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Design and analysis of a new down-the-hole electromagnetic hammer driven by tube linear motor

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Percussive-rotary drilling is an effective method for the hard rock drilling. As hydraulic and pneumatic hammers have many problems especially in the deep core drilling, a new electromagnetic hammer driven by a tube linear motor is introduced in this study. The study mainly covers the mechanical structure and electromagnetic design of the hammer, respectively. Due to the direct use of electric power, the electromagnetic hammer has the advantage of high efficiency while it must meet the minimal percussive energy requirement under the limitation of drill pipe diameter. In this study, the validity of electromagnetic design is verified through the finite-element analysis software Ansoft/Maxwell 2D and numerical calculation of electromagnetic force. The working temperature in deep down-hole is relatively high, the influence of high temperature in deep hole on the electromagnetic thrust of the linear motor is then analysed. A motion model of linear motor hammer is established to calculate and analyse the impact power and frequency under different conditions. The results show that the performance of linear motor hammer can reach or even exceed the one of same diameter hydraulic hammer, especially in the low-frequency range. The prototype test also confirms the validity of the design.
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IET Electric Power Applications
Research Article
Design and analysis of a new down-the-hole
electromagnetic hammer driven by tube linear
motor
ISSN 1751-8660
Received on 6th April 2017
Revised 6th June 2017
Accepted on 23rd June 2017
E-First on 25th July 2017
doi: 10.1049/iet-epa.2017.0208
www.ietdl.org
Tao Wu1,2, Yiru Tang1, Shengwen Tang3 , Yongbo Li1,2, Wangyong He1,2, E Chen4
1School of Automation, China University of Geosciences (Wuhan), Wuhan, People's Republic of China
2Hubei Key Laboratory of Advanced Control and Intelligent Automation for Complex Systems, China University of Geosciences (Wuhan),
Wuhan, People's Republic of China
3State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, People's Republic of China
4Department of Civil and Environment Engineering, The Hong Kong University of Science and Technology, Kowloon, Hong Kong
E-mail: tangshengwen1985@163.com
Abstract: Percussive-rotary drilling is an effective method for the hard rock drilling. As hydraulic and pneumatic hammers have
many problems especially in the deep core drilling, a new electromagnetic hammer driven by a tube linear motor is introduced in
this study. The study mainly covers the mechanical structure and electromagnetic design of the hammer, respectively. Due to
the direct use of electric power, the electromagnetic hammer has the advantage of high efficiency while it must meet the minimal
percussive energy requirement under the limitation of drill pipe diameter. In this study, the validity of electromagnetic design is
verified through the finite-element analysis software Ansoft/Maxwell 2D and numerical calculation of electromagnetic force. The
working temperature in deep down-hole is relatively high, the influence of high temperature in deep hole on the electromagnetic
thrust of the linear motor is then analysed. A motion model of linear motor hammer is established to calculate and analyse the
impact power and frequency under different conditions. The results show that the performance of linear motor hammer can
reach or even exceed the one of same diameter hydraulic hammer, especially in the low-frequency range. The prototype test
also confirms the validity of the design.
1 Introduction
Rotary drilling is widely used in the oil and gas exploration,
however, it is suffered from low penetration rate, high values of
weight-on-bit (W) and excessive bit wear, especially in hard rock
formations. While percussive drilling has the advantages of high
efficiency in hard rock formations [1–3]. Taking into account that
the limitations of the rotary drilling technique can be overcome by
percussive drilling and vice versa, the percussive-rotary drilling has
been considered as a promising approach to improve drilling
performance in hard rock formations [4]. This drilling usually
adopts a down-the-hole (DTH) hammer with a certain impact
frequency and impact power in the rotary drilling tool. The
hammer in most cases is installed at the top of core tube in core
drilling, yet installed at the top of the drill bits in hole drilling.
Nowadays, there are two main types of hammer utilised in
geological and petroleum drilling engineering: hydraulic hammers
and pneumatic hammers. However, it is worth pointing out that
there are three obvious disadvantages of these hammers: (i)
because of the remarkable leakage liquid and gas, the transmission
efficiency of hydraulic and pneumatic hammers is a bit low; (ii) the
application of hydraulic and pneumatic hammers for horizontal and
directional drillings has certain restrictions, caused by gravity force
of drilling fluid; (iii) hammers are dependent on the drilling fluid,
and thus, they cannot be applied to special drilling occasions
without drilling fluid or other circulation without liquid gravity.
