclosed A
feedback loop is connected from the output of the amplifier to a malfunction junction positioned at the input of the amplifier. The signal at the malfunction junction is monitored and compared with a predetermined range of standard reference signal. Whenever the malfunction junction signal is out of the predetermined range, the power amplifier is turned off thereby preventing damage to the tape. A filter means is positioned so as to desensitize the compare means until the amplifier is operating at steady state.

Description

5~6 1 AMPLIFIER ~LFUNCTION DETECTOR

2 Abstract

3 A detection device for sensing malfunctions

4 in a wide band power amplifier, driving the take-up spool motor of a reel-to-reel tape 6 t~ansport system, is disclosed. A feedback loop 7 is connected rom the output of the amplifier to 8 a malfunction junction positioned at the input of 9 the amplifier. The signal at the malfunction junction is monitored and compared with a predetermined range 11 of standard reference signals~ Whenever the malfunction 12 junction signal is out of the predetermined range, 13 the power amplifier is turned of thereby preventing 14 damage to the tape. A filter means is positioned so as to desensitlze the compare means until the 16 ampliEier is operating at steady state.
17 Background of kh _Invention 18 1. F eld of the Invention 19 The in~enti~n relates-generally to circuits for detecting malfunction in amplifiers and more 21 specifically or an improved circuit for detecting . . ~
22 amplifie~ malfunckion over a wide range of input 23 signals.
24 2. Prior Art .
In a general sense, detection circuits 26 may b~ classified into two broad groups. In the 27 first group are circults which protec~ a system - ~- . . . .
28 rom~transient conditlons or disturbances which - 29 occur external to the system. Some of the well . .
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1 known transient conditions are; over-voltage, under-2 voltage, over-current and under-current. The general 3 approach in solving the problems in the first group 4 is to sense the abnormal condition at an external point to the system to be pro~ected and isolating 6 the system once the abnormal condition is sensed.
7 One of the well known procedures is to select an 8 -alternate path to ground once the abnormal condition 9 is sensed.
In the second group are circuits which 11 sense malfunction conditions occurring within the 1~ system to be protected. In other words, the malfunction 13 is generated within the system rather than a mal~unction 14 occurring without the system as in the ~irst group previously mentioned.
16 The present invention may be useful in 17 ~ystems classified in both the fir.st ~roup and second 18 group, but finds its greatest utility in the systems 19 which are classified ~ithin th~ second group.
In general, the present invention is directed 21 to circuits for sensing ~he malfunction in an amplifier.
~2 More specifically, the invention is directed to 2~ circuits for sensin~ tha malfunction in an ampiifier 24 which drives the take~up spool and supply spool .
of a reel-to-reel tape transport. An alternate 26 use lS for sensing malfunction occurring in amplifiers 27 which drive the motor, in an X-Y coordinate library 28 system.
29 ` Reel-to-reel tape transport systems are widely known. In one form a generally cylindrical _z~

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1 mandrel or drum includes a rotating head wheel which 2 carries one or more read/write heads. The magnetic 3 tape engages the mandrel at one point, makes a helical 4 wrap-around at least a portion of the mandrel, and exits the mandrel at a point which is both axially

6 and circumferentially spaced from the entrance point.

