Content uploaded by Tao Wu

Author content

All content in this area was uploaded by Tao Wu on Sep 23, 2019

Content may be subject to copyright.

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 1 Parameters 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 2 Design 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 3 Original 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 4 Main 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

IET Electr. Power Appl.

© The Institution of Engineering and Technology 2017

3

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

=N∫−τp

τp∫r1

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= 2R1∑n= 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)(z−τp/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/τpΔ1∫R1

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)rπ/τp)

+I1((2n− 1)rπ /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 5 Physical 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 6 Permanent 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.

© The Institution of Engineering and Technology 2017

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

IET Electr. Power Appl.

© The Institution of Engineering and Technology 2017

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.

© The Institution of Engineering and Technology 2017

9 References

[1] Wiercigroch, M., Wojewoda, J., Krivtsov, A.M.: ‘Dynamics of ultrasonic

percussive drilling of hard rocks’, J. Sound Vib., 2005, 280, (3–5), pp. 739–

757

[2] Franca, L.F.P.: ‘A bit–rock interaction model for rotary–percussive drilling’,

Int. J. Rock Mech. Min., 2011, 48, (5), pp. 827–835

[3] Saksala, T., Gomon, D., Hokka, M., et al.: ‘Numerical and experimental study

of percussive drilling with a triple-button bit on Kuru granite’, Int. J. Impact

Eng., 2014, 72, (4), pp. 56–66

[4] ‘Advanced Simulation Technology for Combined Percussion and Rotary

Drilling and Cuttings Transport’. Available at http://www.geomechanics-

technologies.com/article/GasTip.pdf, accessed 2005

[5] Engel, L., Erdmann, W.: ‘Electromagnetic linear motor’, Bergbauwiss, 1969,

16, (9–10), pp. 321–325

[6] Roubicek, O., Pejsek, Z., Tusla, P.: ‘Possibilities of applying an electric

translation drive (with linear motion) to mine drilling hammers. (K

moznostem elektrickeho translacniho)primocareho(pohonu dulniho vrtaciho

kladiva)’, Elektrotech Obz, 1971, 60, (4), pp. 202–207

[7] Bekken, R.S.: ‘Experimental testing of a tubular linear permanent magnetic

machine for percussion drilling in hard rocks’. MS thesis, Norwegian

University of Science and Technology., 2011

[8] Zhang, S., Norum, L., Nilssen, R.: ‘Oscillatory motion application of tubular

linear permanent magnet machine’. Proc. IEEE Annual Conf. (IECON),

November 2009, pp. 1223–1227

[9] Zhang, S., Norum, L., Nilssen, R., et al.: ‘Down-the-hole hammer drilling

system driven by a tubular reciprocating translational motion permanent

magnet synchronous motor’. Proc. IEEE Int. Symp. (ISIE), May 2012, pp.

647–651

[10] Melamed, Y., Kiselev, A., Gelfgat, M., et al.: ‘Hydraulic hammer drilling

technology: developments and capabilities’, J. Energy Resour., ASME, 2000,

122, (122), pp. 1–7

[11] Wu, T., Wang, W., Yao, A., et al.: ‘Research on impact stress and fatigue

simulation of a new down-to-the-hole impactor based on ANSYS’, J. Inst.

Eng., 2016, pp. 1–8

[12] Wang, J., Howe, D., Lin, Z.: ‘Design optimization of short-stroke single-

phase tubular permanent-magnet motor for refrigeration applications’, IEEE

Trans. Ind. Electron., 2010, 57, (1), pp. 327–334

[13] Chiang, L.E., Elías, D.A.: ‘Modeling impact in down-the-hole rock drilling’,

Int. J. Rock Mech. Min., 2000, 37, (4), pp. 599–613

[14] Tootoonchian, F., Nasiri-Gheidari, Z.: ‘Cogging force mitigation techniques in

a modular linear permanent magnet motor’, IET Electr. Power Appl., 2016,

10, (7), pp. 667–674

[15] Garcia-Amorós, J., Molina, B.B., Andrada, P.: ‘Modelling and simulation of a

linear switched reluctance force actuator’, IET Electr. Power Appl., 2013, 7,

(5), pp. 350–359

[16] Wang, J., Howe, D.: ‘Tubular modular permanent-magnet machines equipped

with quasi-Halbach magnetized magnets-part I: magnetic field distribution,

EMF, and thrust force’, IEEE Trans. Magn., 2005, 41, (9), pp. 2470–2478

[17] Lv, G., Liu, Z., Sun, S.: ‘Analysis of forces in single-side linear induction

motor with lateral displacement for linear metro’, IET Electr. Power Appl.,

2016, 10, (1), pp. 1–8

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 7 Experiment 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

IET Electr. Power Appl.

© The Institution of Engineering and Technology 2017

7

[18] Schlensok, C., Gracia, M.H., Hameyer, K.: ‘Combined numerical and

analytical method for geometry optimization of a PM motor’, IEEE Trans.

Magn., 2006, 42, (4), pp. 1211–1214

[19] Masoudi, S., Feyzi, M.R., Sharifian, M.B.B.: ‘Force ripple and jerk

minimisation in double sided linear switched reluctance motor used in

elevator application’, IET Electr. Power Appl., 2016, 10, (6), pp. 508–516

[20] Lim, K.C., Woo, J.K., Kang, G.H., et al.: ‘Detent force minimization

techniques in permanent magnet linear synchronous motors’, IEEE Trans.

Magn., 2002, 38, (2), pp. 1157–1160

[21] Zhang, J., Zhang, S., Zhang, H., et al.: ‘Structure, magnetic properties, and

coercivity mechanism of nanocomposite SmCo 5/α-Fe magnets prepared by

mechanical milling’, J. Appl. Phys., 2001, 89, (10), pp. 5601–5605

[22] Dwivedi, A., Singh, S.K., Srivastava, R.K.: ‘Analysis of permanent magnet

brushless AC motor using Fourier transform approach’, IET Electr. Power

Appl., 2016, 10, (6), pp. 539–547

[23] Mohammadpour, A., Gandhi, A., Parsa, L.: ‘Winding factor calculation for

analysis of back EMF waveform in air-core permanent magnet linear

synchronous motors’, IET Electr. Power Appl., 2012, 6, (5), pp. 253–259

[24] Kou, B., Li, L., Zhang, C.: ‘Analysis and optimization of thrust characteristics

of tubular linear electromagnetic launcher for space-use’, IEEE Trans. Magn.,

2009, 45, (1), pp. 250–255

[25] Zhao, J.H., Zhang, X.F., Zhang, J.H., et al.: ‘Field and thrust analysis of

tubular permanent magnet linear synchronous motor’, Electr. Mach. Control,

2010, 22, (3), pp. 60–63 (in Chinese)

[26] Su, C.S., Sun, J.H., An, G.J.: ‘YZ-73 type hydraulic shock device’, Explor.

Eng., 1989, (4), pp. 17–20 (in Chinese)

8IET Electr. Power Appl.

© The Institution of Engineering and Technology 2017