Compared with traditional hydraulic and pneumatic hammers,
DTH electromagnetic hammer has a great deal of advantages: (i)
the hammer is suitable for inner hole space of drill pipe and deep-
hole drilling due to large electromagnetic thrust; (ii) the power of
this hammer and impact frequency can be flexibly adjusted
according to different working requirements; (iii) the hammer
possesses high mechanical efficiency, low cost for manufacture and
follow-up maintenance. As early as 1970s, Engel and Erdmann [5],
and Roubicek et al. [6] considered that electromagnetic thrust had
the potential to drive percussion drilling. They further studied on
the feasibility of utilisation of linear motor for driving impact tool,
and found that this structure of motor was very suitable for the
down-hole electric hammer. However, since the down-hole power
cannot be resolved at that time, the realisation of this kind of motor
or hammer had been laid on the table for a long time. While with
the development of power battery technology, the appearance of
high power density rechargeable battery provides a feasible way
for DTH electromagnetic hammers to work for a comparably long
time, particularly suitable for wire-line core drilling, in which case
power battery can be replaced when the core barrel (tube) is lifted.
Subsequently, Bekken [7] and Zhang et al. [8] put forward a DTH
hammer drilling system that employed a tube permanent magnet
synchronous motor to drive reciprocating and translational motion.
The tubular linear permanent magnetic machine mentioned in [7,
8] was short stroke (<5 mm) with high percussion rates ranging
from 20 to 71 Hz. It produced a highly frequent oscillating motion
while its single percussive energy was very small (when the
diameter of stator was below 70 mm, its energy was <0.5 J). The
winding was on stator, while permanent magnet was on mover that
was connected by springs on the upper and top ends. This structure
had the disadvantage of magnetic loss in motor impacting and
vibrations. Zhang et al. [9] also discussed the mathematical models
and dynamic analysis of this impact procedure of hammer by
winding currents calculation used in common permanent magnet
synchronous motor model.
In this work, a new type of DTH electromagnetic hammer
driven by the tubular linear motor is discussed. The
electromagnetic hammer is designed for reaching actual drill
performance. According to previous investigations by Melamed et
al. [10], the parameters of geological drilling hydraulic hammer are
as shown in Table 1. Main design parameters of our new
electromagnetic hammer are shown in Table 2; its impact power
can reach the target of hydraulic hammer ‘G112V’. When the
supply power decreases about 96%, the diameter and mass also
decrease greatly.
This paper is aimed to design the structure of electromagnetic
hammer to meet drilling requirements, the pipe limitation and
electric–mechanic requests. For example, the structure design must
IET Electr. Power Appl.
© The Institution of Engineering and Technology 2017
1
consider the request of structural strength and fatigue life [11].
Here we mainly concern about the electromagnetic performance.
From the perspective of motor design principle, the windings and
iron core is the mover while the permanent magnet is the stator to
prevent magnet loss in impact and vibrations. According to the
design parameters, the main structural and electrical dimensions of
this hammer is obtained by primary calculation. For the impact
power and frequency are primarily determined by the
electromagnetic force and up-down distance, Ansoft/Maxwell 2D
is used to verify the electromagnetic parameters (force) through the
finite-element analysis. The simulation results show that
electromagnetic force and magnetic field distribution of the motor
are satisfied with the primary design. Next, to analyse the
performance and possibility of the hammer in deep hole, a more
rapid numerical analysis model for calculating electromagnetic
force is introduced. The influence of temperature on the
electromagnetic thrust will be calculated and analysed through this
model. Then the motion dynamic model is established and the
impact power and impact frequency of this electromagnetic
hammer are studied to compare with the hydraulic hammer with
same specifications. Finally, a prototype test also validates the
impact energy and frequency of this new hammer under ambient
temperature.