7 The angle of helical tape wrap can vary in accordance

8 wlth design choice, but is usually hetween 180

9 and 360. The head wheel rotates so as to swèep its magnetLc heads traversely across the tape. The ll angle at which the head enters and exits the tape 12 may vary in accordance with design choice, from 13 slightly less than 90 to a small angle, su~h as 14 15.
A supply spool and a take up spool is 16 positioned so as to bring the tape in transducing 17 relationship with the rotating head. Each spool 18 is driven by a motor. The motor is in turn driven l9 by power amplifiers. ~ue to the accuracy which is required in controlling the tension in ~he tape, 21 any malfunction in the power amplifiers must be 22 instantaneously detected. Failure to detect malfunction 23 will result in snapping or breaking of the tape.
24 ~arious techniques have been used in the prior art for detecting ampliier malfunction or 26 for detecting errox wi~hin a servo control system.
27 In one scheme a predetermined constant error is 28 generaily introduced by conventional means. For 29 example, if a constant load is belng driven by the ". .
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1 servo system, a torque motor is used to apply a 2 torque opposite to the direction in which the load 3 is moved. The torque is used to displace a potentio-4 meter arm and an electrical error signal is introduced into the system. The error signal is fed back into 6 the system and is monitored. Any deviation from 7 the known error signal is a wa nin~ that the system 8 is malfunctioning.
9 Alfhough the prior art devices emboclying the above enumerated scheme operates satisfactorily 11 ~or the intended purpose, there are several problems 12 which plague these devices. One of the problems 13 is the high cost and bulkiness of these prior art 14 devices. The high cost stems from the fact that lS in order to intro~uce the constant error into the 16 system, additional hardware is required; for exampler 17 the torque motor and the circuit for converting 18 the torque into an electrical signal. The aaditional 19 cost of this hardware increases the overall unit cost. Also the additional hardware adds to the 21 overall size and hence, the bulkinPss of the unit.
22 As is well known to those skilled in the 23 art, the present demand is for low cost miniaturized 24 components. With this restraint, the prior art devices are not suitabl~ for several applications.
26 Another drawback with the prior art servo 27 monitoring scheme is that the load which is driven 28 by the servo system has to be constant. In a servo .

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1 system where the load is varying, th0 prior art 2 scheme will not function satisfactorily to detect 3 malfunction in the system. There are several applications 4 wherein the malfunction detector must detect malfunction in a servo system which is driving a variable load.
6 For example, a reel-to-reel servo control system 7 wherein the load (magnetic media) on the take-up 8 spool varies.
g Another prior art scheme which has been used to detect amplifier malunction in a servo 11 controlled system is the so-called random sampling 12 approach. This scheme requires a test run to be 13 undertaken at a predetermined time during normal 14 operation of the system.
Although the random sampling scheme has 16 solved some of the problems which were created by 17 the scheme in which error is introduced into the 18 system, several new problems are creat~d.
19 In ordex to perform a test run on the system, the normal operation of the system has to ~1 - be interrupted~ This interference tends to reduce 2i2 the throughput~ i.e~, the amount of work which is 23 outputted from the system. With a random sampling 24 scheme, the system is interrupted at intervals and a test run is performed. A known signal is introduced 26 into tha system or amplifier; for example, one volt 27 and the output is monitored. If ~he output ~s within 28 a prescribed range, the system is allowed to run.
29 If the output is out of the prescribed range, the system is malfunctioning and corrective steps are 1 taken.
2 Another problem is that if the system 3 has intermittent failures, the random sampling technique 4 may not detect these failures. In several applications S it is important that failures or malfunctions be 6 detected instantaneously. For example, in a reel-7 to-reel servo control system, any failure occurring 8 in the amplif1er means which is driving the take-9 up spool must be detected at once or else the media can be snapped.
1~ Still another problem is the fact that 12 the random sampling technique cannot continuously 13 monitor the system for malfunction detection. In 14 several applications, it is imperative that the malfunction device be capable of continuously monitoring 16 or else intermittent undetected failure will be 17 disasterous.
.
18 Flnally, the prior art malfunction devices 19 and schemes ara operable ovar relatively narrow input signal xange. However, there axe several 21 applications wherein monitoring has to be performed 22 over relatively wide signal range (e.g., -20 volts, 23 +20 volts) due to the signal range limitation, the 24 prior art devices are not suitable.
Objects of the Invention ..
26 It is, therefore, the object of this invention 27 to detect amplifier malfunction in a more efficient Z8 and improved manner than was heretofor possible.

' 1 It is another object of the invention 2 to produce a low C05t miniaturized malfunction detector.
3 It is still another object of the invention 4 to produce a malfunction detector which monitors malfunctions of an amplifier driving a variable load.
6 It is a further object of the invention 7 to continuously monitor to detect malfunctions in 8 an amplifier. -9 Still a further object of the invention is to detect amplifier malfunction over a wider 11 signal range than was heretofor possible.
12 5ummary of the Invention 13 T~e present invention overcomes the drawbacks 14 found in the prior art by means of an improved circuitry which continuously monitors the operation of a wide 16 band amplifier means to detect malfunction. The 17 output of the amplifier means is fed back via an impe-18 dance circuitry to a summing junction positioned 19 at the input to the amplifier means. Whenever the summing junction signal falls out of range of acceptable 21 signal, the power supply is shut down~
22 In one embodiment of the invention, a 23 filter means having a time constant equivalent to~
24 the dominant pole of the amplifier is positioned between the malfunction junction and the error detection 26 means. This embodiment improves the reliability 27 of the error detection scheme in that the error 28 detection means is desensitized unt~l the amplifier .' ~ .

