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Abstract and Figures

Temperature measurement is one of the most important aspects of manufacturing. There have been many temperature measuring techniques applied for obtaining workpiece temperature in different types of manufacturing processes. The main limitations of conventional sensors have been the inability to indicate the core temperature of workpieces and the low accuracy that may result due to the harsh nature of some manufacturing environments. The speed of sound is dependent on the temperature of the material through which it passes. This relationship can be used to obtain the temperature of the material provided that the speed of sound can be reliably obtained. This paper investigates the feasibility of creating a cost-effective solution suitable for precision applications that require the ability to resolve a better than 0.5 • C change in temperature with ±1 • C accuracy. To achieve these, simulations were performed in MATLAB using the k-wave toolbox to determine the most effective method. Based upon the simulation results, experiments were conducted using ultrasonic phase-shift method on a steel sample (type EN24T). The results show that the method gives reliable and repeatable readings. Based on the results from this paper, the same setup will be used in future work in the machining environment to determine the effect of the harsh environment on the phase-shift ultrasonic thermometry, in order to create a novel technique for in-process temperature measurement in subtractive manufacturing processes.
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Manufacturing and
Materials Processing
Journal of
Article
Precision Core Temperature Measurement of Metals
Using an Ultrasonic Phase-Shift Method
Olaide F. Olabode * , Simon Fletcher, Andrew P. Longstaand Naeem S. Mian
Center for Precision Technologies, University of Huddersfield, Huddersfield HD1 3DH, UK
*Correspondence: olaide.olabode@hud.ac.uk
Received: 29 June 2019; Accepted: 2 September 2019; Published: 4 September 2019


Abstract:
Temperature measurement is one of the most important aspects of manufacturing. There
have been many temperature measuring techniques applied for obtaining workpiece temperature in
dierent types of manufacturing processes. The main limitations of conventional sensors have been
the inability to indicate the core temperature of workpieces and the low accuracy that may result
due to the harsh nature of some manufacturing environments. The speed of sound is dependent
on the temperature of the material through which it passes. This relationship can be used to obtain
the temperature of the material provided that the speed of sound can be reliably obtained. This
paper investigates the feasibility of creating a cost-eective solution suitable for precision applications
that require the ability to resolve a better than 0.5
C change in temperature with
±
1
C accuracy.
To achieve these, simulations were performed in MATLAB using the k-wave toolbox to determine
the most eective method. Based upon the simulation results, experiments were conducted using
ultrasonic phase-shift method on a steel sample (type EN24T). The results show that the method gives
reliable and repeatable readings. Based on the results from this paper, the same setup will be used in
future work in the machining environment to determine the eect of the harsh environment on the
phase-shift ultrasonic thermometry, in order to create a novel technique for in-process temperature
measurement in subtractive manufacturing processes.
Keywords:
core temperature measurement; phase-shift method; manufacturing; ultrasonic thermometry
1. Introduction
Dimensional accuracy and the surface integrity of manufactured products are key specifications
that determine conformance to the design intent. The adherence to these specifications is an indication
of the quality of the product and that it will meet the needs of customers. Temperature variation during
manufacturing can influence both the dimensional accuracy and the surface integrity of the product.
Therefore, temperature control or compensation of the thermal eects is essential in order to produce
high quality components.
Exact temperature variation values are not given in the literature, as it is dicult to get a generic
value due to dierent manufacturing processes. However, temperature variation in a workpiece during
machining can reach 10
C. A 10
C variation will result in a diering expansion in dierent materials.
In precision manufacturing, workpieces often need to be machined with a dimensional error of less
than 5
µ
m. The accuracy and resolution with which workpiece temperature needs to be measured for
dierent materials is given in Table 1.
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Table 1. Required accuracy and resolution for the precision manufacturing of dierent materials.
Material
Coecient of
Thermal
Expansion
(×106/C) [1]
Expansion of 200
mm Part at 10 C
Change in
Temperature (µm)
Required
Temperature
Accuracy (C)
Required
Temperature
Resolution (C)
Structural Steel 12 24 ±2 1
Aluminum 24 48 ±1 0.5
Cast Brass 21 42 ±1.2 0.6
Tungsten 4.6 9.2 ±5.4 2.7
Copper alloys 18 36 ±1.4 0.7
The core temperature variation of metals aects their dimensional accuracy. The existing
temperature measurement methods in manufacturing are limited in that they only represent the surface
temperature of the workpiece. This paper investigates the possibility of creating a novel system for
the precision core temperature measurement of metals which can be adapted for use in dierent
manufacturing processes. One such manufacturing process is the machining process. Metal cutting is
arguably the most important aspect of manufacturing and there have been advancements in optimizing
the rate of metal cutting. New technologies have helped to increase the cutting speed, depth of cut
and feed rate. However, with these increments also comes the increase in heat generation originating
near the tool–workpiece interface [
2
]. Combined with eects from change in ambient temperature,
heat sinking to fixtures and non-deterministic heat transfer between the component and cutting fluids,
indirect monitoring of the temperature of the workpiece is a very challenging problem. Most of the
temperature measurement techniques for manufacturing described in literature only deal with the
machine [3] or cutting tool temperature [2].