2 Design of electromagnetic hammer
2.1 Structure of the electromagnetic hammer
The electromagnetic hammer driven by a tube linear motor
transforms electromagnetic energy into kinetic energy to accelerate
the hammer by electromagnetic thrust acting [12]. The coil of
linear motor adopts three-phase symmetry AC winding, generating
a travelling wave magnetic field that moves in the same direction
with rotor when symmetrical three-phase alternating current goes
through into coil. As the magnetic gap is usually fixed at 0.5–1 
mm, the electromagnetic thrust remains constant and maintains
peak value. By changing the electricity frequency and switching
the positive and negative frequency, it is very convenient to control
the movement speed of the secondary mover. The structure of tube
linear motor is suitable for the inner hole space of drill pipe, by
increasing or decreasing longitudinal coil length, it can attain
different impact power [13]. There are many kinds of liner motors
in which the electromagnetic thrust of permanent magnet
synchronous motor is the largest, and the efficiency is also the
highest. There are two structure schematic diagrams of the tube
linear synchronous motor: (i) the structure of moving-iron type: the
coil is fixed, while the magnetic steel and iron core are moved; (ii)
the structure of moving-coil type: the magnetic steel and iron are
fixed, while the coil is moved. In order to prevent the possible loss
of excitation of permanent magnets during the impact, a new type
of electric hammer with moving-coil type structure is developed.
The actual design structure of hammer is shown in Fig. 1. Part 1
is an impact anvil. In order to eliminate the impact rebound stress
wave that may lead to serious damage to the key parts of the motor,
and guarantee the service life of the motor, the rear end of the
hammer is provided with a buffer cushion (part 2). Parts 3 and 5
are the stator magnetic rod (toroidal permanent magnet) and mover
(coil), respectively. In order to ensure the smooth motion of the
mover, the rolling bearing (part 4) is used to reduce the friction
between the mover and the shell of hammer (part 6). Part 7 is the
buffer spring.
2.2 Electromagnetic parameters of the motor
In the preliminary electromagnetic design of motor, the original
data of motor design is given from Table 3 according to the
possible design limitation and conditions [14–16]. The inner
diameter of drilling pipes is 72 mm (small-size pipe).
Considering the structural strength of the hammer, the outer radius
of motor is <62 mm. As there is no limitation on axial length of the
drill pipe, large polar distance and multiple polar are adopted in our
work to enhance the power capacity. These data include the
required technical indicators in the condition of starting and the
synchronisation speed. In this work, the actual main structure
parameters of motor are shown in Table 4.
3 Simulation of electromagnetic thrust by Ansoft/
Maxwell 2D
With the purpose of further validating the model above, the
simulation of electromagnetic thrust of tubular linear motor is
Table 1Parameters of geological drilling hydraulic hammer
Parameter G59V G76V G112V G134V Unit
diameter 59 76 110 134 mm
single impact power 4.5–6.5 8–12 60 100 J
impact frequency 30–40 30–45 15 15 Hz
length 1635 1845 2010 3590 mm
mass 23 39 95 305 kg
supply power (mud pumpa)10–25 25–45 40–60 60–80 kW
aIn drilling project, the mud pump power is generally equipped with much large capacity for the power loss by various reasons.
Table 2Design parameters of electromagnetic hammer
Parameter Value Unit
diameter 67 Mm
length of stator 1200 Mm
length of motor 900 mm
total length 1580 mm
maximum move distance 400 mm
mass 26 kg
impact power 40–80 J
impact frequency 5–10 Hz
supply power 1.6 kW
Fig. 1  Structure of hammer driven by tube linear motor
Table 3Original data of tubular linear motor design
Parameter Description Value Unit
Fstarting electromagnetic thrust 300 N
cos φstarting power factor 0.9
ηstarting synchronisation efficiency 0.9
U1primary phase voltage 220 V
fpower supply frequency 50 Hz
Vssynchronisation speed 9 m/s
2IET Electr. Power Appl.
© The Institution of Engineering and Technology 2017
developed based on finite-element analysis software Ansoft/
Maxwell 2D in this work. The proposed simulation process
consists of three steps: pre-processing, operating calculation and
post-processing. Pre-processing includes model building and
determination of material properties of various parts of motor;
operating calculation incorporates with constructions of boundary
conditions, excitation source, perform parameter and solving
option, mesh generation and motion setting and calculation; post-
processing takes charge of plotting the magnetic figure and
magnetic through distribution map and all kinds of calculations
required [17].
The main parameters in the simulation are selected in Tables 3
and 4. Therefore, the curve of magnetic field of motor and thrust
changing of liner motor along with time can be obtained. Fig. 2a is
a grid map of two-dimensional finite element of liner motor that is
a necessary part of the finite-element analysis of the motor. Fig. 2b
is the distribution of magnetic flux working temperature 20°C.
