Method of measuring previously applied torque to a fastener

The amount of previously applied torque to a fastener is measured by analyzing a series of digital torque values generated during a test of the fastener.

TECHNICAL FIELD 
This invention relates to torque measuring systems and, more particularly, 
it involves techniques for sensing the amount of previously applied torque 
to a fastener. 
BACKGROUND ART 
In a variety of manufacturing applications it is often imperative that a 
predetermined amount of torque be applied to a fastener to form a proper 
joint. For example, in automotive applications, bolts must be tightened 
within a certain prescribed range of torque to properly join two parts 
together thereby assuring good reliability of the joint during expected 
use. A relatively simple test has been used in the past to measure 
fastener torque levels. An operator uses a hand torque wrench to engage 
the fastener to be tested. He then uses the wrench to apply more torque to 
the fastener until it finally begins to rotate in the tightening 
direction. Early techniques called for the operator to merely view the 
reading of the wrench torque indicator just prior to the "give" or 
"breakaway" of the fastener as this torque level was thought to be 
generally associated with the amount of torque originally applied to the 
fastener during the normal assembly process. Later improvements of such a 
test included the use of a wrench which would maintain the position of the 
indicator at the maximum torque level experienced. 
Unfortunately, the prior art methods of sensing the applied torque were not 
very precise and the results were not capable of being accurately 
reproduced from operator to operator. The breakaway torque level was hard 
to accurately measure because it was difficult for the operator to 
instantaneously stop applying any more torque as soon as he noticed 
fastener motion. Hence, the torque reading was often too high due to this 
overshooting problem. 
U.S. Pat. No. 4,244,213 and U.S. Pat. No. 4,319,494 to Marcinkiewicz 
(hereby incorporated by reference) disclose dramatic improvements in 
retorque measuring techniques. These patents broadly disclose the concept 
of electronically and automatically detecting the amount of previously 
applied torque to a fastener. In general, electrical circuitry is used to 
automatically detect a change in slope of the torque signal. The torque 
value associated with the occurrence of the slope change is displayed as 
being representative of the amount of torque previously applied to the 
fastener. Preferably, the circuitry is adapted to detect the torque signal 
value associated with a negative valley occurring after the breakaway 
point. This negative valley torque, when it occurs, provides an even 
better indication of the amount of torque applied to the fastener during 
its original tightening process. 
While the above commonly assigned patents certainly advanced the state of 
the art, the particular embodiments disclosed therein for carrying out 
their broad teachings can be even further improved. Spurious peaks or 
spikes in the torque signal are often encountered under true operating 
conditions. These spikes can be generated by things like electrical noise 
but generally they are due to the operator "jerking" the wrench during the 
test instead of smoothly applying the torque to the fastener. 
Unfortunately, the analog circuit approach of the previous patents cannot 
readily filter out those signals. Since their detection schemes look for 
changes in relative torque values these spikes could trigger false 
readings. 
The present invention is directed to solving one or more of these problems. 
DISCLOSURE OF THE INVENTION 
The present invention is broadly directed to a digital torque detection 
scheme centering around the use of a microprocessor to convert an analog 
torque signal into discrete digital sample values which are stored and 
then examined in more detail for given characteristics. During the 
retorque or retightening process, the microprocessor is devoted almost 
exclusively to the task of converting the analog input signal into 
discrete samples. It is not burdened with the chore of making relatively 
sophisticated calculations during the time that the input data is being 
received. 
In a preferred embodiment, only increasing digital sample values are stored 
in a sequential memory thereby conserving memory requirements. If valley 
regions do occur during the retorquing operation, only the negative peak 
valley occurring therein and a limited amount of associated information is 
stored. When the operator notices fastener rotation and ceases applying 
more torque thereto, the digital sample values fall below a given 
threshold and the microprocessor enters into a search routine. 
During the search routine, the microprocessor scans the stored samples 
beginning with the peak and looking backwards to determine if a valley 
region has occurred within a predefined window. If so, the negative peak 
value of the valley occurring within that window is displayed as the 
indication of the amount of previously applied torque to the fastener. In 
a second mode of operation where the valley torque is not desired, the 
value of the digital sample occurring just before the valley region is 
displayed. 
