Dynamic force measuring instrument for foundation and casing

An attenuation circuit for a pile driving force measurement device is disclosed. The attenuation circuit allows for measurements to be made of the force being applied to a driven pile at a reduced voltage compatible with a voltage sensitive measurement device, such as a personal computer. The forces measured create measurable electrical impulses via magnetostriction. The attenuation circuit conditions the measured impulses so that no separate electrical power circuit or battery is required to power the sensor. At the same time the attenuation circuit protects the measurement device from voltages which might be overly high and damage the equipment.

FIELD OF THE INVENTION 
This application is for an improvement over U.S. Pat. No. 3,931,729, 
entitled DYNAMIC FORCE MEASURING INSTRUMENT FOR FOUNDATION AND CASING, 
issued to LEONARD L. FREDERICK, on Jan. 13, 1976. The improvement allows 
for the acquisition of data, such as depth driven and force applied, from 
the driven pile with simpler electric circuitry and without the use of a 
separate battery or other power circuit as was required by the patented 
invention. 
BACKGROUND OF THE INVENTION 
As a pile is driven, the operator of the pile driver must be careful to 
control the force of the blows applied to the pile so as not to exceed the 
elastic limit of the pile material in order to minimize costly tip damage. 
Those who are familiar with pile driving are well acquainted with the 
condition known in the art as "overdriving" and the ultimate damage that 
results therefrom. To minimize such damage, drive caps are fitted over the 
head of the pile to evenly transmit the hammer blows to the pile, while at 
the same time maintaining the head of the pile in alignment with the 
hammer by guiding the head parallel to the leads frame and retaining the 
pile in a straight predetermined path. 
Though this helps to mitigate some of the damage, the burden in most cases 
falls largely upon the experience of the operator to determine the driving 
force required. For a given set of conditions, tests may be made to help 
the judgment of the operator and further strain gauges may be used to 
determine the force of the blow and the dynamic forces within the pile. 
Strain gauges, however, must be cemented or otherwise attached to the 
beam, and it is a relatively time consuming and costly process to mount 
them properly. The strain gauge is also a very fragile device and its 
reliability under the repetitive dynamic shock loading to which the pile 
is subjected is, to say the least, questionable. 
After a pile is driven to the proper depth, there remains a need for 
measurement of the static bearing load which the pile will support, which 
is usually done by loading a test pile with weight until it moves. This is 
called a dead load bearing test and is also a time-consuming and expensive 
process. By accurate measurement of the dynamic forces below which the 
pile will not move, the bearing capacity may be reasonably estimated thus 
saving considerable time and expenses. 
The patented invention disclosed both a novel apparatus and a novel method 
for utilizing magnetostrictive principles for accurately and quickly 
determining the driving force applied to the end of the pile, the visual 
indication of which can be displayed directly to the operator of the pile 
driver for proper adjustment and accurate control of the driving force. 
The principles of this invention can be further used to provide a means 
for determining the equivalent static bearing capacity of the pile after 
it is driven to a position where it should have attained its bearing 
capacity. In this application, those principles have been carried further 
in a simple, novel way to achieve data readouts to a computer or other 
instrument without the complexity of a separate power circuit. 
Magnetostriction can be described as the deformation of a material causing 
the generation of an electric current which influences a magnetic field, 
or vice versa, i.e., deforming a ferrous material under the influence of 
the magnetic field and inducing a current, which accordingly changes the 
magnetic field. Further, the change in the magnetic field is proportional 
to the deformation within the elastic limit, thus such deformation may be 
determined by the magnitude of the change. Normally, this effect is small 
and noticeable under most conditions, however, the tremendous driving 
force used in driving a pile gives rise to usable signals which can be 
sensed with instruments of normal sensitivity and be used to determine the 
force with which the pile was struck. Some explanation of the terms and 
properties used in this application would be useful at this point. 
MAGNETOSTRICTION: This term defines the effect whereby a material will 
change shape in the presence of an external magnetic field. This effect is 
brought about by the re-ordering of the magnetic dipoles within the 
material. The British physicist, J. P. Joule, discovered this effect in 
1847. Joule later discovered the VILLARI EFFECT in 1864. 
VILLARI EFFECT: This term defines what happens when one applies an external 
stress to a magnetostrictive material, such as iron; thus, a corresponding 
strain will develop, which strain will in turn induce a magnetic field. 