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1 is operating at steady state. With this design 2 false alarm due to amplifier slew rate or high 3 frequency triggering is eliminated.
4 In another embodiment of the invention a gain switch means is positioned between the mal-6 function junction and the filter means. The switch 7 is in a non-inverting gain configuration. This 8 gain switch means widens the reference band so 9 that the amplifier may operate with an overdrive siynal in a saturation state without generating 11 a malfunction signal.
12 The foregoing and other objects, features 13 and advanta~es of the invention will be apparent 14 from the following more particuli~r description o~ preferred embodiments of the invention, as illustrated 16 in the accompanying drawings.
17 Brief Description of the Drawin~s 18 FIGURE 1 is a perspective view of a reel-19 to-reel tape control system with the malfunction d~tector of ~he present invention in block diagram 21 form. ~
22 FIGURE 2 shows a more detailed view of 23 a motor power amplifier and the malfunction detector 24 with filtering means.
FIGQRE 2A shows an alternate embodiment 26 of a filter means.
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27 FIGU~E 2B shows one type of voltage-follower ~
28 filter means~ `
29 FIGURE 3 shows ~he malfunction detector with gain switch means.

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1 DescrIption of the Preferred Embodlment 2 FIGURE 1 illustrates the simplest form 3 of the malfunction detector as it is incorporated 4 in a reel-to-reel tape transport system. In the drawings similar elements are identified by the same 6 numbers~ In its simplest form, the tape transport 7 unit comprises magnetic media 10. One end' of magnetic 8 media 10 is wound onto take-up spool 12. Driving 9 means 14 can rotate spool 12 in a clockwise or counterclock-wise direction. Whenever spool 12 is rotated in 11 the clockwise direction, the amount of maynetic 12 media 10 on the spool increases. Likewise, if spool 13 12 is rotated in khe counterclockwise direction, 14 the amount of magnetic media 10 on the spool decreases.
With this configuration, the combination of spool 16 12 and magnetic media 10 represents a variable load 17 means for driving means 14. The other end of magnetic ' 18 media 10 is wound onto a supply spool (not shown).
~9 A more detailed description of a reel-to-reel system in which the malfunction detector can be utilized 21 is'found in U. S. Patent 3,809,335 entitled ''Web ~ -22' Movement Controlled in a Reel-to~Reel Web Transport"
23 issuad to J. P. Mantey and assigned to the assignee 24 of the present invention. Also, publication entitled "Tachometer Feedback in a Reel-to-Reel Control System"
26 by N. H. Hanse~, Jr., published in IBM TechnicaI
27 Disclosure Bulletin, Volume 17, No. 11, April, 1975, 28 page 3194, discloses a similax reel-to-reel'system.

_9_ , 1 Although the invention is described in 2 relation-ship with a reel-to-reel tape transport 3. system, this should be construed as illustrative 4 and not a limitation on the scope of the in;ention;
since the malfunction detector finds use in any :
6 application where a pow~r amplifier has to be monitored 7 or malfunctioning~
8 Still referring to FIGURE 1, the malfunction 9 detection,is comprised of power amplifier 160 The output of power amplifier 16 is connected to driving 11 means 14 and output signals which control driving 12 means 14. Power amplifier 16 is comprised of operational 13 amplifier 18 connected in series with power booster 14 20. Operational amplifier 18 is comprised of a negative input terminal 22 and a positive input .16 terminal 24. Positive input terminal 24 is tied ~ .
17 to ground. . : .
18 Malfunction junction 26 hereinafter 19 called malfunction detection means 26 is connected to negative input terminal 22. Basically, malfunction 21 detection means 26 is a summing junction. It accepts 22 a feedback signal from terminal 28 which ties output 23 of powsr amplifier 16 to malfunction detection means 24 26 via .feedback resistive means 30. -Also, malfunction detection means 26 accepts a signal from terminal 26 32. The signal on terminal 32 is derived from an .
27 external driving source, for example, Ein tsee FIGURE
28 2). The external driving source is connected by input - : ~