From this figure, the maximal magnetic flux density in the air gap
is about 0.773 T and the average flux density is 0.5 T. Fig. 3 is the
electromagnetic thrust curve of tube linear motor, and it can be
seen that the thrust of tube liner motor fluctuates slightly which is
mainly due to the detent force and thrust ripple. The formation of
detent force is mainly composed of two forces caused by end and
slot effects, respectively. A lot of literature has been carried out on
the analysis of the permanent magnet linear synchronous motor slot
effect force [18–19]. While thrust ripple comes from the primary
coil current and back-electromotive force (EMF) harmonics, and it
generally contains six times' harmonic [20]. So the simulation
result is reasonable, and it can be concluded that the average thrust
is about 310 N which is basically consistent with the design criteria
(F= 300 N), and the fluctuation range is about 80 N.
4 Performance analysis of the hammer in drilling
especially in deep hole
Currently the widely used hydraulic hammer has many problems in
deep hole drilling for high temperature and other technical
difficulties that cannot be overcome. So it is significant to discuss
the performance of the electric–magnetic hammer in deep hole.
Here we first discuss the properties of permanent magnets under
high temperature and a numerical analysis model of thrust of the
motor is introduced. Then the variety of thrust under different
temperature is analysed. By comparison of the results of Ansoft/
Maxwell and numerical analysis model, it can be concluded that
the results of both are basically the same. According to the latter,
we can easily analyse the performance of electric–magnetic
hammer in different depths.
4.1 Magnetic properties of permanent magnets in high
temperature
With the increase of depth in deep hole drilling, the ambient
temperature of impact motor may reach to 120°C or even 300°C.
However, the maximum operating temperature of NdFeB magnet is
generally below 150°C, so SmCo magnet (Sm2Co17YXG − 24),
which is produced by Ningbo Ninggang Permanent Magnet
Material Co., Ltd, is selected as the permanent magnet material in
this design. The main performance parameters of the material are
shown in Table 5 and Fig. 4 [21].
The relative permeability of permanent magnetic materials can
be expressed as
Table 4Main dimensions of prototype
Description Value Unit
outer radius of stator permanent magnet pole 10 mm
inside radius of stator permanent magnet pole 4 mm
outer radius of rotor core 31 mm
inside radius of rotor core 11 mm
primary core length 900 mm
polar distance 90 mm
mechanical air gap length 1 mm
polar logarithm 5
Fig. 2  Simulation results of motor by Ansoft/Maxwell 2D software
(a) Two-dimensional diagram finite-element mesh of liner motor, (b) Flux distribution of the linear motor
Fig. 3  Electromagnetic thrust curve of tube linear motor
Fig. 4  Analytical calculation of electromagnetic thrust curve under
different temperature
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μr=μrec/μ0=Br/(HcBμ0)
(1)
where μ0 is vacuum permeability and μ0= 4π× 10−7 H/ m . As is
shown in Fig. 4, the remanence Br and coercive force HcB of
permanent magnet under different temperatures can be obtained,
and thus, according to (1), the relative recovery permeability of
different temperatures is calculated as shown in Table 6.
4.2 Accurate numerical analysis model of electromagnetic
thrust
Although the permanent magnet motor has outstanding advantages,
they are susceptible to the service temperature. It is well
documented that the remanence of permanent magnetic materials
will reduce significantly with increasing temperature, leading to the
reduction of magnetic flux density of the air gap magnetic field of
the motor and declining electromagnetic thrust [22–24]. Therefore,
the theoretical calculation is needed to investigate the influence of
high temperature on electromagnetic impact of motor. The tubular
permanent magnet linear synchronous motor (TPMLSM) without
slot axial magnetisation is shown in Fig. 5. As shown in Fig. 5, Ri
is radius of primary axis, R1 is outer radius of secondary magnet, R2
is outer radius of primary winding, hm is the thickness of
permanent magnet, τp is the polar distance, τw is axial width of the
coil, τwp is coil axial distance, g is air gap length.
