fo

logy,

msanthep vto

ly sutifu

a exible brake for a belt conveyor.

downwcoal snologys, coalarge dian dec

cost [710]. We will show that control of such a disc braking sys-tem is the key technology to guarantee reliability in its operation,but also suggest that this issue requires an urgent solution.

2. Braking velocity curves

Because a conveyor belt is a visco-elastic body, uctuations instress should be avoided in the process of braking in order to

eration, shortens its service life and is not good for braking control.Therefore, we have used a Harrison curve to analyze the brakeprocess of a downward belt conveyor.

3. Disc braking device

A disc braking device, composed of a disc brake, a brake disc anda frame is shown in Fig. 3, where 1 the is the cylinder and piston, 2the disc spring, 3 the brake lining and 4 the brake disc; P is the oilpressure and S the gap between brake lining and brake disc. The

Mining Science and Technology (China) 21 (2011) 491494

Contents lists availab

T

.e Corresponding author. Tel.: +86 15262925872.mines considerably, reduce investment in capital construction,shorten construction periods and generate marked social and eco-nomic benets [1,2]. The principle of a downward belt conveyortransmission scheme is shown in Fig. 1 [3].

At present, the braking system of a downward belt conveyorused domestically, largely consists of hydraulic pressure, hydraulicpower and a disc braking system [46]. Such disc braking systemshave been widely used, due to their outstanding advantages of asingle-stage brake, simple structure, mature technology and low

Current commonly used braking velocity curves are shown inFig. 2. It is seen that the linear curve has a sudden change in decel-eration both at the starting and the end points of the curve, wherethe derivative of deceleration approaches innity. The Nordellcurve has a sudden change in deceleration at point T/2, wherethe derivative of deceleration is discontinuous. The Harrison curveis continuous without a sudden change in deceleration. The impactof tension within a conveyor, caused by a sudden change of decel-Optimal PIDControl strategy

1. Introduction

Towards the end of the 1980s aused, for the rst time, on an inclinedven our progress in science and techin production at inclined coal seammore long-distance, large-capacity, lconveyors. The use of conveyors c1674-5264/$ - see front matter 2011 Published bydoi:10.1016/j.mstc.2011.06.005

E-mail address: cumtxfw@126.com (F. Xie). 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

ard belt conveyor waseam in our country. Gi-as well as the increasemines need more andp angle downward beltrease the workload in

reduce the dynamic load on the belt, drum, roller and frame ofthe conveyor. The ideal braking process of a downward beltconveyor should have the following characteristics [11,12]:

(1) avoid braking for too long when the deceleration of thebrake is less than the stipulated deceleration;

(2) makethemaximumbrakingdecelerationassmallaspossible;(3) avoid sudden changes in braking deceleration.Downward belt conveyorDisc braking systemHarrison velocity curve

brake system. Our experimental results demonstrate that the optimal PID control can make the outputvelocity to follow a preset velocity correctly with only small uctuations, meeting the requirements ofControl strategy of disc braking systems

Hou Youfu a, Xie Fangwei b,, Huang Fei aa School of Mechanical and Electrical Engineering, China University of Mining & Technob School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China

a r t i c l e i n f o

Article history:Received 15 November 2010Received in revised form 10 December 2010Accepted 4 January 2011Available online 13 July 2011

Keywords:

a b s t r a c t

Reliability of braking systeveyor brakes. By analyzingthat the Harrison curve isthe control in a closed-looWe used MATLAB/Simulinkmal PID control is especialysis and simulation, a mulMining Science and

journal homepage: wwwElsevier B.V. on behalf of China Unr downward belt conveyors

Xuzhou 221008, China

is a key requirement to ensure the safety of in using downward belt con-d comparing three commonly used braking velocity curves, we concludebest. Given the characteristics of a downward belt conveyor, we studiedelocity, a conventional PID method and an optimal PID control method.simulate the three control methods. Our simulation results show that opti-itable for disc braking systems. To verify the results from theoretical anal-nctional test-bed was developed to simulate the braking process of a discle at ScienceDirect

echnology (China)

l sevier .com/locate /mstcdisc braking device has a normal closed hydraulic control, i.e.,

iversity of Mining & Technology.

an input signal, the velocity curve of a conventional PID control isshown in Fig. 5. It can be seen that this conventional PID controlis characterized by a quick response with a small steady-state error,while the overshoot increased by 24%. The increase in the overshootmay cause uctuations in braking velocity, which impacts the con-

U0

UVelocity feedback

+ controllerhydraulic

valve system conveyor

Fig. 4. Flow chart of disc braking control system.