-10-l resistiv~ ~leans 34 to terminal 32. Malfunction detection 2 means 26 sums the two signals, one on terminal 28 3 and the other on terminal 32 and outputs an error 4 signal. The error signal is used for driving power amplifier 16. The error signal is also fed into 6 malfunction detector 36 via terminal 38. As will 7 be explained subsequently, the error signal on terminal 8 38 lS compared with a reference range of signal 9 in malfunction detector 36 and a malfunction signal is outputted on terminal 40 whenever the error signal ll is not within a predetermined range. The malfunction 12 signal is used tv either shut down the power supply 13 or inform a microprocessor that power amplifier 14 16 is defective.
As will be explained suhsequently, the 16 input signal is picked off by terminal 42 and fed 17 into malfunction detector 36. Mal~unction detector 18 36 has circuitry which will monitor and control 19 the signal on terminal 42 so that power amplifier 16 can be driven into saturation without outputting 21 a malfunction signal on terminal 40.
22 Referring now to FIGURE 2, section 44 23 of the malfunction circuitry of malfunction deteetor ~4 36 is shown in gxeater detail. Malfunction circuitry 44 is comprised of filter means 46, compare means 26 48 and OR circuit means 50. Filter means 46, compare 27 means 48 and OR circuit means 50 are connectad in 28 tandem. The output from OR circuit means 50 appears 29 on terminal 40 and indicates when-there is a malfunction.

35~P~

The main theory underlined detection of 2 circuit malfunction is that in normal operation 3 of power amplifier; the error voltage ES (see FIGURE.
4 2) which appears on terminal 38 from malfunction detection means 26 will be very close to zero (offset 6 term plus output/open loop gain). A malfunction 7 will cause Es to rise above or fall below a few 8 millivolts of ground. By monitoring the fall or 9 rise and comparing it with a standard range of reference voltages, one can determine when malfunction occurs

11 in power amplifier 16.

12 ,However, it is well known that at initial

13 startup; i.e., whenever the ampli~.'ier is running

14 at high fxequencies or if the slew rate of the amplifier is exceeded, compare means 48 will output a signal 16 which'will activiate OR circui~ me~ans 50 and output 17 a malfunction signal on terminal 40 when in fact 18 there is no malfunction.ing in amplifier 16. To 19 alleviate this false alarm, filter means 46 is incorporaked as shown in FIGU~E 2.
-21- ~ Referring now to FIGURE 2A and FIGURE
22 2B alternate embodiments of filter means 46 are 2~ shown.
24 . ' In FIGURE 2A, filter means 46 is comprised .-of a conventional RC filter 56. Filter 56 is comprised 26 ' of terminal BB and resistor 60 in series with capacitor 27 62. The time constan~'of filter 56 is substantially -.
28 ' equivalent ~o~the.dominant pole of the ampliier.

~ ' . -12 :