This application capitalizes upon this Villari effect. 
OBJECTS OF THE INVENTION 
Accordingly, it is the principal object of the invention to provide a novel 
circuit for a force-measuring transducer that does not require a separate 
power circuit in order to allow for the measurement of the force data from 
the hammer that is applied to the pile with extreme accuracy, which data 
readout will allow an operator to prevent damage to the tip of the pile. 
It is also an object of this invention to provide a transducer having 
simplified electric circuitry versus the state of the art and yet that 
will still accurately measure the maximum threshold force from the pile 
driving hammer that can be applied to the pile without causing permanent 
penetration of the pile into the ground. Thus, the maximum bearing 
capacity of the pile can be determined immediately and thus eliminates the 
need for a costly dead load bearing test. 
A still further object of this invention is to provide an attentuation 
circuit that improves pile bearing measurement techniques and that reduces 
the required coil voltage so that the output of the circuit is compatible 
with a computer or an oscilloscope. 
SUMMARY AND ADVANTAGES OF THE INVENTION 
The invention comprehends the use of a passive sensor and attenuator 
equalizer to provide electrical isolation between the sensor and cable. 
The magnetic flux sensor is a passive coil placed over the pile and does 
not require physical contact to the pile for measurement. 
No electrical stimulus to measurement sensors or the pile being measured is 
required. It is not necessary to generate a magnetic field to measure the 
field produced in the pile by the impact of the hammer. By virtue of 
"inverse magnetostriction", the strain in the pile produced by the 
compression force of the hammer produces a magnetic field that is 
proportional to the resistance that the pile encounters during driving. 
The attenuator/equalizer provides a simple means of scaling the signal 
voltage for compatibility with the recording device, i.e., oscilloscope, 
computer monitor, chart recorder, etc. The attenuator/equalizer can be 
composed of any combination of elements, passive or active circuits, 
suitable for equalization of the signal and cable properties. The 
attenuator/equalizer provides a means for impedance matching, 
equalization, and termination of the sensor coil, cable, and measurement 
instrument. 
Further objects and advantages will become more apparent from a reading of 
the following specification taken in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to FIG. 1, there is shown a view of the customary mobile pile 
driving crane 10 on which is mounted a boom II, which is so mounted as to 
swivel at its upper end by hammer leads 12, and at its lower end by a 
bottom brace 13. For a better understanding of the function of the bottom 
brace, reference is made at this time to my U.S. Pat. No. 3,817,091 
granted Jun. 18, 1974. The pile 14, which is shown in position preparatory 
to be driven into the earth, is supported at its lower end by the ground 
at the point generally denoted as 15, and at its upper end by the drive 
cap 16 which is slidably associated with the leads 12 so that the cap 
maintains the pile 14 in alignment with the leads at all times, the drive 
cap arranged to slide down as the pile is driven into the ground. 
Supported adjacent to the top of the leads and above the drive cap is a 
pile driving hammer 17, which is slidably associated with the leads and 
thereby arranged to follow the drive cap 16 and pile 14 as the pile is 
driven into the ground as indicated. Loosely fitted adjacent to the bottom 
of pile 14 without any physical or direct contact 25 there is the 
transducer 18. Electrically connected to the transducer 18, as generally 
shown at 19, is a cable 60 which may lie on the ground or be arranged to 
be carried along the longitudinal extent of the bottom brace 13 and into 
the cab 21 where it is attached to the visual readout unit 22 positioned 
in front of the operator so that he can maintain a constant surveillance 
of the progress of the pile while he controls the operation of the pile 
driver hammer 17. 
Applicant believes that it should be apparent that since the transducer is 
positioned substantially at ground level, it is easily situated prior to 
erecting the pile into place, but must be lifted up over the top of the 
pile after the driving operation is completed and therefore the cable 60 
should be sufficiently long enough to accomplish this purpose or the cable 
must be provided with some type of disconnect means, not shown. 