According to a feature of this invention a method is provided to pinpoint 
the exact value of the breakaway torque in those instances where valleys 
do not occur or, if they occur, at unreliable points on the torque curve. 
A digital sample associated with a change in slope of the torque curve 
occurring before the peak is chosen as an initial coarse breakaway point. 
After this coarse breakaway point is found, an exact breakaway value is 
pinpointed by examining the angles of the torque curve associated with 
samples adjacent to the coarse breakaway point. The sample having the 
smallest associated angle or arc tangent on the torque curve is chosen as 
the exact breakaway point and displayed. 
The displayed torque readings are expected to be very accurate using the 
techniques of this invention and minimizes the chances of error which may 
otherwise result from the operator not smoothly pulling on the torque 
wrench. This is because the method of this invention utilizes only the 
portion of the torque curve where the fastener actually begins to move 
until the time that the operator quits supplying more force. Everything 
else is effectively ignored and thus, irregular torque readings not 
containing valid information occurring, for example, during early portions 
of the test will not adversely effect the accuracy of the measurement. 
BRIEF DESCRIPTION OF THE DRAWINGS 
Those skilled in the art will come to appreciate the full range of 
advnatages of various features of this invention by reading the following 
specification and by reference to the drawings in which:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates one example of a torque wrench device suitable for 
incorporating and using the teachings of the present invention. Torque 
wrench 10 includes a handle 12 on which housing 14 is mounted on 
intermediate portions thereof. The interior portion of housing 14 includes 
the components making up the electronic circuitry which will be described 
in detail later on in this specification. An LCD display 16, keyboard 18, 
rotation switch 20 and on/off switch 22 are provided on the top panel of 
housing 14. A shaft 24 attached to an opposite end of handle 12 includes a 
cylindrical head 26 at its end. Head 26 includes suitable strain gauges or 
other transducers therein for sensing the amount of torque applied to a 
fastener by wrench 10. A more detailed description of torque wrench 10 may 
be obtained by reference to U.S. Pat. No. 4,124,016 to Lehoczky et al 
issued Nov. 14, 1978, which is hereby incorporated by reference. 
Torque wrench 10 is typically used to test the amount of previously applied 
torque to a fastener such as bolts 28. Head 26 of torque wrench 10 
includes a suitable socket in its lower end for receiving the head of one 
of the bolts 28. The wrench is then rotated by the operator in the 
fastening or clamping direction until further rotational movement of bolt 
28 is noted. This is commonly referred to in the industry as the 
"breakaway" of the fastener under test. 
FIG. 2A shows a typical torque level signal curve that may be encountered 
in this type of retorqueing operation. The torque level generally 
increases with applied force until such time as the fastener begins 
further rotational movement. This point shall be referred to as the 
breakaway torque level and is noted by the reference letter B. In many 
fasteners the torque level actually decreases for a short period of time 
even though the operator is still applying force to the fastener. This 
point is labeled with the reference letter V and shall be referred to as 
the valley torque. As set forth in the above referenced patents to 
Marcinkiewicz, valley torque V provides a very close approximation of the 
amount of torque previously applied to the fastener. In some instances, 
however, the particular fastener under test does not develop a torque 
curve with a well defined valley. Instead, the slope of the torque curve 
merely changes as shown in the curve of FIG. 2B. The torque level will 
then increase to some peak P until the operator ceases to apply further 
force to the wrench. 
According to the teachings of the present invention, the valley torque 
value is automatically and precisely identified or, if no valley occurs, 
the breakaway torque level is identified and displayed. The latter, while 
not being quite as accurate as the valley torque level, still does provide 
a close approximation of the amount of torque previously applied to the 
fastener under test. 
Unfortunately, the input torque curve often encounters highly fluctuating 
torque readings which often occur during the early phases of the 
retorqueing process as shown in FIG. 2A. As noted above, these 
fluctuations can be caused by electrical noise or by operator error in not 
smoothly applying force to the fastener under test. As will appear later 
herein the present invention provides the capability of precisely 
detecting the valley or breakaway torque levels in spite of these 
occurrences. 