1 Whenever filter 56 is substituted for filter means 2 4 6 in FIGURE 2, the signal on terminal 64 will not 3 be outputted until power amplifier 16 is running 4 ~t steady state. With this configuration, any transient signal which is caused by s~artup will not be able 6 to reach compare means 48 and therefore the reliability 7 of the system is enhanced.
8 FiLter 58 (FIGURE 2B) is an a~ternative 9 to filter 56. Basically filter 58 is comprised of 10 - terminal CC~ voltage follower 66, resistor 68 and 11 capacitor 70. Voltage follower 66 is connected 12 in series with resistor 68, and the combination 13 is in series wi*h capacitor 70.
14 ! , Although filter 56 and ~ilter 58 are needed for accurate malfunction detection, there are several 16 applications wherein a filter is not required. For 17 example,-if the input signal (Ein on terminal 32) 18 is restricted (i.e.; no high frequencies and/or 19 low slew rate) then a through wire, instead of ilter 56 and filter 58, càn be used to connect terminal 21 38 to termlnal 64. In one embodiment a straight 2? ~hrough wire is used to connect terminal 38 to terminal 23 64 and there was no degradation in the malfunction 24 detection-circuitry.
The signal from filter means 46 appears 26 on terminal 64. Termlnal 64 .i5 attached to comparator 27 means 48. Comparator means 48 is comprised of comparator 28 72 and comparator 74. Comparakor 72 is the positive 29 comparator and will output an error slgnal on terminal 76 whenever the error voltage ES from malfunction .
.. ' ' 1 detection means 26 exceeds the allowable few millivolts 2 range in the positive directionO Comparator 72 3 has a positive terminal which is tied to terminal 4 64 and a negative terminal which is tied to ground via positive reference batterv 78. The voltage 6 on battery 78 is equivalent to the maximum possible 7 voltage which is allowable for proper operation 8 of the ampliiier 16. In other words, whenever errox 9 voltage Es exceeds the vo].tage on battery 78 a malfunction si~nal will appear on terminal 40. For example, 11 in one embodiment battery 78 was set with 50 millivolts.
12Likewise, comparator 74 is a conventional 13 negative comparator wi.th its negative terminal tied 14 to terminal ~4 and its positive terminal tied to ground via negative reference batt:ery 80. Negative 16 reference battery 80 stores a negative voltage, 17 the value of which is equivalent t:o the maximum lB negative swing which.can he experienced by malfunction 19 detection means 26. Whenever the swing in th~ negative direction at malfunction detection means 26 exceeds 21. the value on negative reference battery 80,.an error .
22signal is outputted on terminal 82. The signals ~:

23 on terminal 76 and terminal 82 are ~ed into OR circuit 24 means 50 which in turn outputs a signal on terminal 25 40 indicative of a malfunction.

26. Referring now to FIGURE 3, malfunction 27 detection circuitry B4 of malfunction detector 36 28 is shown~ As was mentioned previously, in some 29 operation ~for exampler when amplifier 16 is used to control the drive motor of a library system~, 1 it is advan~ageous to operate the amplifier in a 2 saturation state during peak acceleration periods.
3 This can be done at the expense of reduced sensitivity 4 to malfunct~on detection. To overcome this lack of-sensitivity, malfunction detection circuitry 6 84 hereinafter called gain switch stage 84, may 7 be incorporated as shown in FIGURE 3. Essentially, 8 gain switch stage 84 monitors the error signal on 9 terminal 38 and the input signal on terminal 42.
As long as the input signal on terminal 38 is below 11 a predetermined level, amplifier 16 should not be 12 in saturation, and buffer amplifier 86 will provide 13 a relatively high gain for the ma:Lfunction signal . . .
14 on terminal 38. Therefore, for relatively low and moderate signal levels, a high degree of sensitivity 16 ma~ ~e maintained. At higher input signal levels;
17 i.e., signal levels which drive amplifier 16 into 18 sat~uration, the gain switch s age will reduce the 19 gain of buffer amplifier 86. This has the effect of widening the sample window or reference band ~1 to handle the overdrive signal. However, the reduced 22 malfunction detection sensitivity will only be felt 2~ at high signal levels. By positioning the gain ~4 switch stage prior to filter means 46, switch transient will not cause false signals.
26 Essentially, gain switch stage 84 is comprised 27 of buffer amplifier 86, a single pole double throw 28 gain switch means B8 and a~ magnitude threshold circuit 29- ,means 90. Gain switch means 88 can also be a single pole single throw switch. Magnitude threshold circuit

-15- -.

l means 90 is a compare circuitxy which is substantially 2 analogous to compare means 48. It stores the minimum 3 or threshold value of the input signal which wîll 4 drive amplif.ier 16 into saturation. The stored value is then compared with the actual input signal 6 which appears on terminal 42, and whenever the value 7 of the signal on terminal 42 is equal to or greater 8 than the threshold value a signal is outputted 9 .on terminal 92 which switches gain switch means .10 88 to the high terminal and thereby forming a conductive ll path between ground and buffer amplifier 86. Likewise, 12 if the value of the input signal on terminal 42 13 is below the value stored in threshold circuit means 14 90, gain switch means 88 will be in contact with the low terminal and therefore there will be no

16 . conductive path between ground and buffer amplifier

17 86.

