**6.1 Load sharing**

Although the motors have the same power, there are few necessary reasons to do the load distribution: different wheel diameter, unequal adhesion, geometrical imperfection of the construction, slipping of the pinion wheel due to wet or frozen rails. Load distribution is resolved by using speed trim load sharing configuration, Fig.1c). Load distribution controller is realized by PLC.

In the Fig.19 the principle block scheme for load distribution between two rail coupled induction motors (IM1 and IM2) fed by frequency converters (FC1 and FC2) is shown. Starting point at design of load sharing controller is that the less loaded motor should accelerate in order to take over the part of load from the more loaded motor. Information about the load can be obtained in different ways. The easiest one is by motor current. Modern converters used in drives, enable to obtain information about the motor torque in percentage in relation to rated torque.

As we can see in the Fig.19, the speed reference of only one motor (*n*\* 2) is updated in relation to the main speed reference (*n*\*=*n*\* 1). Reference correction *n*\* is proportional to the

In the Fig.18 gantry crane with indicated drives is shown. All motors are three phase fed by

Basic requirements set in front of this drive are: equal load distribution between motors located on the same side, as well as skew elimination between fixed and free gantry leg.

Although the motors have the same power, there are few necessary reasons to do the load distribution: different wheel diameter, unequal adhesion, geometrical imperfection of the construction, slipping of the pinion wheel due to wet or frozen rails. Load distribution is resolved by using speed trim load sharing configuration, Fig.1c). Load distribution

In the Fig.19 the principle block scheme for load distribution between two rail coupled induction motors (IM1 and IM2) fed by frequency converters (FC1 and FC2) is shown. Starting point at design of load sharing controller is that the less loaded motor should accelerate in order to take over the part of load from the more loaded motor. Information about the load can be obtained in different ways. The easiest one is by motor current. Modern converters used in drives, enable to obtain information about the motor torque in

2) is updated in relation

1). Reference correction *n*\* is proportional to the

Certainly, the most complicated is the gantry drive, from following reasons:

plant is located outdoor so the influence of the wind may be considerable,

that is multi motor drive which consists of two motors on each side,

frequency converters.

the span is wide,

construction is lattice, therefore it is elastic,

the length of runway rail path is 300 m.

Fig. 18. Gantry crane with indicated drives.

**6.1 Load sharing** 

controller is realized by PLC.

percentage in relation to rated torque.

to the main speed reference (*n*\*=*n*\*

As we can see in the Fig.19, the speed reference of only one motor (*n*\*

difference of estimated electromagnetic torque (*Te*=*Te*1-*Te*2). Proportional gain of load sharing regulators is denoted as *KLS* .

In order to ensure the stabile operation of the motors during the large external disturbances, especially at low speed when estimation of electromagnetic torque in speed sensorless drives looses on accuracy, it is necessary to limit the correction value *n*\*, as shown in Fig.19.

For the purpose of suggested algorithm verification the trolley load sharing is analyzed. Because of the short distance between left and right side the skew may be neglected. Trolley drive consists of four motors, two on each side (IM1-IM2 on left and IM3-IM4 on right side). Frequency converters are set on speed sensorless vector control mode. Motors have the common reference speed. In the Fig.20a) motors torque without load distribution is shown. At reference speed, in steady state, we can see that even the motors have the same rated power, load torques are different. Estimated motor torque is not applied in control algorithm. Speed between left and right side is different because it depends of motor characteristics and load, as shown in Fig.20a).

Fig. 19. The principle of load sharing based on estimated torque.

Effect of load sharing is shown in Fig.20b). The approximately equal motors torque on the same leg can be easily seen. Used system enables that speed of every motor is regulated, but also the load difference is controlled. In this way the load difference is being maintained on the desired accuracy.

Depending on the purpose of drives and needed accuracy of maintaining load distribution, load controller can be with only proportional effect, but also with proportional integrated effect. In our case only proportional controller with *KLS*=1 p.u. is used. Output from the load controller is restricted on only several percentages of maxsimum speed reference (in our example *n*min-max=2%). That is quite enough to provide necessary load regulation and not to "break" the drive speed regulation by too big effect on the speed reference. This solution can be applied for all kinds of multi motor drives on cranes.