One of the temperature measurement methods for workpieces reported in literature is the
tool/workpiece thermocouple [
4
6
]. The main diculties reported concerning the use of this method
are the parasitic electromotive force (EMF) from secondary joints, the necessity for the accurate
calibration of the tool and workpiece as a thermocouple pair, the need to isolate the thermocouple
from the environment and the lack of clarity on what the EMF represents [
5
,
6
]. Also, this method does
not indicate the core temperature of the workpiece. Infrared thermometry is another method that has
been used in both dry conditions and with the presence of coolant [
7
]. However, the measurement
only represents the surface temperature, and the accuracy that can be achieved with this method is
less than the required accuracy for the precision machining of some materials, as infrared cameras’
stated accuracy is typically
±
2
C [
8
]. Moreover, this value is only valid in ideal conditions, as the
accuracy will greatly reduce in harsh machining environments. Infrared cameras also require a good
line of sight to work. Of all the methods previously used, none gives a direct indication of the core
temperature of the workpiece, which is the parameter that aects the dimensional expansion.
The speed of sound in any material is dependent on the temperature of the medium of propagation.
This dependence has been used in a variety of ways to measure the temperatures of dierent media.
The pulse-echo technique uses the time-of-flight method to measure ultrasonic velocity which can
then be related to the temperature of the medium [
9
]. The pulse-echo method is relatively simple, but
the resolution of measurement may reduce with distance due to attenuation of the echo signal [
10
].
Another main technique is the continuous wave method which evaluates the distance of ultrasonic
travel by computing the phase-shift between transmitted and received signals [
11
]. Some modifications
and combinations of the two techniques have also been used, such as the two-frequency continuous
wave method [
11
,
12
] and multiple frequency continuous wave method [
13
,
14
]. These techniques make
use of two or more ultrasonic frequencies to increase the range of measurement and improve resolution.
Hu et al., using the temperature range of 25 to 200
C, modified the ultrasonic velocity equation
based on the eects thermal expansion has on the travel path of ultrasonic waves. They reported an
increase in accuracy based on the compensations made for expansion [
15
]. Another recent work by
Ihara et al. is the use of laser ultrasonic thermometry to measure the internal temperature of heated
J. Manuf. Mater. Process. 2019,3, 80 3 of 12
cylindrical rods. The authors reported that the results almost agree with those measured using a
thermocouple, however the accuracy of the method is not suciently described [
16
]. Dierent methods
of ultrasonic measurement have been used and modified for use in dierent fields of measurement,
however, their use in precision manufacturing is not suciently described in the literature.
This paper explores the use of ultrasonic waves for the precision core temperature measurement
of a steel workpiece. The results from simulation using the two main techniques are evaluated to find
the best option for core temperature measurement in metals. The initial work consists of simulations
in the k-Wave MATLAB toolbox—an open source toolbox for time domain acoustic and ultrasound
simulations [
17
]. This was used to study the two main ultrasonic thermometry methods—the ultrasonic
pulse-echo and phase-shift methods. k-Wave was chosen for simulation because of its flexibility for
defining dierent parameters and media of interest—it also gives a real-time A-scan of the propagation
medium during simulation, as well as its propagation plot. In an A-scan, the amplitude of an ultrasonic
pulse is represented as displacement in y-axis and the corresponding travel time is represented on
the x-axis. Based on the simulation results, ultrasonic phase-shift experiments were conducted in a
metrology laboratory and the results of these experiments will serve as input to a future experiment
which involves the use of the developed techniques in subtractive manufacturing processes.
2. Materials and Methods
The Pulse-echo method is the traditional means of ultrasonic measurement. It uses the principle of
time-of-flight (tof ), where an ultrasonic pulse is propagated through a medium and the pulse is reflected
when it encounters a medium of dierent physical property [
18
]. The tof from the ultrasonic transmitter
to the receiver and the length of the travel path is used to compute the ultrasonic velocity [
19
]. This
relationship is given as:
c=d
to f (1)
where cis the ultrasonic velocity, dis the distance travelled and tof is the time of flight [20].