18 Bu~fer amplifier 86 is comprised of a l9 positive terminal which is connected to terminal 38 and a negative input which is connected to terminal 21 94. The output of buffer amplifier 86 is fed back .22 to terminàl 94. The output of buffer amplifier 23 . 86 is fed back to terminal 96 via a feedback resistor 24 98. Terminal 96 is connected to gain switch means 88 via lead lO0 through resistor 102 hereinafter ~ .
26 called second resistive means 102. Likewise, terminal 27 96 is tied to terminal 94 via lead 104, while terminal 28 94 is tled to gro-md through resistor 106, here- -2g inafter.called first resistive means 106. The output 1 from buffer amplifier 86 appears on terminal 108.
2 This completes the detailed description of the preferred 3 smbodiment 4 peration As was previously mentioned, the malfunction 6 detector is based on the theory that during normal 7 operation of power ampliier 16, the voltage Es 8 . at malfunction detector means 26 should be ideally 9 zero. However, due to the offset term and output/open loop gain, Es would swing about the zero point.
11 The swing is generally in the area of plus or minus 12 a few millivolts. By monitoring the.swing in voltage 13 Es and comparing the swing with standard reference 14 .voltage, one is abla to pinpoint malfunction in amplifier 16 over its entire operational range.
.16 In operation, an input signal Ein is applied 17 to terminal 32. This signal can be derived from 18 any conventional driving means, for example, a voltage

-19 sourceO The inP~t signal is then registered at malfunction detection means 26. Simultaneouslyr any signal 21 appearing at the output of amplifier 16 i5 fed back 22 through feedback terminal 28 to malfunction detection 23 means 26. Malfunction detection means 26 outputs . 24 an error signal which is the weigh~ed difference in ~he sum of the input signal on terminal 32 and 26 the signal which appears on terminal 28. lf the 27 error signal Es is within a few millivolts (i.e., 28 ~ positive or negative) of ground, the amplifier is 29 operating normally and will not be disturbed.

-17- , . --' 5~

1 The permissible voltage swing of Es is 2 stored on positive reference battery 78 and negative 3 reference battery 80. The voltage Es from malfunction 4 detection means 26 is constantly compared with the reference voltages stored on positive reference 6 battery 78 and negative reference battery 80. Whenever 7 Es exceeds either of these voltages, an error signal 8 will appear on terminal 40. The signal is used 9 to close down the power supply and/or to signal the microprocessor.
11 In order to pxevent false alarm due to 12 slew rate or high frequency triggering of the amplifier, 13 filtering means 46 is incorporated so that the compare 14 means will not be activated until the system is running at steady state. The time constant of the 16 filter is substantially equivalent to the dominant 17 pole of the amplifier.
18 ~s was previously stated, in some operation 19 it is advantageous for one to operate the amplifier when it is saturated. In order to achieve this 21 goal, gain switch stage 84 is incorporated. In 22 operation, the error signal which appears on terminal 23 38 is fed ~nto buffer amplifier 86 Simultaneously, 24 the input signal Ein is fed into magnitude threshold circuit means 90. Magnitude threshold circuit means -26 90 will compare the input signal on terminal 42 27 with a standard predetermined signal. As long as 28 the input signal is not greater than or equal to -29 the predetermined threshold value, gain switch means . .

-18- ;

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1 88 will attach to the high terminal which results 2 in a closed circuit. With the closed circuit configuration, 3 the gain of buffer amplifier 86 is one plus the 4 feedback resistor 98 divided by the parallel combination o resistive means 106 and resistive means 102:
6 1 ~ R98/ R106 R102 R106 ~ R102 7 Where R98 = feedback resistor 98 8 . and R106 = resistive means 106 9 and R102 resistive means 102 Buffer ampli.fier 86 enables the malfunction detPctor 11 to maintain its sensitivity as long as amplifier 12 16 is not driven into saturation.
13 If the input signal (Ein~ exceeds or is 14 equal to the stored predetermined signal of magnitude threshold circuit 90, the gain of 'buffer amplifier 16 86 is reducedO Although the malfunction de~ector 17 is le~s sensitive, it will detect any malfunction 18 which will occur while the amplifier is being driven ~9 at a saturated rate. In order to reduce the gain :
of ~uffer amplifier 86, the signal which appears 21 .on terminal 92 will switch gain switch means 88 22 to its low terminal thereby rendering a nonconductive 23 path between ground and buffer amplifier 86. In 24 other words, there ls an open circuit. In the open configuration, the gain of buffer amplifier 86 is 26 reduced. Theoretically, the gain is equivalent 27 ~: 1 + R98~R106 By effectively increasing the ~ -28 resistance in the circuit, the gain of buffer ampli~ier ~9 86 is reduced.