Electrical Drives for Crane Application 153

master for gantry drive skew elimination algorithm; while in this case, the speed reference for the frequency converter on the free leg (FC2) is modified with the anti-skew controller

The control scheme for skew elimination between the master and slave motor of gantry drive is shown in detail in Fig.22). As it can be seen, we propose a simple proportional (P) controller acting as an additional, outer correction loop, which supplies speed control loop.

In order to ensure the stabile and safe operation of the motors during the large external disturbances and at low speed, when estimation of electromagnetic torque in speed sensor-

A reliable operation (even in terms of key components failure - for example encoders) requests an additional external disturbance compensator (EDC) which includes several pairs of position bars (or markers, M) and inductive proximity sensors (IPS). The EDC takes into account all external influences on the position difference of the two encoders: the free

The proximity sensors are fitted on the end truck holders, while the position bars are equidistantly mounted along the rails. During the crane movement, proximity sensors detect the moment when the fixed (or free) leg passes above the markers and so register the crane actual skew. Now when both legs are positioned on the markers, absolute encoders measure the trajectory difference, as shown in Fig.22). In fact, this difference is the real skew (*s*) of the crane, determined at each crossing over the markers. If the difference is greater than the length of the markers (*lm*) that means the crane skew is bigger than allowed. For this reason it is required that the length of markers matches the allowed skew of the crane. The distance between successive markers (*lms*) depends on the length of marker and

wheels diameter difference and an accidental wheel and encoder joint slipping.

maximum expected liner speed difference between the legs.

less drives looses on accuracy, it is necessary to limit the correction value *n*\*.

1) with reference correction value *n*\*. If the encoder position difference *E* related to maximum allowed skew is known, the controller gain *KSC* can be calculated, (Mitrovic et al.,

2) is updated in relation to the main speed reference

output.

(*n*\*=*n*\*

2010).

The speed reference of one motor (*n*\*

Fig. 21. Block scheme of gantry drive.

Fig. 20. Motors torque and speed: a) without load sharing; b) with load sharing.

#### **6.2 Skew elimination**

On most rails mounted wide span gantry cranes skewing problem is associated with poor rail conditions, uneven wheel wear, wind influence, wheel slippage or unequal load conditions when the trolley is operating at one end of the crane bridge. The skewing of the crane can cause excessive wheel abrasion and stress, especially to the wheel flanges. It can also produce horizontal or lateral forces that can result in unusual stresses to the crane runway beams and building structure. This often results in differing diameters of drive wheels, which subsequently cause the crane to skew.

The crane construction consists of opposed pairs of end truck assemblies (left hand side is named as free leg and right hand side is named as fixed leg). These are movable along a track and a long transverse support member between the end truck assemblies. Each end truck assembly includes two sets of trolleys and an upper load bar laterally interconnects the two sets of trolleys.

The hardware for skew elimination consists of a PLC with Field-bus communication, two absolute multi-turn encoders, two proximity sensors and four frequency converters for motor supply of trolley drives, as shown in Fig.21). On each end truck, one of the converters is master and the other one is slave. The master-slave references distribution is modified according to the load sharing principle as shown in Fig.1c).

The main devices for skew tracking are two absolute encoders (E1 and E2) installed on a special, non-tractive wheel (so called free wheel), in order to avoid slipping. Encoders measure the traveled distance, and absolute position is transferred to the anti-skew control subsystem in PLC, as shown in Fig.21). The fixed leg frequency converter (FC1) is set as a

a) b)

On most rails mounted wide span gantry cranes skewing problem is associated with poor rail conditions, uneven wheel wear, wind influence, wheel slippage or unequal load conditions when the trolley is operating at one end of the crane bridge. The skewing of the crane can cause excessive wheel abrasion and stress, especially to the wheel flanges. It can also produce horizontal or lateral forces that can result in unusual stresses to the crane runway beams and building structure. This often results in differing diameters of drive

The crane construction consists of opposed pairs of end truck assemblies (left hand side is named as free leg and right hand side is named as fixed leg). These are movable along a track and a long transverse support member between the end truck assemblies. Each end truck assembly includes two sets of trolleys and an upper load bar laterally interconnects

The hardware for skew elimination consists of a PLC with Field-bus communication, two absolute multi-turn encoders, two proximity sensors and four frequency converters for motor supply of trolley drives, as shown in Fig.21). On each end truck, one of the converters is master and the other one is slave. The master-slave references distribution is modified

The main devices for skew tracking are two absolute encoders (E1 and E2) installed on a special, non-tractive wheel (so called free wheel), in order to avoid slipping. Encoders measure the traveled distance, and absolute position is transferred to the anti-skew control subsystem in PLC, as shown in Fig.21). The fixed leg frequency converter (FC1) is set as a

Fig. 20. Motors torque and speed: a) without load sharing; b) with load sharing.

wheels, which subsequently cause the crane to skew.

according to the load sharing principle as shown in Fig.1c).