Thrust force (Fz) on the armature is produced by the interaction
between winding current and permanent magnet magnetic field,
and it can be expressed as [25]
dFz(r,z) = 2πrBr0(r,z)Jmdrdzor Fz
=Nτp
τpr1
r2
2πrJmBr0(r,z)drdz
(2)
where Jm is the current density in coil conductors, Br0 is the radial
magnetic density in air gap. N is the number of the unit motor. The
distribution of magnetic motive force on the surface of inner iron
core can be considered as a trapezoidal wave. It can be
decomposed by 2n − 1 times' Fourier series as following:
Br0(r,z) =
n= 1
μ0(2n− 1)π|HpmF2n−1|
τpΔ1Δ3(r)sin (2n− 1)πz
τp
(3)
Where F2n− 1 = 4τpsin((2n− 1)πb/τp)/(2n− 1)2π2, Hpm is
permanent magnet working point magnetic field strength which
can be calculated by
Hpm =Br/[μ0(μr+σ)]
(4)
where
σ= 2R1n= 1
[(2n− 1)F2n− 1Δ3(R1)cos((2n− 1)πb/2τp)/(τpΔ1)]/(R1
2
Ri
2),
σ is a parameter that is only related to the geometry of the motor.
According to (2), the thrust of TPMLSM without slot can be
described as follows:
F=2Jm
n= 1
K2n− 1[cos((2n− 1)(zτp/2)π/τp)sin(ωt)
+cos((2n− 1)(z− 7τp/6)π/τp)sin(ωt − 2π/3)
+cos((2n− 1)(z+τp/6)π/τp)sin(ωt + 2π/3)]
(5)
where
K2n− 1 = − 4πpKwn(2n− 1)πμ0HpmbF2n−1τwN/τpΔ1R1
R2rΔ3(r) dr,
and Kwn =K1, 2n− 1K2, 2n− 1 is the winding coefficient of (2n − 1)
times, and K1, 2n− 1 = sin((2n− 1)πτwp/(2τp)) is the (2n − 1) times'
harmonic winding coefficient,
K2, 2n− 1 = sin((2n− 1)πτw/(2τp))/((2n− 1)π τw/(2τp)) is the (2n − 1)
times winding distribution coefficient. N is turns-in-series per-
phase; b is half of the axial length of the magnet; R1 is the
equivalent inner radius of winding and R2 is the outer radius of
winding
Δ1=I0((2n− 1)R2π/τp)K0((2n− 1)R1π/τp)
I0((2n− 1)R1π/τp)K0((2n− 1)R2π/τp)
(6)
Δ3(r) = I0((2n− 1)R2π/τp)K1((2n− 1)/τp)
+I1((2n− 1) /τp)K0((2n− 1)R2π/τp)
(7)
where I0 and K0 are the first and second kind of zero-order
modified Bessel functions, respectively; I1 and K1 are the first and
second kind of first-order modified Bessel functions, respectively.
With the increase of temperature, the permanent magnet
remanence and coercivity will change considerably, which affects
the motor air gap magnetic field and average current density to
some degree. According to (5), under the condition of current
density J= 40, 000 A /m2 in the coil conductor, the analytical
Table 5Physical properties of Sm2Co17YXG − 24
Description Parameter Value Unit
remanence Br0.95–1.02 T
coercive force HcB 692–764 kA/m
intrinsic coercive force HcJ ≥1433 kA/m
maximum magnetic energy product (BH)max 175–191 kJ/m3
curie temperature TC 800 °C
maximum operating temperature TW 300 °C
temperature coefficient of remanence α(Br)−0.025 %/°C
intrinsic coercivity temperature
coefficient
α(BcJ)−0.20 %/°C
Table 6Permanent magnet parameters under different temperatures
Parameter Unit Parameters under different temperatures
20°C 100°C 150°C 200°C 250°C 300°C
BrT 1.020 0.975 0.963 0.953 0.926 0.913
HcB kA/m 764 748 732 716 699 680
μr 1.062 1.0373 1.0469 1.0592 1.0542 1.0669
Fig. 5  Three-phase winding distribution for slotless axial magnetised
TPMLSM
4IET Electr. Power Appl.
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calculation results of electromagnetic thrust of TPMLSM under
different temperatures are shown in Fig. 6. It is shown that: at
20°C, the electromagnetic thrust fluctuates around 300 N; while the
service temperature increases from 100 to 300°C, thrust fluctuates
around from 280 to 190 N. It should be also noted that there is six
times fluctuating force ripple in thrust waveform, which is mainly
due to the presence of five and seven harmonics in the EMF
waveform.
