Stress monitoring system

The stress monitoring system includes a transducer assembly coupled to the member to be monitored for providing an electrical analog signal having an amplitude proportional to the stress forces imposed on the member and a polarity corresponding to whether the member is in tension or compression, a panel meter responsive to the analog signal for recording the level of stress imposed on the member and automatic zero circuitry for nulling out any reading in the panel meter under no load conditions.

This invention relates to a stress monitoring system with automatic zero 
control. 
The ability to monitor and control stress in a structural member under 
tension or compression is of critical importance to die casters, injection 
molders and other users of highly stressed mechanical machinery. 
Maintenance of proper stress levels can eliminate costly downtime due to 
breakage of an overstressed member and should materially contribute toward 
increased machine longevity by decreasing unnecessary component wear. This 
is accomplished in accordance with the present invention by providing 
means to monitor the level of stress in the structural member under load 
such that it can be safely loaded to its optimum level required for a 
particular job. In a die casting machine, for example, employing a 
plurality of tie bars each may be separately monitored in accordance with 
the present invention to prevent unequal loading thereby minimizing 
problems unique to the particular die casting operation, such as, flash 
and hazardous hot metal expulsion. 
Other important features of the stress monitoring system of the present 
invention include; the ability to automatically cancel out potential 
errors in the stress reading due to thermal expansion of the stressed 
structural member, the elimination of the need for periodic recalibration 
of the system and the simplicity of installation requiring only a minimum 
of skill and time without the need for a precise adjustment of the zero 
position under no lead, i.e., when the die is open. 
Accordingly, it is the principal object of the present invention to provide 
a system for monitoring stress in a structural member with automatic zero 
control. 
Other objects and advantages of the present invention will become apparent 
from the following detailed description of the invention when read in 
conjunction with the accompanying drawing illustrating schematically the 
system of the present invention.