1.2

0.8

0.4

0 1 2 3 4 5

Conventional PID control

Velocity closed- loop control

Optimal PID control

Time (s)

y

Fig. 5. Step response of control system.

492 Y. Hou et al. /Mining Science and Technology (China) 21 (2011) 491494the brake lining is pressed to the brake disc by the disc springwhen there is no oil pressure, producing a maximum braking mo-ment. Regulating the input voltage (or current) of the proportionalelectro-hydraulic valve, the output oil pressure P can continuouslyadjust the braking moment. In this way, the braking velocity of adownward belt conveyor can be controlled in a exible manner.

4. Control strategy of disc braking system

4.1. Closed-loop velocity control

The disc braking system studied by us is a closed-loop velocitycontrol system, its ow chart described in Fig. 4. In this system theproportional electro-hydraulic valve is a core element; the qualityof this valve has an important effect on the performance of thesystem. The output oil pressure and the braking moment can beadjusted by changing the input voltage (or current) of the propor-tional electro-hydraulic valve, in order to control the brakingvelocity of the downward belt conveyor. Using a step function asan input signal, the dynamic performance of the system can besimulated by MATLAB/Simulink [1315]. The velocity curve of aclosed-loop control is shown in Fig. 5. It is seen that the responseis too slow and the steady-state error too large with a closed-loopvelocity control. Therefore, measures must be taken to correct thecontrol system.

4.2. Conventional PID control

To obtain a more ideal dynamic performance of the disc brakingcontrol system, a PID (Proportion Integration Differentiation)controller should rst be designed and installed for correcting thesystem. The adjustment of its parameters is very important in theuse of the PID controller. Because of its simple operation and out-

Fig. 1. Principle of downward belt conveyor transmission. (1) Tail bend pulley; (2)idler pulley; (3) bend pulley; (4) progressive drum; (5) driving drum and (6) headbend drum.standing effect, we used the ZieglerNichols method to adjust thePID parameters. The adjusted results show that the proportionalcoefcient kp 208:044, the integral coefcient ki 889:077 andthe differential coefcient kd 12:191. Using a step function as

Velocity curve

Deceleration curves Dec

Velo

T/2t

T

v0v0

,v v.

,v v

(a) Linear curve (b) Harr

.

Fig. 2. Braking velocity anFig. 3. Structural scheme of disc braking device.

Preset v U p Mz vConventional /optimal PID

Proportional electro - Disc braking

Downward belt veyor belt, implying that the requirements for exible control of thedownward belt conveyor are not met. Therefore, the parameters ofthe PID controller must be further optimized in order to decreasethe overshoot and reduce the impact on the braking system.

eleration curvesDeceleration curves

city curve Velocity curve

T/2T T

t

v0

,v v

ison curve (c) Nordell curve

.

d deceleration curves.

PID control are clearly reduced, where the overshoot is reducedfrom 24% to 4%. Reducing the overshoot causes the output velocityto follow the preset velocity quickly, while reducing the uctuationand impact on the conveyor belt in its braking process.

The velocity curve of the downward belt conveyor wassimulated by MATLAB and is shown in Fig. 6, where the initial beltvelocity was v0 = 2 m/s and the braking time t = 20 s. In this gure,the preset curve is a Harrison curve and the simulation curve thevelocity curve controlled by an optimal PID. It can be seen thatthere is a certain lag between the simulation and preset curves,but the lag is quite small. In general, the output velocity cancorrectly follow the preset velocity; the control method can actual-ize exible control of the downward belt conveyor.

The time for the simulation experiments was set at 40 s. The

1.6

2.0

1.2

0.8

0.4

0 4 8 12 16 20

Preset curve

Simulated curve

Time (s)

Vel

ocity

(m/s

)

Fig. 6. Velocity curve of optimal PID control.