Turning then to FIG. 3 there is disclosed a block diagram of the major 
functional components of the hardware for carrying out the method of the 
present invention. The analog input torque signal is supplied over line 40 
to one input of a comparator network 42. The analog torque signal is 
representative of the amount of torque applied to the fastener. Typically, 
strain gauges in torque wrench head 26 are configured in a Wheatstone 
Bridge circuit whose output forms the analog torque signal. 
The system employs a microprocessor 44 which forms the heart of a 
microcomputer system. Microprocessor 44 has an output which is connected 
to a digital to analog converter 46 whose output is coupled back to 
another input of comparator 42. Under the control of a program within 
program storage memory 48 the microprocessor 44 uses a reiterative process 
to generate discrete digital samples from the analog input signal. The 
microprocessor 44 converts the output signal from comparator 42 into a 
binary number which is, in turn, converted back to an analog signal by way 
of D/A converter 46. The analog output of converter 46 is compared to the 
torque signal and fed back to the input of microprocessor 44. This 
interactive process is repeated until a binary number is found which is 
equivalent to the analog torque signal. 
As each new digital sample value is generated it is compared with the 
previously generated digital sample. If it is greater than the earlier 
sample it is stored in a sequential memory 50 on a first in/first out 
(FIFO) basis. Accordingly, only progressively increasing digital sample 
values will be stored in memory 50. If a valley is detected, a flag is 
placed in the next memory 50 location. All of the digital sample values 
associated with the valleys or after the peak is reached are not stored in 
memory 50 as illustrated by the shaded portions in FIG. 2A. Instead, just 
pertinent information such as the negative peak torque sample value for 
each valley is stored. This valley information is stored in valley torque 
memory 52. 
This sample and storing process continues throughout the test until the 
operator stops pulling on the wrench 10 and the sample values become less 
than a given threshold level. During the retorquing operation the largest 
digital sample value is stored in a peak register 54. When the retorquing 
operation is completed, the contents of the peak register 54 has a value 
associated with the point "P" on the torque curves of FIGS. 2 (A-C). There 
are several advantages to this sample and storing process. First, is that 
a substantial savings in memory space is obtained. Secondly, and perhaps 
more importantly, fluctuations of the torque curve due to mishandling of 
the wrench by the operator will not significantly effect the accuracy of 
the system's ability to accurately determine the breakaway or valley 
torque level. 
The next broad step is for the microprocessor 44 to examine or search the 
data sample values stored within sequential memory 50. A subset or window 
of data samples within the sequential memory is defined. This window is 
chosen to be wide enough to encompass the expected breakaway and valley 
torque values but should not be any larger than necessary. In the 
preferred embodiment, this window is chosen to include those digital 
samples having values less than 99% and greater than 80% of the peak 
value. 
The microprocessor 44 then scans the samples in sequential memory 50 
beginning with the peak value and progressing to decreasing values, i.e. 
looking backward, to determine if a valley region has occurred within the 
window. If so, the negative peak value stored in valley torque memory 52 
is generally displayed on display 16 as the indication of the amount of 
previously applied torque to the fastener. Alternatively, or in addition 
to display 16, there may be provided a printer for generating a hard copy 
of the test results. The system utilizes buzzer 58 to alert the operator 
to various conditions. 
If a recognizable valley has occurred within the window but the wrench 10 
has been placed in a breakaway only mode (as will be explained) then 
microprocessor 44 will choose the digital data sample occurring just 
before the valley to be displayed on display 16. In FIG. 2A this point is 
labeled "B". A more difficult task is the determination of the breakaway 
point where no discernible valley has occurred as shown in FIG. 2B. The 
detection of the breakaway point under these circumstances will be 
described in detail later herein. The K slope register 56 will be used in 
performing this calculation. 
FIGS. 4(A-B) are electrical schematic of the components making up the 
system of the preferred embodiment. Microprocessor 44 is an eight bit 
microprocessor such as the Motorola MC146805. As known in the art, 
microprocessor 44 includes various input/output ports for receiving and 
sending information. Among the inputs to microprocessor 44 are the 
switches associated with keyboard 18. Keyboard 18 allows the user to 
select various modes of operation and to enter control data values. For 
example, one mode that can be selected will cause the system to detect the 
absolute peak torque (peak mode) that is applied to the fastener under 
test. Another modes of operation adapt the system to track or display the 
instantaneous torque value. Of particular concern to the present invention 
is the retorque mode to detect breakaway torque only or in a second 
retorque mode where the valley torque is displayed if one occurred during 
the test and, if not, then to display the breakaway valve. 