The phase-shift method, on the other hand, uses the dierence in the phase of steady state
frequency ultrasonic wave between transmitted and received signals [
13
]. For an ultrasonic wave of
known frequency travelling through a known distance, the phase dierence between the transmitted
and received signals can be used to compute the ultrasonic velocity through the medium. The
relationship between the ultrasonic velocity and phase-shift is given in the equation below:
L=n+ϕ
2πc
f(2)
where Lis the distance between the transmitter and receiver, nis the integer number of wave periods,
φ
is the phase-shift, fis the ultrasonic frequency and cis the ultrasonic velocity through the medium [
19
].
A modification of the phase-shift method which considerably improves both the range and
resolution of measurement is the two-frequency continuous wave method (TFcw). It uses two
frequencies for ultrasonic velocity computation. The TFcw equation is given as:
c=2πLf
ϕ(3)
where cis the ultrasonic velocity, Lis the length of travel,
fis the dierence between the two
frequencies (f1–f 2) and φis the dierence between the phase-shifts given as [13]:
ϕ=ϕ1ϕ2,i f ϕ1> ϕ2,
ϕ=ϕ1+2πϕ2,i f ϕ1< ϕ2
(4)
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By adding a third frequency, the range and the resolution can both be further improved. This
technique is known as the multiple frequency continuous wave method (MFcw) and the MFcw equation
is given as [21]:
c=L
Intϕ1
2π
f2
f1c
f2+Intϕ2
2π
f1
f2c
f1+ϕ1
2πc
f1
(5)
2.1. Simulations
Three simulations were performed in MATLAB R2017b using the k-Wave toolbox. The first
simulation was set up to resolve 0.1
C change in temperature with tof of ultrasonic wave. Steel was
chosen as the medium of propagation with a nominal length of 200 mm. The k-Wave grid (Nx) was
defined as 6.561
×
10
3
grids, the spacing (dx) was defined as 1.2
×
10
4
. The ultrasonic wave parameters
were set up to achieve sensitivity of 0.1
C. The sensor position for the simulation is given in Figure 1.
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∆𝜑 = 𝜑𝜑, 𝑖𝑓 𝜑𝜑
,
∆𝜑 = 𝜑+2𝜋𝜑,𝑖𝑓 𝜑
𝜑
(4)
By adding a third frequency, the range and the resolution can both be further improved. This
technique is known as the multiple frequency continuous wave method (MFcw) and the MFcw
equation is given as [21]:
𝑐= 𝐿
𝐼𝑛𝑡Δ𝜑
2𝜋 Δ
𝑓
Δ
𝑓
𝑐
Δ
𝑓
+𝐼𝑛𝑡Δ𝜑
2𝜋
𝑓
Δ
𝑓
𝑐
𝑓
+𝜑
2𝜋 𝑐
𝑓
(5)
2.1. Simulations
Three simulations were performed in MATLAB R2017b using the k-Wave toolbox. The first
simulation was set up to resolve 0.1 °C change in temperature with tof of ultrasonic wave. Steel was
chosen as the medium of propagation with a nominal length of 200 mm. The k-Wave grid (Nx) was
defined as 6.561 × 10
3
grids, the spacing (dx) was defined as 1.2 × 10
4
. The ultrasonic wave parameters
were set up to achieve sensitivity of 0.1 °C. The sensor position for the simulation is given in
Figure 1.
Figure 1. A-scan image of simulation.
The ultrasonic velocity used is based on the temperature–velocity relationship given by
Ihara et al. [22]. This is given as:
𝑣𝑇= 0.636𝑇 +5917.6 (6)
where v(T) is the temperature-dependent ultrasonic velocity and T is the temperature.
The simulation was run over the range of 25 to 25.5 °C to observe if the corresponding change
in time of flight can be reliably measured. Equation (6), which was used for the simulation, is almost
linear for a temperature range of 0–200 °C [22]—hence, in this simulation, the sensitivity is prioritized
over range. The peak detection technique was used to record the time the ultrasonic pulse strikes the
sensor at both the transmitting and receiving positions [20].
The second simulation was carried out to observe the individual effect of change in ultrasonic
velocity and change in material dimension (expansion) on tof. This was performed in order to verify
if the tof can be reliably estimated from the change in velocity alone, expansion alone or by combining
both. The ultrasonic velocity was varied using Equation (6), while material expansion was varied
using Equation (7) given below:
Δ𝐿 = 𝛼𝛥𝑇𝐿 (7)
where ΔL is the change in length, α
L
is the linear coefficient of thermal expansion, ΔT is the change in
temperature and L is the original material length.
Figure 1. A-scan image of simulation.