Referring now to the single FIGURE schematic drawing of the system embodied 
in FIGS. 1A and 1B of the present invention in which a structural member 
10 is partially shown extending from the foundation or housing 12 of a 
machine (not shown). The structural member 10 may represent a single tie 
bar in a die casting machine which is intended to impose a substantial 
compressive or tensile load upon the structural member 10 in the normal 
operation of the machine. The end 14 of the structural member 10 is 
fixedly held in abutment against the housing 12 of the machine by means of 
a nut 16. 
A drilling 18 is made through the end 16 of the structural member 10 a 
distance "L" beyond the abutting face 19 of the nut 16 and the housing 12. 
Stress in the structural member 10 is measured by sensing axial deviations 
from the preestablished distance "L". To sense such deviations a long 
probe 22 is inserted into the drilling 18 up to the drilled end 23. The 
probe 22 extends from a slug 24 which is magnetically coupled to a 
plurality of coils 25, 26 and 27 wound about the slug 24 to form a linear 
variable differential transformer 20. The linear variable differential 
transformer 20 is a commercially available transducer for sensing 
differential displacement of the core material, i.e., the slug 24 relative 
to the wound coils 25 and 27. In the present instant the slug 24 is 
connected to the probe 22 which, in turn, is biased by a spring 28 for 
urging the probe 22 against the drilled end 23 of the structural member 
10. The spring 28 is adapted to be held in compression in a housing (not 
shown) in common with the probe 22 and the transducer slug 24. An end 
plate 29 is affixed to the end 14 of the structural member 10 for mounting 
the common housing (not shown) for the linear variable differential 
transformer elements and the spring 28 to the structural member 10. 
The coils 25, 26 and 27 of the linear variable differential transformer 20 
are connected to a hybrid integrated detection circuit 30. The detection 
circuit 30 generates a transducer analog output voltage 40 corresponding 
to the position of the transducer slug 24 relative to the wound position 
of the coils 25 and 27. Any stress encountered in the structural member 10 
will cause a longitudinal movement of the probe 22 and a corresponding 
displacement of the transducer slug 24. The detection circuit 30 comprises 
an oscillator 32 for generating, for example, a 10 KH.sub.z alternating 
signal which is imposed on the center coil 26 surrounding the transducer 
slug 24. The coils 25 and 27 are coupled through diode detector circuits 
D1, C1 and D2, C2 to the positive and negative terminals 34 and 36 of an 
operational amplifier 38. The operational amplifier 38 generates the 
transducer output voltage 40 which is a DC signal representing the 
algebraic difference between the signals induced in the coils 25 and 27. 
The induced signal in coils 25 and 27 should be equal and opposite for a 
perfectly centered slug 24 relative to the coils 25 and 27. 
The output 40 of the operational amplifier 38 is connected to the positive 
terminal 41 of a comparator 42 having an output 43 which is applied to a 
conventional digital panel meter 44. The digital panel meter 44 provides 
an LED display of, for example, 3 digits corresponding to the output 
voltage 43 of the comparator 42. The panel meter 44 can be calibrated to 
read out in any desired dimensional units. Unless the slug 24 is precisely 
positioned to provide a balanced output under a no load machine condition 
an output voltage 40 will be generated of either positive or negative 
polarity which will register and be recorded in the panel meter 44 as a 
stress reading. As will be discussed in more detail hereafter the present 
system provides for automatic zero compensation to eliminate the need for 
field calibration of the transducer 20. 
The digital panel meter 44 has a seven line output with four lines 50, 51, 
52 and 53 providing a binary coded decimal "BCD" four bit output 
corresponding to the number recorded by the meter 44 and another 3 lines 
54, 55 and 56 for use as a 3 line multiplex strobe. The four output BCD 
lines 50, 51, 52 and 53 are connected to three storage latches 47, 48, and 
49 respectively. The three output multiplex lines 54, 55 and 56 are used 
as clock signals for the storage latches 47, 48, and 49 in order to store 
the number recorded in the panel meter 44 in the storage latches 47, 48, 
and 49 respectively. The number recorded in the panel meter 44 is stored 
most significant digit first in the storage latch 47, followed by storage 
of the second most significant number in the storage latch 48 and, in 
turn, followed by storage of the least significant number in storage latch 
49. Each of the storage latches 47, 48, and 49 are commercially available 
and represent a multiple number of flip-flops for storing each four bit 
BCD digit on lines 50, 51, 52, and 53. Accordingly, the three storage 
latches 50, 51, and 52 provide a 12 line output with each four lines 
representing one of the three BCD digits corresponding to the recorded 
number in the panel meter 44. The four line output 55, 56, 57, and 58 of 
the storage latch 47 is connected to a polarity detector 60 which may 
represent simply an And gate for providing a logical high or low signal 62 
corresponding to a positive or negative DC output signal 40. The output 52 
of the polarity detector 60 is applied to an "up/down" counter 64 with the 
direction of count being controlled by the logical state of the output 
signal 62. The up/down counter 64 is itself conventional and counts 
respectively in synchronizing with each input pulse 65 delivered from the 
Nand gate 66. A clock generator 68 is connected to the input 69 of Nand 
gate 66. The other input 70 is connected to a manually operated switch SW1 
which when closed provides the automatic zero compensation of the present 
invention as will hereafter be explained in greater detail. 
The output of the up/down counter 64 is applied to a conventional digital 
to analog counter 72 for providing an output analog signal 75 
corresponding to the numerical count of the up/down counter 64. The analog 
signal 75 is applied to the negative terminal of the comparator 42. The 
automatic zero becomes effective when the operator closes switch SW1. The 
up/down proceeds to count the clock pulses generated by the clock 
generator 68 in a direction corresponding to the logical state of the 
output 62 from the polarity detector 64. This varies the analog voltage 
input 75 on the negative terminal of the comparator 42 thereby 
algebraically decreasing the comparator output voltage 43 to the panel 
meter 44. When the panel meter reaches zero any further increase or 
decrease will reverse the polarity detector output to maintain an 
automatic zero reading. The automatic zero switch SW1 can be opened after 
a null condition has been established. The count stored in the up/down 
counter 64 will continue to provide the zero offset input voltage 75 to 
the comparator 42. Accordingly, the panel meter 44 will now provide a true 
reading corresponding only to the stress imposed on the structural member 
10. The up/down counter 64 can be reset by closing the transducer 
calibrate switch SW2 to permit an initial calibration adjustment of the 
transducer assembly 20. 
The stress is monitored during operation of the machine by the 12 line 
comparator 80 which receives end of the 3 BCD digit inputs from the 
storage latches 47, 48, and 49. The comparator 80 is a conventional device 
which permits the operator to dial in (not shown) an input corresponding a 
high and low numerical value. These values are converted to BCD digits for 
comparison with the input BCD from the storage latches 47, 48, and 49. 
Accordingly, by closing switch SW3; e.g., when the die is closed, an 
automatic comparison is made to determine if the stored BCD digits are 
above or below the set values. A high fault counter 82 and a low fault 
counter 84 are used to count the number of high and low faults that may 
occur over a given interval of time. Each time a fault occurs the Nand 
gate 86 or 88 triggers the respective fault counters 82 or 84. An Or gate 
90 is used to sound an alarm 92 for each fault whether high or low. The Or 
gate 90 energizes the relay 94 which closes the relay contact 96 for 
energizing the alarm.