The operator can program in a threshold torque value and a KSLOPE value 
that are selected for the particular fastener characteristics under test. 
As will become apparent later herein, the threshold torque value is the 
value above which the microprocessor will generate digital samples from 
the analog torque signal. Normally, the threshold is set at a sufficiently 
high level that extraneous input signals generated during set up are 
effectively ignored. The KSLOPE value is an optional parameter which may 
be used to modify the reference slopes used to identify the general or 
coarse breakaway point. Normally, it is set to one. The importance of this 
KSLOPE value will become apparent later herein. Suffice it to say that the 
user has a considerable degree of flexibility in defining the particular 
parameters of the test to be performed. This flexibility is especially 
advantageous due to the fact that the same torque wrench and detection 
system may be used for a wide variety of different fasteners, each having 
their own particular tightening characteristics. 
The output from the strain gauge bridge or analog input signal is sensed by 
a differential amplifier 62 whose inputs are coupled to the two outputs of 
the bridge. The output of differential amplifier 62 thus is a voltage 
whose absolute magnitude is proportional to the amount of torque applied 
to the fastener. The output of differential amplifier 62 is connected to 
the noninverting input of comparator 42. The inverting input of comparator 
42 is coupled to the output of digital to analog converter 46. The output 
of comparator 42 will either be a logical one or zero depending upon the 
relationship between the voltage values at its inputs. As long as the 
analog torque signal on the noninverting input is greater than that 
supplied by D/A converter 46 to the inverting input, microprocessor 44 
will see a logical 1 at its input. As will be described in connection with 
the conversion routine, the microprocessor generates a binary number and 
sends this number to the input of D/A converter 46. D/A converter 46 is a 
CMOS binary multiplying digital to analog converter using conventional 
ladder switching techniques to effect the conversion process. In this 
particular embodiment D/A converter 46 utilizes a DAC1232 component 64 
made by National Semiconductor. The output of component 64 is coupled to 
an op amp 66 in the manner suggested by the component manufacturer. Op amp 
66 serves as an inverting amplifier whose output has an absolute magnitude 
proportional to the digital value at the input to D/A converter 46. 
Other inputs to microprocessor 44 may include circuitry generally 
designated by the numerals 68 and 70 for communicating with an optional 
printer. The circuitry 68 provides outputs to the printer whereas 
circuitry 70 accepts acknowledgement signals from the printer. 
Circuitry 74 operates as a calibration circuit. When the system enters the 
calibration mode the relay in the circuit activates the switch which, in 
turn, couples the precision calibration resistor to amplifier 62 so that 
its output is equivalent to a full scale reading. Suitable calibration 
techniques may be then used to calibrate the system. The oscillator 
circuitry 76 generates the master clock signal for driving microprocessor 
44 in the manner known in the art. Suitable circuitry for driving buzzer 
60 is also connected to microprocessor 44. 
The output of microprocessor 44 is connected to external memory devices and 
display 16 as well as to the D/A converter 46. The memory devices include 
a programmable read only memory 78 which contains the operating program 
for the microprocessor 44 and a random access memory (RAM) 80. Display 16 
includes a display driver component 84 for controlling the operation of a 
multidigit liquid crystal display (LCD) 86. The transfer of data within 
the system including the reading and writing of the memories are carried 
out in a manner known in the art and may include such devices as address 
buffer 88 and a binary to BCD decoder 90 serving as a chip selector. 
Selected sections of RAM memory 80 are used as the sequential memory 50, 
valley torque memory 52, and the various registers 54-59. Those skilled in 
the art will appreciate that the purpose of registers 54-59 is to 
temporarily store data and thus, the registers may be made up of 
individual storage devices or, as in the preferred embodiment, dedicated 
locations within a larger RAM memory. In fact, the internal memory (not 
shown) in microprocessor 44 may be used in some instances. 