The ultrasonic velocity used is based on the temperature–velocity relationship given by
Ihara et al. [22]. This is given as:
v(T)=0.636T+5917.6 (6)
where v(T) is the temperature-dependent ultrasonic velocity and Tis the temperature.
The simulation was run over the range of 25 to 25.5
C to observe if the corresponding change in
time of flight can be reliably measured. Equation (6), which was used for the simulation, is almost
linear for a temperature range of 0–200
C [
22
]—hence, in this simulation, the sensitivity is prioritized
over range. The peak detection technique was used to record the time the ultrasonic pulse strikes the
sensor at both the transmitting and receiving positions [20].
The second simulation was carried out to observe the individual eect of change in ultrasonic
velocity and change in material dimension (expansion) on tof. This was performed in order to verify if
the tof can be reliably estimated from the change in velocity alone, expansion alone or by combining
both. The ultrasonic velocity was varied using Equation (6), while material expansion was varied
using Equation (7) given below:
L=αLTL (7)
where
Lis the change in length,
αL
is the linear coecient of thermal expansion,
Tis the change in
temperature and Lis the original material length.
The third simulation was performed using the phase-shift method, with the aim of obtaining the
frequency pair which can consistently sense a 0.1
C change in temperature. This same simulation was
modified for the MFcw method and, for this simulation, the length of material was scaled down by
J. Manuf. Mater. Process. 2019,3, 80 5 of 12
a factor of ten to reduce computational load. Also using the phase-shift method, a simulation was
performed for a 15 mm piece of steel—this was performed to predict the possibility of using the 5 MHz
transducer to obtain a 10 C range and 0.1 C resolution.
2.2. Simulation Results
In order to achieve a measurement resolution of 0.1
C, a tone burst of 1.2 MHz and sampling
frequency of 10 GHz were used. The recorded tone burst at 25
C and the tof for the whole range of
simulations are given in Figure 2and Table 2respectively.
Figure 2. Recorded tone burst.
Table 2. Time of flight variation with temperature.
Temperature (C) Velocity (m/s) Tof (µs)
25.0 5901.70 33.4372
25.1 5901.63 33.4376
25.2 5901.57 33.4380
25.3 5901.51 33.4384
25.4 5901.45 33.4388
25.5 5901.38 33.4392
With 10 GHz sampling frequency, a 0.1
C change in temperature will cause a change in time
of flight that can be resolved at the fourth decimal place of tof in microseconds. The costs of
pulsers/receivers that sample at 10 GHz frequency are considerably high.
From the findings of Ihara et al., the relationship between ultrasonic velocity and temperature
is almost linear within the range of 0 to 200
C [
22
], as is the eect from the change in distance from
thermal expansion. The simulations for expansion and ultrasonic speed were performed over this
linear range. The result of this simulation is shown in Figure 3.
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Figure 3. Effect of change in speed and expansion on tof.
Figure 3 shows that the time of flight varied largely due to changes in velocity, while the
variation due to expansion is relatively smaller. Expansion across the range of 0 to 200 °C is 480 µm
in total for 200 mm steel. The tof can be reliably estimated by considering ultrasonic velocity while
compensating for expansion.
The MFcw technique was used in the third simulation for the estimation of ultrasonic velocity.
First, relatively low frequencies of 0.6 and 0.5 MHz were used to estimate ultrasonic velocity through
phase-shift—this is the TFcw technique. Thereafter, based on Equation (3), 0.5, 0.51 and 10 MHz were
used to improve the sensitivity of the simulation for a 0.1 °C change in temperature (MFcw). The
results for the simulations are given in the Figure 4a,b and Table 3 respectively.
(a) (b)
Figure 4. Two-frequency continuous wave method (TFcw) technique (a) 11.6 phase-shift at 0.6 MHz;
(b) 110.3 phase-shift at 0.5 MHz.
Figure 3. Eect of change in speed and expansion on tof.
Figure 3shows that the time of flight varied largely due to changes in velocity, while the variation
due to expansion is relatively smaller. Expansion across the range of 0 to 200
C is 480
µ
m in total for
200 mm steel. The tof can be reliably estimated by considering ultrasonic velocity while compensating
for expansion.
The MFcw technique was used in the third simulation for the estimation of ultrasonic velocity.
First, relatively low frequencies of 0.6 and 0.5 MHz were used to estimate ultrasonic velocity through
phase-shift—this is the TFcw technique. Thereafter, based on Equation (3), 0.5, 0.51 and 10 MHz
were used to improve the sensitivity of the simulation for a 0.1
C change in temperature (MFcw).
The results for the simulations are given in the Figure 4a,b and Table 3respectively.
J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 6 of 12
Figure 3. Effect of change in speed and expansion on tof.