Referring now to FIG. 2, there is shown a pile 14 which has been partially 
driven into the earth 23 by means of the pile driving hammer 17 shown 
supported above it. Loosely wound around pile 14 is the transducer 18, 
which comprises a single conductor 24, both ends of which terminate in a 
connector means 25. Extending from the connector means 25 and electrically 
connected to the transducer 18 is a pair of conductor cables 20 with one 
wire 20' being connected to ground at point 26. Also connected to ground 
at point 26 by a wire 22 is the negative terminal of battery 28 with the 
positive terminal connected by wire 29 to one end of the primary winding 
30 of impedance matching transformer 31. Connected in parallel with 
battery 28 is capacitor 32, one end being connected to the negative 
terminal and the other end connected to the positive terminal, so that AC 
signals bypass the battery. The second wire 20" of cable 20 is connected 
from the connector means 25 to the other end of primary winding 30 of 
impedance matching transformer 31. 
One end of the secondary winding 33 of impedance matching transformer 30 is 
connected to ground as at 34, the other end connected by wire 35 to the 
input terminal 36 of oscilloscope 37. The second terminal 38 of 
oscilloscope 37 is connected by wire 39 to ground as at 40. 
It will be clear to those skilled in the art that a signal is induced into 
the transducer 18 by the magnetic field produced in the steel pile when it 
is struck and that this signal bypasses the battery through the capacitor 
32 and is then amplified by the transformer action in the impedance 
matching transformer 31 to a usable voltage level to drive the 
oscilloscope 37. The signal level is proportional to the force with which 
the pile is struck and therefore the amplitude of the signal displayed on 
the oscilloscope is also proportional to the force. The battery 28 is used 
to establish a magnetic field level in the pile which changes when the 
pile is struck, but does not affect the actual signal that is induced into 
the transducer 18. 
It should also be apparent to those skilled in the art that the signal 
level induced into the transducer 18 is also governed by the number of 
turns and the gauge of the wire, and the same principles apply as when 
designing transformers, i.e. for high frequencies, a generally low 
impedance winding is required, whereas the input to the oscilloscope 
requires a high impedance to prevent loading. 
Other methods of isolating the energizing voltage of the transducer 18 from 
the oscilloscope can be used. 
Referring now to FIG. 3, there is also shown a transducer 18' as a bifilar 
winding and in which a pair of insulated conductors 41-46 are wound into a 
coil and the four ends terminate in a connector 25'. Electrically 
connected to conductor 41 of transducer 18' at connector 25' is wire 42, 
which in turn is connected at its opposite end to the input terminal 36 of 
the oscilloscope 37. Electrically connected to the other end of winding 41 
of transducer 18' at connector 25' is a wire 43 which is connected at its 
opposite end to the grounding terminal 38 of oscilloscope 37 with this 
terminal also being grounded as at 44 by wire 45. The second conductor 46 
is electrically connected at one end at connector 25' then to wire 47 
which is in turn connected at its opposite end to the negative terminal 48 
the energizing battery 28. This negative terminal is also grounded as at 
49 by a wire 50. Electrically connected to the other end of the conductor 
46 at connector 25' is the wire 51, which is connected at its opposite end 
to the positive terminal 52 of the battery 28. Connected in parallel with 
battery 28 and extending across its terminals 48 and 52 is a bypass 
capacitor 32 which serves as previously stated to bypass the induced high 
frequency signal past the battery in a manner well known to the art. 
It will be apparent to those skilled in the art that this construction 
isolates the oscilloscope circuit from the battery circuit, while the 
intimacy of the two conductors of the bifilar-wound transducer 18 closely 
couples the sensing winding 41 into the magnetic field produced by the 
energizing winding 46. Other components of the system, which are shown but 
not presently described, function in the manner previously described. 
A further method of isolating the energizing voltage of transducer 18" from 
the oscilloscope by use of a blocking capacitor is shown in FIG. 4, 
wherein the transducer 18" is wound with a single conductor 24, its ends 
terminating at connector 25 as clearly described in connection with FIG. 
2. Electrically connected to one end of the winding conductor 24 at 
connector 25" is one conductor 20' of cable 20 which is connected to the 
negative terminal 48 of battery 28 and also grounded as at 49, as 
hereinbefore described. 