Y. Hou et al. /Mining Science and Technology (China) 21 (2011) 491494 4934.3. Optimal PID control

There are two major parameter optimization technologies for

Fig. 7. Experimental equipment.PID control systems, i.e., a multivariable optimization technologyand a random optimization technology. Both optimization technol-ogies are based on complex calculations, requiring complexprogramming. The PID parameters were optimized by the SignalConstraint module in MATLAB/Simulink. This method can obtaingood optimization results.

The PID parameters adjusted by Signal Constraint are bound upwith the initial ranges of kp, ki and kd. After repeated trials, theirranges were established with the Tuned Parameters window,where kp=5 6 kp 6 kp; ki=20 6 ki 6 5ki; kd=5 6 kd 6 5kd: The opti-mized initial values (kp 208:044, ki 889:077, kd 12:191) wereour PID parameters adjusted by the ZieglerNichols method andentered into the Command Window of MATLAB. In our simulation,the size of the step was variable, the time 5 s. The optimal results ofour simulation were kp 207:805, ki 44:454, kd 19:423. Thevelocity curve of the optimal PID control is shown in Fig. 5. It canbe seen that the overshoot and adjustment time of the optimal

20

0 10

40

60

80

100

20 30 40 50t (s)

V(r/

min

)

Measured

Preset velocity20

0 10

40

60

80

100

20t (s

V(r/

min

)

M

Preset veloci

(a) Velocity curve of closed-loop control (b) Velocity curve of conFig. 8. Velocity curves of tsystem was controlled by a closed-loop velocity control withoutcorrection. The experimental velocity curve obtained is shown inFig. 8a. There is a large lag between the measured and preset veloc-ity curves with wide uctuations, although the measured velocitycurve is similar to a Harrison curve. Because the system was onlycontrolled by the closed-loop velocity without correction, a goodbraking control performance could not be obtained. Fig. 8b is thevelocity curve of a conventional PID control, in which the mea-sured velocity tracks the preset velocity better; the response veloc-ity is faster and uctuation smaller than with the closed-loopcontrol. Fig. 8c is the velocity curve of the optimal PID control, inwhich the measured velocity correctly tracks the preset velocityand the uctuation is relatively small. This meets the requirementfor exible control of a disc braking system.

6. Conclusions

(1) By analyzing and comparing three commonly used brakingvelocity curves, i.e., a linear curve, a Harrison curve and aNordell curve, we found the Harrison curve to be the best.

(2) Considering the characteristics of the disk brake system, i.e.,a closed-loop velocity control, a conventional PID controland an optimal PID control were simulated by MATLAB/Sim-ulink. By analysis and a comparison, we established the con-trol strategy of the optimal PID for the disc brake system.

30 40 50)

easured

ty20

0 10

40

60

80

100

20 30 40 50t (s)

V(r/

min

)

Measured

Preset velocity5. Experiments

To verify the simulation and control strategy of the optimal PID,a multifunctional test-bed (shown in Fig. 7) was developed tosimulate the braking process of the disc brake system.We only sim-ulated the braking process under overload conditions, which is themost incident prone and serious condition. In the experiments, theoverload of the downward belt conveyor was simulated by ahydraulic winch.ventional PID control (c) Velocity curve of optimal PID controlhree control methods.

(3) Experiments were carried out on a multifunctional test-bed.The experimental results demonstrate that the optimal PIDcontrol can make the measured velocity correctly track thepreset velocity and satisfy exible control requirements ofa downward belt conveyor. Our experimental results alsoprove that the theoretical analysis is correct and the controlstrategy effective.

Acknowledgments

The authors gratefully acknowledge the assistance and valuablesuggestions provided by Prof. Zhang Yongzhong and Prof. GuoChuwen.

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494 Y. Hou et al. /Mining Science and Technology (China) 21 (2011) 491494

Control strategy of disc braking systems for downward belt conveyors1 Introduction2 Braking velocity curves3 Disc braking device4 Control strategy of disc braking system4.1 4.2 Conventional PID control4.3 Optimal PID control

5 Experiments6 ConclusionsAcknowledgmentsReferences