With additional reference to the flow chart of FIGS. 5 (A-J), the operation 
of the system of this invention will be described. When the user chooses 
either of the retorque modes of operation the microprocessor is instructed 
by the program shown in FIGS. 5 (A-J). Initially, all of the counters, 
registers and flags pertinent to this routine are cleared as illustrated 
in steps 102-104 (FIG. 5(A)). As the operator uses wrench 10 to apply 
torque to the fastener under test the analog torque signal is converted 
into digital values by way of the conversion routine shown in FIGS. 5G-J. 
The A/D conversion routine is entered by way of a software interrupt (SWI) 
120 which occurs about once every two milliseconds to generate a digital 
sample with a value corresponding to the analog torque signal value 
occurring at the time the sample is taken. The microprocessor is designed 
to convert the analog signal into a precision twelve bit data value, even 
though the microcomputer system employs conventional eight bit processing 
techniques. FIGS. 5H shows the steps used to generate the first eight bits 
of the digital sample value. Briefly, the most significant eight bits of 
the previous value which was stored in memory is fetched and fed to the 
input of D/A converter 46. After waiting about 2 .mu.sec for the output of 
the D/A to be generated, the microprocessor determines whether that signal 
is greater or less than the analog torque signal. Depending on the outcome 
of those tests the microprocessor increases or decreases the value of the 
most significant eight bits of the sample value until approximate matching 
occurs. Then in FIG. 5J the microprocessor uses a successive approximation 
technqiue to set the lower four bits to the precise value. The upper eight 
bits are saved for the next conversion routine. 
Returning to FIG. 5A, each new torque reading is tested in step 152 to 
determine whether it is greater than the user programmed threshold value. 
Until that time, the system will continue in loop 1 merely displaying the 
generated torque value. Once the value is greater than the threshold, a 
flag is set (step 154) and the system begins saving selected sample 
values. 
Turning then to FIG. 5B, the microprocessor 44 will sample the analog input 
signal at about a two millisecond rate and will store the digital samples 
in sequential memory 50 with the new torque sample values replacing the 
older values. Peak register 54 is used to store the highest digital sample 
value generated during the test. As represented by step 160, each 
successively generated digital sample is compared with the contents of the 
peak register 54. If the new digital sample value is greater than the 
contents of the peak register 54 a peak flag is set and the new torque 
value is stored in register 54 along with its location in PKLOC. (steps 
162-164). Assuming that the samples not associated with a valley (steps 
166-169) the contents of the peak register 54 is also stored in sequential 
memory 50 and the memory pointer, i.e. address register, is decremented 
ready to receive the new sample value (steps 170-172). The program now 
jumps (step 182) back to the A to D conversion routine via the software 
interrupt 120 of FIG. 5A to thereby generate the next sample. 
If the next sample is less than the peak register 54 and the peak flag is 
set (step 184, FIG. 5B) this is considered as the start of a valley 
region. At this point, the peak flag is cleared (step 186). The next step 
188 is to store the smaller sample value in negative peak register 55. A 
valley counter or timer 57 associated with memory 52 is cleared and a 
debounce register 59 is set or loaded with the value of 12. The debounce 
register effectively acts as a filter allowing subsequent digital sample 
values below the peak to continue to be stored into sequential memory 50 
until 12 consecutive torque values are found that are less than the peak. 
Electrical noise or other factors may create a small number of decreasing 
sample values and, thus, the debounce operation and valley timer are used 
to disregard such occurrences as true valleys. 
In step 190 a test is made to determine whether the next torque value (less 
than the peak) is less than the previous negative peak stored in negative 
peak register 55. If so, that torque replaces the previous contents of 
negative peak register 55. In either event, the debounce register is 
decremented in step 194 and a test is made in step 196 to determine 
whether the contents of the debounce register is less than zero. If the 
debounce register has not been fully decremented the torque values are 
also stored in sequential memory 50 (step 170). However, once the debounce 
register has been decremented to zero then the digital samples less than 
the peak will not be stored in sequential memory 50. Valley timer 57 is 
incremented and the debounce register is cleared (step 198). The purpose 
of the valley time counter is to determine the length of time that the 
torque curve is in a valley region; i.e. samples having values less than 
the previously generated peak value. 
Once a true valley region has been detected no further samples are stored 
in sequential memory 50 until the value of a subsequent sample exceeds the 
value stored in peak register 54. Only the least positive digital sample 
occurring during a valley is stored in the negative peak register 55. The 
valley timer 57 is also incremented during this time. 