5 Motion dynamic model and impact performance
Impact power and impact frequency play important roles in the
performance evaluation of the hammer. In the following section,
impact power and impact frequency of the electromagnetic
hammer under the normal temperature (20°C) are analysed from
the viewpoint of theory. Some typical parameters are adopted in
this analysis: the effective stroke of the mover (L) is 400 mm, and
the hammer (mover and impact device base) weight (m) is 6 kg.
According to the electromagnetic thrust F= 300 N, we can get the
down-stroke acceleration a= 6g and up-stroke acceleration
a= 4g. As the electromagnetic thrust is much larger than the
gravity, the reset stroke of the electromagnetic hammer will not use
reset spring, as shown in Fig. 1, just rely on the electromagnetic
thrust to complete the reset stroke. On the end of the up-stroke,
there is a spring with large elastic coefficient, which can buffer
momentum and convert it into the initial velocity of the down-
stroke.
The maximal velocity at the end of the shock device and the
maximal impact frequency are principally related to the effective
stroke of the mover. Fig. 7 is the working stroke of the hammer:
Fig. 7a is the up-stroke working diagram, the initial velocity (v0) of
hammer mover in lower limit is zero, and the hammer accelerates
upward under the electromagnetic and gravity force (G). Through
the effective stroke of the mover, the mover decelerates to a stop
when it touches the spring whose elastic coefficient is k in upper
limit (the speed is v1). Then under the action of gravity and spring
force (Fs), the mover begins to lower the stroke. Fig. 7b is the
down-stroke working diagram, the spring is in free status when the
mover is back to the upper limit, at this time the moving speed is
v3, and the electromagnetic thrust begins to work. The mover
accelerates downward under the action of gravity and downward
electromagnetic thrust, the end speed is vt.
The spring oscillator period (T) can be calculated by the
formula: T= 2π m/k, k is the elasticity coefficient of the spring, m
is the weight of the mover and impact device base. So a large
elastic coefficient of spring can be used to speed up the reverse
process, here the value of k is assumed as 250 N/mm. The
electromagnetic thrust of hammer motion should ignore the starting
process of the upper and lower parts. Using the constant torque
control mode can maintain electromagnetic thrust as maximum
value as possibly. For the sake of simplicity, when impact stroke is
100 mm, the double stroke time (t) is 0.1133 s, the end speed (vt)
is about 4.47 m/s, the impact kinetic energy (E) is 60 J and the
hammer frequency is about 8.83 Hz.
Fig. 8 is the relationship among impact stroke and impact
frequency and impact power that are calculated by MATLAB
program. The line for impact frequency decreases sharply with the
impact stroke range between 0 and 50 mm. The data trend becomes
smooth gradually with the increase of impact stroke. The main
reason may be attributed to that the speed of mover in initial stroke
is relatively low. Besides, it is noted that the linearity of blue line
(impact power) is presented in Fig. 8. The unique relation is
explained as that only electromagnetic thrust does effective work
during the whole impact process and the value of the thrust remains
constant.
From Fig. 8, several typical features can be found: (i) the
maximal impact kinetic energy can reach 240 J in 400 mm stroke
when the impact frequency is about 4.73 Hz; (ii) the impact
frequency is about 12 Hz and impact power is about 30 J when the
stroke is 50 mm; (iii) the impact frequency is about 18 Hz and
impact power is about 12 J when the stroke is 20 mm. It seems that
the theoretical calculation value of impact power is better than the
general performance index (the impact power of 73 mm domestic
hydraulic hammer is about 8 J when the impact frequency is 15 
Hz) of 73 mm Chinese domestic hydraulic hammer in the case of
low impact frequency ( < 20 Hz) [26]. Nevertheless, when the
frequency becomes higher, the performance of this hammer is
almost same or slightly worse, compared with the one of domestic
hydraulic hammer.
Fig. 9 is the curve of average electromagnetic thrust and impact
power of hammer under different temperatures in the deep hole
drilling. The date line for impact power is calculated by the one for
electromagnetic thrust in the condition of 400 mm impact stroke.