Conductor 20" of the cable 20 is electrically connected at connector 25 to 
the other end of the transducer conductor 24 and connects to the positive 
terminal 52 of the battery 28, thus the transducer 18" is energized by 
battery 28 as explained earlier herein. Connected to positive terminal 52 
of the battery 28 is a blocking capacitor 53 which is connected at its 
other end to the input terminal 36 of oscilloscope 37, and a lead 54 is 
arranged to be connected from the negative terminal 48 of battery 28 to 
the ground terminal 38 of the oscilloscope 37. It is believed to be clear 
from the foregoing that the battery voltage is effectively isolated from 
the oscilloscope and, further that the blocking capacitor 53 must have a 
low impedance at the signal frequency produced by the hammer blow, thus 
allowing the magnetostrictive signal to be passed into the oscilloscope. 
Turning now to FIG. 5, there is shown a pile 14 which has been partially 
driven into the earth 23 by means of the pile driving hammer 17 shown 
supported above it. Loosely wound around pile 14 is the transducer, which 
comprises a single conductor both ends of which terminate to a shielded 
conductor 20, to form conductors 20' and 20". The shield is grounded at 
49. The conductors 20' and 20" terminate in female connector 58. The male 
end 59 of attenuator plug 61 sockets into female connector 58, as shown in 
FIG. 6. The attenuator 61 is comprised of inline resistor 57 for line 20' 
and resistor 55 for line 20". Across terminals 64 and 65 is placed 
crossline resistor 56. The conductors 20' and 20" are then terminated in 
the female end of the attenuator plug 61 at 62. Male connector 63 which 
terminates shielded cable 60 sockets into female 62. As indicated in FIG. 
5, shielded cable 60 containing wires 60' and 60" run to a computer or an 
oscilloscope 37, and are terminated at 36 and 38. Cable 60 shield is 
grounded at 49. 
One skilled in the art should immediately grasp the importance of the 
improvements described herein upon U.S. Pat. No. 3,931,729. To ensure that 
those improvements are fully understood, applicant would advance the 
following features of this invention as being novel and non-obvious: 
1. A simple electric circuit as described in FIGS. 5 and 6. This circuit 
functions better than that shown in the prior art, denoted by FIGS. 2, 3, 
and 4 of U.S. Pat No. 3,931,729, which figures of drawings are 
re-disclosed hereinabove. 
2. A circuit that does not use a power source or battery. 
3. A circuit that utilizes the advantages of an attenuator to make the 
voltage compatible to reading by a computer or an oscilloscope. 
The improvements in the operation of the dynamic force measurement 
instrument over those provided by the applicant's above noted patent are 
numerous. Those improvements result from the benefits of the 
above-described attenuator circuit. The benefits provided by the 
attenuator/equalizer and the removal of the battery circuit and the 
secondary coil circuit are as follows: 
(1) The passive sensor and attenuator/equalizer provide electrical 
isolation between the sensor and cable. The magnetic flux sensor is a 
passive coil placed over the pile and does not require physical contact 
with the pile for measurement. No electrical stimulus to measurement 
sensors or the pile being measured is required. The elimination of the 
battery circuit and secondary coil that was used to produce a constant 
magnetic field resulted in a more consistent and clear measurement of the 
field produced by the pile hammer stress induced "inverse 
magnetostriction". 
(2) Applicant has discovered that it is not necessary to generate a 
magnetic field to measure the field produced in the pile by the impact of 
the hammer. By virtue of "inverse magnetostriction", the strain in the 
pile produced by the compression force of the hammer produces a magnetic 
field that is proportional to the resistance that the pile encounters 
during driving. 
(3) The attenuator/equalizer provides a simple means of scaling the signal 
voltage for compatibility with the recording device, whether it is an 
oscilloscope, a computer monitor, a chart recorder, or a similar 
measurement device. 
(4) The attenuator/equalizer, which can be composed of any combination of 
elements, passive or active circuits, is suitable for equalization of the 
signal and cable properties. 
(5) The attenuator/equalizer provides a means for impedance matching, 
equalization, and termination of the sensor coil, cable, and measurement 
instrument. 
FIG. 7 illustrates a mobile pile driving crane with which the invention can 
be carried out.