Once a digital sample has a value greater than the stored value in peak 
register 54, the end of the valley has been reached and the test 160 will 
become true again. The program progresses to step 166 which determines 
whether a valley has previously occurred by checking the contents of the 
debounce register 59. If it is zero then the valley parameters are stored 
in valley torque memory 52 as represented by step 168. The valley 
parameters are the length of time in the valley provided by the contents 
of timer 57, and the negative peak associated with that valley provided by 
negative peak register 55. A valley memory pointer is decremented ready to 
save new valley parameters if they occur during the test. In step 169 a 
valley detected flag is stored in sequential memory 50 to indicate that a 
valley occurred before the next larger sample is stored therein (step 
170). 
It will be appreciated that this method of storing torque values generally 
allows only increasing values to be stored in sequential memory while only 
a limited amount of compact data is stored for each of the valley regions. 
This technique operates to conserve memory space and thus decrease costs. 
In this embodiment, memory 50 has a capability of storing 256 different 
torque values. When the memory becomes half full (step 176) it is assumed 
that the operator is really performing a retorquing operation on the 
fastener and that sufficient data has been obtained to determine breakaway 
or valley torque values. Accordingly, a search flag is set in step 178 
signifying that the system is ready to perform the breakaway or valley 
search routine once the retorquing operation is finished. Returning to 
FIG. 5A, test 152 will become false once the operator stops applying any 
more force to the wrench 10 and the torque values fall below the threshold 
level. The routine then branches up through loop 106 and if the search 
flag has been set (step 108) then the program jumps to the search routine 
shown in FIGS. 5(C-F). 
Before describing the search routine it may be advisable to summarize the 
contents of the various memories and registers. Sequential memory 50 will 
contain progressively increasing digital sample values that were generated 
during the retorquing operation. Valley torque memory 52 will contain the 
smallest or negative peak sample value occurring for each valley as well 
as the length of time that the valley region occurred. Peak register 54 
will contain the largest digital sample value that was generated during 
the operation. 
Turning then to FIG. 5C, the search routine will initialize itself (step 
202) by calculating 99% of the peak value and 80% of the peak value, with 
the program storing these parameters in suitable working registers. A 
slope is then calculated by taking the slope of a line containing the peak 
and 80% of the peak value on the torque curve. This slope is shown in FIG. 
2A as KSLOPE and is stored in the Kslope register 56 (FIG. 3). This slope 
may be multiplied by an optional fraction parameter which may be entered 
by the user via keyboard 18 although it is normally set to 1. 
The next step is to begin to look for a valley region if a recognizable 
valley has occurred during the retorquing operation. In step 202 pertinent 
flags are cleared and a sequential pointer is loaded with an address 
associated with a digital sample occurring just prior to the peak. In this 
example, the address is associated with the digital sample in sequential 
memory 50 occurring ten samples before the peak. Now the entire sequential 
memory 50 is scanned backwards starting from this location until a valid 
valley region is found which is less than 99% of the peak and greater than 
80% of the peak, these two values defining a window where accurate values 
of the breakaway or valley torque level will normally be found. 
In step 204 the contents of the sequential memory location addressed by the 
pointer will be read. It will be remembered that step 169 in the sample 
generation and storage routine (FIGS. 5A and B) will have placed a flag in 
memory 50 at the location where a valley has occurred. If this flag is 
detected the microprocessor 44 increments a pointer for the valley memory 
52 and reads the parameters stored at that address locations. In 
particular, steps 208 and 210 determine whether the negative valley peak 
value is within the 99-80% window. If so, a valley flag is set and the 
parameters are transferred to a working register referred to as VALLOC in 
step 212. Test 214 causes this process to loop back and search for all of 
the valleys that may have occurred. However, only those valleys having 
negative peak values within the window will be considered as valid and, 
more particularly, the first valley occurring prior to the peak will be 
selected as the most accurate even if two or more negative peak values 
satisfy the above criteria. 
In summary, the routine just described is used to determine the first 
valley, if any, that has occurred just prior to the peak of the torque 
curve. 