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1 The advantage of the above described invention 2 enables one to monitor a wide band amplifier for 3 malfunction over its entire signal range.
4 By varying the gain of gain switch stage 84 as a function of the input voltage, one can allow 6 the amplifier to be operated in a saturation mode.
7 While the invention has been particularly ~ :
8 shown and described with reference to a preferred 9 embodiment thereof, it will be understood by those skilled in the art that various changes in form -ll and detail may be made therein without departing 12 from the spirit and scope of the invention.

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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for detecting malfunction in a power amplifier comprising:
amplifier means;
control means for producing a controlled signal to drive said amplifier;
malfunction means operably connected to said control means for summing a plurality of signals;
feedback means operably connected for feeding back a signal from the output of the amplifier to the malfunction means;
compare means to compare the signal from the malfunction means with reference signals: and means for outputting an error signal indicative of the amplifier malfunction when the range of reference signals is exceeded.

2. The apparatus as claimed in Claim 1 wherein the amplifier means comprises:
an operational amplifier having two input terminals and one output terminal;
a power booster stage operably connected in series with said operational amplifier.

3. The device as claimed in Claim 1 further including means for desensitizing the compare means so as to prevent the output means from outputting an error signal until the amplifier is running at steady state.

4. The device as claimed in Claim 3 wherein the disable means is a filter.

5. The device as claimed in Claim 4 wherein the filter has a time constant approximately equivalent to the dominant pole of the amplifier.

6. The device as claimed in Claim 4 wherein the filter comprises:
a voltage follower means having an input and an output terminal;
resistor means connected in series with said voltage follower means; and a capacitor means connected in series with said resistor means.

7. The device as claimed in Claim 1 wherein the malfunction means is a summing junction which accepts a plurality of signals and outputs an error signal representative of the amplifier malfunction.

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8. The device as claimed in Claim 1 wherein the feedback means comprises an impedance feedback network.

9. The device as claimed in Claim 1 wherein the compare means comprises:
a first comparator for comparing two signals, said comparator establishing the upper limit of a predetermined range;
a first reference means operably connected to determine the upper limit of the range to be compared;
a second comparator positioned in parallel with the first comparator to establish a lower limit of a predetermined range; and a second reference means operably connected to determine the lower limit of the range to be compared.
.

10. The device as claimed in Claim 1 wherein the output means is an OR circuit.

11. An improved malfunction detector for a reel-to-reel magnetic tape transport having a supply spool and a take-up spool, the improvement comprising:
a first drive means operably connected for driving the take-up spool; and supply spool amplifier means operably connected for controlling said drive means, a second drive means operably connected to drive said amplifier;
malfunction junction operably connected to said amplifier means for detecting malfunction in the amplifier;
feedback means operable connected to tie the output of said amplifier means to the malfunction junction;
gain switch means operably connected for limiting the second drive means signal;
filter means connected to the gain switch for desensitizing the malfunction signal;
compare means to compare the malfunction signal with reference signals; and means for outputting an error signal indicative of the amplifier malfunction when the range of reference signal is exceeded.

12. The apparatus as claimed in Claim 11 wherein the gain switch means comprises:
circuit means to determine when a signal is out of a predetermined range;
switch means positioned within the path of said signal to be operative when the signal is out of the predetermined range;
amplification means positioned to receive a plurality of signals; and gain control means operably associated with said amplification means whereby the gain of said amplification means can be controlled by closing or opening the switch means.

13. The device as claimed in Claim 12 wherein the gain control means comprises a plurality of resistors positioned so as to form a plurality of combination wherein one of said combination being a series combination and another of said combination is a series parallel combination.