Figure 3 shows that the time of flight varied largely due to changes in velocity, while the
variation due to expansion is relatively smaller. Expansion across the range of 0 to 200 °C is 480 µm
in total for 200 mm steel. The tof can be reliably estimated by considering ultrasonic velocity while
compensating for expansion.
The MFcw technique was used in the third simulation for the estimation of ultrasonic velocity.
First, relatively low frequencies of 0.6 and 0.5 MHz were used to estimate ultrasonic velocity through
phase-shift—this is the TFcw technique. Thereafter, based on Equation (3), 0.5, 0.51 and 10 MHz were
used to improve the sensitivity of the simulation for a 0.1 °C change in temperature (MFcw). The
results for the simulations are given in the Figure 4a,b and Table 3 respectively.
(a) (b)
Figure 4. Two-frequency continuous wave method (TFcw) technique (a) 11.6 phase-shift at 0.6 MHz;
(b) 110.3 phase-shift at 0.5 MHz.
Figure 4.
Two-frequency continuous wave method (TFcw) technique (
a
) 11.6 phase-shift at 0.6 MHz;
(b)110.3 phase-shift at 0.5 MHz.
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Table 3. Simulation results using multiple frequency continuous wave method (MFcw) technique.
Temperature
(C)
Phase-Shift
(0.5 MHz) ()
Phase-Shift
(0.51 MHz) ()
Phase-Shift (10
MHz) ()
Used
Ultrasonic
Speed (m/s)
Calculated
Ultrasonic
Speed (m/s)
20 23.33 98.54 106.95 5904.88 5904.86
20.1 23.261 98.61 105.64 5904.82 5904.8
20.2 23.2 98.67 104.32 5904.75 5904.73
20.3 23.13 98.74 103.01 5904.69 5904.67
20.4 23.06 98.81 101.7 5904.63 5904.61
20.5 23 98.88 100.38 5904.56 5904.54
20.6 22.93 98.94 99.07 5904.5 5904.48
20.7 22.87 99.01 97.76 5904.44 5904.41
20.8 22.8 99.08 96.44 5904.37 5904.35
20.9 22.74 99.14 95.13 5904.31 5904.29
21 22.67 99.21 93.82 5904.24 5904.22
The result of the simulation with 5 MHz transducer and 15 mm steel plate is given in Figure 5.
J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 7 of 12
Table 3. Simulation results using multiple frequency continuous wave method (MFcw) technique.
Temperature
(°C)
Phase-
Shift (0.5
MHz) (°)
Phase-
Shift
(0.51
MHz) (°)
Phase-
Shift (10
MHz) (°)
Used
Ultrasonic
Speed (m/s)
Calculated
Ultrasonic
Speed (m/s)
20 23.33 98.54 106.95 5904.88 5904.86
20.1 23.261 98.61 105.64 5904.82 5904.8
20.2 23.2 98.67 104.32 5904.75 5904.73
20.3 23.13 98.74 103.01 5904.69 5904.67
20.4 23.06 98.81 101.7 5904.63 5904.61
20.5 23 98.88 100.38 5904.56 5904.54
20.6 22.93 98.94 99.07 5904.5 5904.48
20.7 22.87 99.01 97.76 5904.44 5904.41
20.8 22.8 99.08 96.44 5904.37 5904.35
20.9 22.74 99.14 95.13 5904.31 5904.29
21 22.67 99.21 93.82 5904.24 5904.22
The result of the simulation with 5 MHz transducer and 15 mm steel plate is given in Figure 5.
Figure 5. Phase-shift vs. temperature for 15mm steel.
From the results of the simulations, both the pulse-echo method and the phase-shift method
were able to resolve 0.1 °C change in temperature. However, a pulser/receiver is needed to use the
pulse-echo method. For a 0.1 °C change detection, the pulser/receiver needs samples at up to 10 GHz
and the cost of such device can reach the €20,000 mark. However, for the phase-shift method, the cost
of a phase detector is under €400. The phase-shift method was chosen because of its cost effectiveness,
therefore, the experiments described in Section 3 are all based on the phase-shift method.
3. Experiments
3.1. Experimental Setup
Based on results from the k-Wave simulations, a phase-shift experiment was set up as shown in
Figure 6.
Figure 5. Phase-shift vs. temperature for 15mm steel.
From the results of the simulations, both the pulse-echo method and the phase-shift method
were able to resolve 0.1
C change in temperature. However, a pulser/receiver is needed to use the
pulse-echo method. For a 0.1
C change detection, the pulser/receiver needs samples at up to 10 GHz
and the cost of such device can reach the
20,000 mark. However, for the phase-shift method, the cost
of a phase detector is under
400. The phase-shift method was chosen because of its cost eectiveness,
therefore, the experiments described in Section 3are all based on the phase-shift method.