After this valley detection subroutine is completed, microprocessor 44 then 
will search for and calculate a breakaway torque. Turning then to FIG. 5D, 
the sequential memory 50 pointer is loaded again with the address location 
(PKLOC) of the peak sample value. (step 216) The addressed torque value is 
read and compared with the 80% peak criteria (steps 218 and 220). If the 
latter test is true, a slope is calculated by taking the difference of two 
torque values ten samples apart. This is performed in step 222 where the 
torque of the present sample is subtracted from the torque associated with 
a sample located at a tenth earlier address in sequential address memory 
50. This slope is then compared with the previously calculated KSLOPE in 
step 226. When four slopes are found that are less than KSLOPE a flag 1 is 
set (steps 228-232). After flag 1 is set, microprocessor 44 continues to 
scan sequential memory 50 until four consecutive segments are found with 
slopes greater than KSLOPE. Briefly, this is accomplished by step 236 
which compares each segment with K slope. The largest torque value 
associated with the first segment having a slope greater than KSLOPE is 
defined as the coarse breakaway torque value B.sub.c and its location is 
stored in a register referred to as BRKLOC. (steps 238-230) Step 242 
increments the debounce register 59 and determines whether it has reached 
the number four. Once four consecutive segments have slopes less than 
KSLOPE test 242 becomes true and a breakaway flag is set (step 244). 
The purpose of the coarse breakaway routine described in connection with 
FIG. 5D is to select from sequential memory 50 an approximate value of the 
torque associated with breakaway. The routine operates to identify that 
point on the torque curve generally associated with a knee where the curve 
begins to undergo a change in slope. A generalized approximation of this 
point is provided by the routine described above by first checking for a 
consecutive number of slopes less than the predefined KSLOPE and then 
selecting the torque value associated with a slope segment transitioning 
to slopes greater than KSLOPE. Remember, that in this embodiment the 
memory is scanned backwards. Under some instances the 99-80% KSLOPE 
criteria will not result in detection of the course breakaway point having 
a value greater than 80% of the peak as required by test 220. In such 
instances, the program branches to step 246 of FIG. 5E. If the memory 
pointer has read a torque value below 80% of the peak before the breakaway 
flag is set, the KSLOPE value is either incremented or decremented, 
depending upon the status of the flag 1 (steps 248-256). The coarse 
breakaway routine of FIG. 5D is then repeated via step 260 until the 
KSLOPE value has been incremented or decremented beyond acceptable limits 
as determined by steps 252 and 256, respectively. In such case an error 
signal is provided as represented by step 258 and the retorque program is 
reinitialized from the beginning. This may happen when there is no clear 
breakaway on the torque curve or if the torque increases linearly from 
threshold to peak. The error signal thus indicates to the operator that he 
should run another test on the fastener. 
Under normal conditions the breakaway flag will be set because the 
microprocessor is capable of detecting a change in slope of the torque 
curve. As noted in the background portion of this invention, the torque 
curves for some fasteners will exhibit recognizable valleys as shown in 
FIG. 2A whereas others will generate a torque curve similar to that shown 
in FIG. 2B with no discernible valley. In those instances where a valley 
does occur, the most accurate breakaway point occurs just before the 
valley. Sometimes, however, the generated torque curve will have a valley 
which occurs before the detected course breakaway point. Such instances 
will result in an erroneous measurement and may be due to such things as 
the operator jerking the torque wrench, electrical noise, or various 
anomalies in the particular fastener being tested. Thus, the selection of 
the actual torque value to be displayed as an indication of the previously 
applied torque to the fastener must be carefully determined. 
In this embodiment, the coarse breakaway torque is checked to determine 
whether it is greater than 80% of the peak torque value. (step 264) If so, 
and if the valley flag is set indicating that a recognizable valley has 
been generated, then the microprocessor determines whether the valley is 
closer to the peak than the coarse breakaway point as represented by the 
test of step 268. This may be accomplished by comparing the relative 
address locations of the valley detected flag and the coarse breakaway 
sample in memory 50. If the valley is closer to the peak than the coarse 
breakaway point, as it should be, then the valley location (VALLOC) is 
obtained and a check is made to determine what retorque mode has been 
programmed by the operator (steps 270-272) If the valley mode has been 
set, then the negative peak valley torque is displayed as the indication 
of previously applied torque to the fastener (steps 274, 276). This value 
would correspond to the point labeled V in FIG. 2A. If the valley mode is 
not set then the point labeled B in FIG. 2A is displayed. This point 
corresponds with the torque value in sequential memory 50 which is located 
just before the flag which identified the valley. Conveniently, this may 
be accomplished by utilizing VALLOC to address the location in sequential 
memory 50 containing the torque value occurring must before the valley. 