As is shown in Fig. 9, under the premise of keeping the air gap
magnetic field, the change of electromagnetic thrust and impact
power with the temperature is almost constant. However, under
normal circumstances, the air gap magnetic field will gradually
become weak with the increase of temperature. The
electromagnetic thrust decreases about 10.3% when the deep hole
temperature rises from 20 to 120°C, at the same time, impact
power reduces from 240 to 176 J.
6 Experiments of prototype
To validate the feasibility of the design above, we have used the
linear motor of electromagnetic hammer prototype to simulate the
simple impact test, and the experimental platform is shown in
Fig. 10a. The impact process of motor is controlled by a servo-
driver with the power line and hall line. In experiments, we use the
current loop control mode and two laser diffuse reflection
proximity switches as the up and down travel switch. The impact
stroke is about 100 mm, and the maximum operating current is set
from 7 to 10 A in the whole upper and lower strokes except for
short spring buffering switch time.
In this prototype, the end speed method is adopted to measure
the impact power, the experiment data is demonstrated in Table 7
and the curve of lower limitation sensor is shown in Fig. 10b. The
mass of the mover (m0) is 6 kg in this prototype, meanwhile, the
value of average end speed (vend) can be calculated by Table 7, so
the impact power (E) is calculated as following:
E=m0vend
2/2
(8)
According to (8) and Fig. 10b, the impact power is about 36.15 
J and impact frequency is 5 Hz. It can be found that the actual
impact power and frequency are smaller than the theoretical
calculation claimed in Section 5 (when the impact stroke is 100 
mm, the impact power and impact frequency is about 60 J and
8.83 Hz, respectively) which may result from the mechanical
friction and wind resistance in high-speed condition.
Notwithstanding, by compared with the similar diameter hydraulic
hammer, our electromagnetic hammer prototype has excellent
performance in impact power.
7 Conclusion
In this work, a new type of electromagnetic hammer is studied. By
constructing the model of the mechanical structure of the motor
and the electromagnetic calculation, the main parameters of the
motor are determined. Moreover, finite-element analysis software
Ansoft/Maxwell 2D is used to verify the validity of the
electromagnetic design. The analytic method is applied to
investigate the influence of high temperature on the
electromagnetic thrust of the linear motor, it can be concluded that
with the increase of temperature, the electromagnetic thrust
decreases gradually. Finally, a motion model of linear motor shock
is established to calculate and analyse the impact power and impact
frequency. The results show that the linear motor hammer
performance can reach or even exceed the hydraulic hammer with
same diameter, especially in the low-frequency range. It is
estimated by calculation that the linear motor hammer may drop
about 10.3% of electromagnetic thrust and impact power when the
deep-hole drilling reaches 3000 m, which can still meet the
requirements of impact drilling. The impact test of prototype also
validates the hammer design under ambient temperature. New type
of electromagnetic hammer has high drilling efficiency and is
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5
promising for the development of petroleum exploration toward the
deeper underground.
Indeed, some parameters of hammer are still uncertain, such as
electromagnetic thrust that is obtained only by the theoretical
calculation. Besides, the selection of permanent magnet material
has a great impact on the analysis results, which is needed to be
polished in the future work. Notwithstanding, this work may offer
a new perspective on deep-hole drilling.
8 Acknowledgments
The work was supported by the Open Research Fund of Research
Center for Advanced Control of Complex Systems and Intelligent
Geoscience Instrument, China University of Geosciences (Wuhan)
(no. AU2015CJ018), Natural Science Foundation Project of Hubei
Province (2014CFB903) and Geological Survey Project by China's
Ministry of Land and Resources (NO.1212011120255).
Fig. 6  Analytical calculation of electromagnetic thrust curve under different temperature
Fig. 7  Working stroke of the hammer
(a) Up-stroke working diagram, (b) Down-stroke working diagram
Fig. 8  Impact frequency and impact power with different impact strokes
6IET Electr. Power Appl.
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9 References
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Fig. 9  Average electromagnetic thrust and impact power curves under different temperatures and conditions
Fig. 10  Impact test platform
(a) Prototype and test platform, (b) Test measurement
Table 7Experiment data of end speed
Number End speed value, m/S
1 3.49
2 3.42
3 3.51
4 3.44
5 3.47
6 3.48
7 3.50
8 3.46
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8IET Electr. Power Appl.
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