3. Experiments
3.1. Experimental Setup
Based on results from the k-Wave simulations, a phase-shift experiment was set up as shown in
Figure 6.
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Figure 6. Ultrasonic phase-shift experimental setup.
A sinusoidal waveform was generated using a Spectrum M2i.6022-exp arbitrary waveform
generator (Spectrum Instrumentation, Ahrensfelder, Grosshansdorf, Germany) . The waveform was
then sent to the input A port of the Analog Devices AD8302 (Analog Devices, Norwood, MA, USA)
phase detection board and the transmitter probe of a 5 MHz ultrasonic transducer. Using steel (type
EN24T) as the medium of propagation, the received signal from the receiver probe of the transducer
was then sent to the input B port of the phase detector. The phase detector calculates the difference
in phase between the transmitted and the received signal and sends the voltage equivalent as the
phase difference value through the phase-out port of the phase detector board. To reduce noise from
the ‘phase’ value, a low pass filter was used to cut off high frequency signals. A 3.4 Hz filter was
used, however, a cut off frequency of similar value will give satisfactory results. An NI-9239 analogue
input card (National Instruments, Austin, TX, USA) was used for data acquisition which was then
stored using NI LabVIEW. A calibrated Maxim DS18B20 digital temperature sensor was used as
reference and the measured temperature was recorded using WinTCal software, which is proprietary
to the research group. A PT100 temperature sensor was used for monitoring finer resolution changes
of 0.1 °C. Using a thermostatic temperature controller, a ceramic heater was used to gradually change
the temperature of the steel workpiece in predefined steps. An ultrasonic transducer with a center
frequency of 5 MHz and approximate bandwidth of 3 MHz was used. The effect of the change in
temperature on the phase of the ultrasonic wave was recorded and will be discussed in the results
section.
3.2. Experimental Results
Using the setup described in Section 3.1, the AD8302 board was used to obtain the voltage
equivalent values of phase-shift. A typical curve showing the relationship between the phase output
(V) and the equivalent phase difference (Degrees) is given in Figures 7 and 8, respectively. Figure 7
is the response specified in the datasheet while Figure 8 was obtained experimentally using the same
parameters as those used for the phase-shift experiments 2 V input, 40 mV received signal and
5 MHz ultrasonic frequency.
Figure 6. Ultrasonic phase-shift experimental setup.
A sinusoidal waveform was generated using a Spectrum M2i.6022-exp arbitrary waveform
generator (Spectrum Instrumentation, Ahrensfelder, Grosshansdorf, Germany) . The waveform was
then sent to the input A port of the Analog Devices AD8302 (Analog Devices, Norwood, MA, USA)
phase detection board and the transmitter probe of a 5 MHz ultrasonic transducer. Using steel (type
EN24T) as the medium of propagation, the received signal from the receiver probe of the transducer
was then sent to the input B port of the phase detector. The phase detector calculates the dierence in
phase between the transmitted and the received signal and sends the voltage equivalent as the phase
dierence value through the phase-out port of the phase detector board. To reduce noise from the
‘phase’ value, a low pass filter was used to cut ohigh frequency signals. A 3.4 Hz filter was used,
however, a cut ofrequency of similar value will give satisfactory results. An NI-9239 analogue input
card (National Instruments, Austin, TX, USA) was used for data acquisition which was then stored
using NI LabVIEW. A calibrated Maxim DS18B20 digital temperature sensor was used as reference and
the measured temperature was recorded using WinTCal software, which is proprietary to the research
group. A PT100 temperature sensor was used for monitoring finer resolution changes of 0.1
C. Using
a thermostatic temperature controller, a ceramic heater was used to gradually change the temperature
of the steel workpiece in predefined steps. An ultrasonic transducer with a center frequency of 5 MHz
and approximate bandwidth of 3 MHz was used. The eect of the change in temperature on the phase
of the ultrasonic wave was recorded and will be discussed in the results section.
3.2. Experimental Results
Using the setup described in Section 3.1, the AD8302 board was used to obtain the voltage
equivalent values of phase-shift. A typical curve showing the relationship between the phase output
(V) and the equivalent phase dierence (Degrees) is given in Figures 7and 8, respectively. Figure 7is
the response specified in the datasheet while Figure 8was obtained experimentally using the same
parameters as those used for the phase-shift experiments
2 V input, 40 mV received signal and 5 MHz
ultrasonic frequency.
J. Manuf. Mater. Process. 2019,3, 80 9 of 12
J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 9 of 12
Figure 7. Phase Output vs. Input Phase Difference [23].