If there has been no discernible valley or if the valley occurs before the 
coarse breakaway point, then a program branches to the routine shown in 
FIG. 5F. Steps 280-288 may be optionally used to define a new coarse 
breakaway torque value by calculating the arc tangent of every tenth point 
from peak to the original coarse breakaway location. The location of the 
same associated with the minimum arc tangent is saved in a storage 
location labeled BRKLOC. Regardless of whether the coarse breakaway point 
B.sub.c is recalculated via steps 280-288 or the original point is 
utilized, the next major step is to pinpoint the exact or final breakaway 
torque. With additional reference to FIG. 2C, the final breakaway value 
calculation will be described. 
FIG. 2C is an enlarged portion of a typical breakaway curve adjacent to the 
chosen coarse breakaway point B.sub.c. Microprocessor 44 operates to 
calculate the arc tangent for every point on the torque curve within a 
given range of the coarse breakaway point. In this embodiment, arc tangent 
calculations are made for each digital sample within the range of ten from 
the sample associated with the coarse breakaway sample. In FIG. 2C, the 
outer range of this arc tangent process is defined by the points B.sub.c 
-10 and B.sub.c +10. The calculation of the arc tangent for each of these 
points may be accomplished in a variety of manners. In this embodiment, 
microprocessor 44 operates to calculate the angle between segments on 
opposite sides of the point. Ten sample wide segments are chosen in this 
particular example and are illustrated in dotted lines for point B.sub.c 
+10 in FIG. 2C. For point B.sub.c +2, these segments are shown in solid 
lines. Briefly, the arc tangent routine calculates the angles .phi.1 and 
.phi.2 for each of the segments and then takes the difference between 
these angles. The difference is subtracted from 180 degrees to determine 
the angle defined by segments on either side of the selected point. This 
process is analogous to taking the second derivative of each of the 
points. 
In FIG. 5F steps 292-298 are utilized to determine the minimum arc tangent 
calculated for each of the points on the curve. The torque of the digital 
sample associated with the point having the minimum arc tangent is chosen 
as the final breakaway value. In FIG. 2C this torque corresponds to the 
point B.sub.c +2. 
As noted before, under some circumstances the valley torque or negative 
peak will be displayed whereas in other circumstances the final breakaway 
value will be displayed in step 302. If the valley torque is displayed 
buzzer 58 is activated to beep twice whereas it will operate to beep once 
if the breakaway value is displayed (steps 304.308). Then, the program 
returns back to loop 1 and is ready for measurement of another fastener 
during another subsequent test. 
The method of determining the so called "retorque" value as just described 
is designed to optimize the accuracy of the measurement while at the same 
time minimizing manufacturing costs. It should be realized that the torque 
curves shown in the drawings are idealized and that, in actual use, 
individual digital sample values may vary quite dramatically from the 
normal progression of the average spectrum of values. Such abberations are 
to be expected when it is realized that the torque wrench is designed to 
be used in an industrial environment by a human operator. Thus, it is 
expected that a certain amount of electrical noise and operator induced 
error can be expected. The present invention takes great pains to 
eliminate as many of these noninformative data samples from effecting the 
accuracy of the ultimate measurement. Of course, the operator error can be 
minimized by using automated machines to apply the retorquing forces to 
the fasteners under test and such a modification falls within the spirit 
of the invention. However, in many quality control applications it is 
desirable to provide a manually operated device as shown in the preferred 
embodiment. 
Therefore, while this invention has been described in connection with 
particular examples thereof, no limitation is intended thereby except as 
defined in the appended claims. For example, instead of a torque verses 
time curve, torque-angle calculations for the digital samples may be 
employed if an angle decoder device is used to detect the rotation of the 
wrench. Various other modifications will become apparent to one skilled in 
the art upon a study of the drawings and specification as well as the 
claims.