Figure 8. Phase Output vs. Input Phase Difference using the same parameters for the experiment.
Based on the phase-shift simulation, a steel sample of 15 mm (type EN24T) was used for the
ultrasonic experiment. With a 5 MHz transducer signal, measurements were made for a temperature
range of 20 to 30 °C. The choice of this temperature range is based on a typical temperature variation
during manufacturing processes. For the first experiment, temperature was varied in steps of 1 °C—
thereafter, another experiment was carried out from the range of 21.5 to 23.5 °C in 0.1 °C steps. The
‘Phase Out’ values are the output from AD8032 phase detection board, which represents the phase-
shift between the transmitted and the received ultrasonic signals. The recorded phase out vs.
temperatures for both experiments are given in Figures 9 and 10.
Figure 7. Phase Output vs. Input Phase Dierence [23].
J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 9 of 12
Figure 7. Phase Output vs. Input Phase Difference [23].
Figure 8. Phase Output vs. Input Phase Difference using the same parameters for the experiment.
Based on the phase-shift simulation, a steel sample of 15 mm (type EN24T) was used for the
ultrasonic experiment. With a 5 MHz transducer signal, measurements were made for a temperature
range of 20 to 30 °C. The choice of this temperature range is based on a typical temperature variation
during manufacturing processes. For the first experiment, temperature was varied in steps of 1 °C—
thereafter, another experiment was carried out from the range of 21.5 to 23.5 °C in 0.1 °C steps. The
‘Phase Out’ values are the output from AD8032 phase detection board, which represents the phase-
shift between the transmitted and the received ultrasonic signals. The recorded phase out vs.
temperatures for both experiments are given in Figures 9 and 10.
Figure 8. Phase Output vs. Input Phase Dierence using the same parameters for the experiment.
Based on the phase-shift simulation, a steel sample of 15 mm (type EN24T) was used for the
ultrasonic experiment. With a 5 MHz transducer signal, measurements were made for a temperature
range of 20 to 30
C. The choice of this temperature range is based on a typical temperature variation
during manufacturing processes. For the first experiment, temperature was varied in steps of
1C—thereafter, another experiment was carried out from the range of 21.5 to 23.5 C in 0.1 C steps.
The ‘Phase Out’ values are the output from AD8032 phase detection board, which represents the
phase-shift between the transmitted and the received ultrasonic signals. The recorded phase out vs.
temperatures for both experiments are given in Figures 9and 10.
J. Manuf. Mater. Process. 2019,3, 80 10 of 12
J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 10 of 12
(a) (b)
Figure 9. Results of 20 to 30 °C range in 1 °C steps (a) Phase output vs. temperature; (b) Residual plot.
(a) (b)
Figure 10. Results of 21.5 to 23.5 °C range in 0.1 °C steps (a) Phase output vs. temp; (b) Residual plot.
4. Discussion
The simulation, as well as experimental results, confirm that with the phase-shift technique,
temperature change in a material can be measured, these results agree with the reviewed literature
and other existing studies based on the dependence of ultrasonic velocity on temperature. The
achievable measurement resolution as well as the range of measurement depend on the choice of
frequency. Ultrasonic frequencies for phase-shift measurement need to be chosen to suit the expected
range of measurement and the use of wideband transducers covering the expected range of
measurement can reduce the cost of the test set up. For this experiment, where a 15 mm steel plate
was used, the range can be increased with the selection of suitable frequencies based on
Equations (3) and (5). Lower frequencies will result in a higher range, but with lower resolution. It is
possible to use the TFcw method with low and high frequency transducers—this will improve both
the resolution and the range. Two major limitations of the AD8302 board are the lack of clarity on the
sign of the phase difference based on the reading obtained from the phase output in voltage and the
non-linearity experienced at the extremes as shown in Figures 7 and 8. It is possible to avoid the non-
linear regions by carefully selecting the transducer frequency for the length of the material whose
temperature is to be measured. Changes in temperature can be measured reliably over the linear
region, even when the sign of the phase is not known.
Figure 9.
Results of 20 to 30
C range in 1
C steps (
a
) Phase output vs. temperature; (
b
) Residual plot.
J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 10 of 12
(a) (b)
Figure 9. Results of 20 to 30 °C range in 1 °C steps (a) Phase output vs. temperature; (b) Residual plot.
(a) (b)
Figure 10. Results of 21.5 to 23.5 °C range in 0.1 °C steps (a) Phase output vs. temp; (b) Residual plot.
4. Discussion
The simulation, as well as experimental results, confirm that with the phase-shift technique,
temperature change in a material can be measured, these results agree with the reviewed literature
and other existing studies based on the dependence of ultrasonic velocity on temperature. The
achievable measurement resolution as well as the range of measurement depend on the choice of
frequency. Ultrasonic frequencies for phase-shift measurement need to be chosen to suit the expected
range of measurement and the use of wideband transducers covering the expected range of
measurement can reduce the cost of the test set up. For this experiment, where a 15 mm steel plate
was used, the range can be increased with the selection of suitable frequencies based on
Equations (3) and (5). Lower frequencies will result in a higher range, but with lower resolution. It is
possible to use the TFcw method with low and high frequency transducers—this will improve both
the resolution and the range. Two major limitations of the AD8302 board are the lack of clarity on the
sign of the phase difference based on the reading obtained from the phase output in voltage and the
non-linearity experienced at the extremes as shown in Figures 7 and 8. It is possible to avoid the non-
linear regions by carefully selecting the transducer frequency for the length of the material whose
temperature is to be measured. Changes in temperature can be measured reliably over the linear
region, even when the sign of the phase is not known.
Figure 10.
Results of 21.5 to 23.5
C range in 0.1
C steps (
a
) Phase output vs. temp; (
b
) Residual plot.
4. Discussion
The simulation, as well as experimental results, confirm that with the phase-shift technique,
temperature change in a material can be measured, these results agree with the reviewed literature and
other existing studies based on the dependence of ultrasonic velocity on temperature. The achievable
measurement resolution as well as the range of measurement depend on the choice of frequency.
Ultrasonic frequencies for phase-shift measurement need to be chosen to suit the expected range of
measurement and the use of wideband transducers covering the expected range of measurement can
reduce the cost of the test set up. For this experiment, where a 15 mm steel plate was used, the range
can be increased with the selection of suitable frequencies based on Equations (3) and (5). Lower
frequencies will result in a higher range, but with lower resolution. It is possible to use the TFcw
method with low and high frequency transducers—this will improve both the resolution and the range.
Two major limitations of the AD8302 board are the lack of clarity on the sign of the phase dierence
based on the reading obtained from the phase output in voltage and the non-linearity experienced at
the extremes as shown in Figures 7and 8. It is possible to avoid the non-linear regions by carefully
selecting the transducer frequency for the length of the material whose temperature is to be measured.
Changes in temperature can be measured reliably over the linear region, even when the sign of the
phase is not known.
J. Manuf. Mater. Process. 2019,3, 80 11 of 12
5. Conclusions
This study showed that an ultrasonic measurement of the speed of sound in a metal based on
the phase-shift method can be used to obtain the core temperature of the metal with a resolution of
up to 0.1
C. Based on simulation results of the two main ultrasonic measurement techniques—the
pulse-echo and the phase-shift techniques—phase-shift is the less expensive technique for high
resolution ultrasonic thermometry in metals. The two-frequency continuous wave method (TFcw) and
multiple frequency continuous wave method (MFcw) are two improvements on the general phase-shift
method for longer range and finer resolution measurements. A simulation was performed to observe
the individual eects of expansion and change in ultrasonic velocity on time-of-flight. Based on the
simulation, ultrasonic velocity can be relied upon for measuring time of flight and, where necessary,
compensations can be made for the material expansion. Using a 5 MHz transducer, 15 mm steel
plate and varying the temperature from 20 to 30
C, a voltage equivalent of phase dierence was
obtained. Overall, the results demonstrate that phase-shift ultrasonic thermometry can be used for
core temperature measurement with a resolution of 0.1
C. A possible application of this study would
be for temperature monitoring during co-ordinate metrology, such as on a co-ordinate measuring
machine. As part of future work to deploy this setup in subtractive manufacturing, more experiments
will be undertaken to understand the eects of swarf and coolant on ultrasonic thermometry. Future
experiments will also be carried out to understand the eect of temperatures above 200
C on ultrasonic
velocity. Also, as dierent materials have dierent physical properties, ultrasonic thermometry must be
calibrated for the material of which the measurement is to be made. Future work will also address the
possibility of using ultrasonic thermometry to measure the temperature of a region or a point within
dierent materials. The use of switching algorithms to dierent frequencies for dierent materials and
material sizes will also be researched.
Author Contributions:
Formal analysis, O.F.O.; Funding acquisition, S.F.; Methodology, O.F.O. and S.F.; Software,
O.F.O.; Supervision, S.F., A.P.L. and N.S.M.; Writing–original draft, O.F.O.; Writing–review and editing, O.F.O.,
S.F., A.P.L. and N.S.M.
Funding:
This research was funded by the UK’s Engineering and Physical Sciences Research Council (EPSRC),
grant number EP/P006930/1.
Conflicts of Interest: The authors declare no conflicts of interest.
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©
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article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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