**6.2 Skew elimination** 

the two sets of trolleys.

master for gantry drive skew elimination algorithm; while in this case, the speed reference for the frequency converter on the free leg (FC2) is modified with the anti-skew controller output.

The control scheme for skew elimination between the master and slave motor of gantry drive is shown in detail in Fig.22). As it can be seen, we propose a simple proportional (P) controller acting as an additional, outer correction loop, which supplies speed control loop. The speed reference of one motor (*n*\* 2) is updated in relation to the main speed reference (*n*\*=*n*\* 1) with reference correction value *n*\*. If the encoder position difference *E* related to maximum allowed skew is known, the controller gain *KSC* can be calculated, (Mitrovic et al., 2010).

In order to ensure the stabile and safe operation of the motors during the large external disturbances and at low speed, when estimation of electromagnetic torque in speed sensorless drives looses on accuracy, it is necessary to limit the correction value *n*\*.

Fig. 21. Block scheme of gantry drive.

A reliable operation (even in terms of key components failure - for example encoders) requests an additional external disturbance compensator (EDC) which includes several pairs of position bars (or markers, M) and inductive proximity sensors (IPS). The EDC takes into account all external influences on the position difference of the two encoders: the free wheels diameter difference and an accidental wheel and encoder joint slipping.

The proximity sensors are fitted on the end truck holders, while the position bars are equidistantly mounted along the rails. During the crane movement, proximity sensors detect the moment when the fixed (or free) leg passes above the markers and so register the crane actual skew. Now when both legs are positioned on the markers, absolute encoders measure the trajectory difference, as shown in Fig.22). In fact, this difference is the real skew (*s*) of the crane, determined at each crossing over the markers. If the difference is greater than the length of the markers (*lm*) that means the crane skew is bigger than allowed. For this reason it is required that the length of markers matches the allowed skew of the crane. The distance between successive markers (*lms*) depends on the length of marker and maximum expected liner speed difference between the legs.

Electrical Drives for Crane Application 155

acceleration/deceleration, due to different loads between the fixed and free leg temporarily skew can be observed. The skew controller action eliminates this start-up disturbance in a few seconds. Simultaneously, with the action of a skew regulator, load-sharing controllers provide motor loading in proportion to their rated power. At constant speed operation, the trolley moves between the fixed and free leg, which causes additional differences in loads, but the proposed controller successfully compensates for these disturbances. In the case of crane deceleration, it can be seen that the characteristic case of the free leg stopping is postponed in order to complete the elimination of skew and for the fine position adjustment.

a) b)

The application of squirrel cage induction motors supplied from the frequency converters (also known as adjustable speed drive) have become the standard solution for the modern crane drives. However, the standard configuration of the inverter can not be used for some drives primarily due to regenerative operation, which in some cases may be intermittent (long travel and cross travel) and continuous (lowering). The power and torque requirements in details are described and analyzed for such drives. From the aspect of the required power crane drives are often implemented as a multi motor. One of the important issue in this case is load distribution between the motor proportional to the motor power rating which can be resolved

This chapter describes the solutions that are commonly used in modern crane drives. In case that it is a casual recuperating the dynamic braking is used. If continious regeneretation occur active front end rectifier capable to returning energy into the supply network is used. The following two case studies are selected. Case study 1 is typical because the AFE is used which in addition of power recovery possibility also serves to supply all the drives on the

Fig. 23. Behavior of gantry drives: a) without skew controller; b) with skew controller.

by applying the modern converters in one of the master-follower configuration.

**7. Conclusion** 

Fig. 22. The principle block diagram of skew controller.

The limited number of the necessary input data for the calculation and design of the skew controller allows quick adjustment of parameters and the choice of EDC components, and the proposed algorithm makes it suitable for industrial applications.

In the analyzed example, the loads of fixed and free legs are different, partly because of asymmetry of gantry, but mostly because of the trolley moving along the gantry. The calculated critical skew of gantry structure is 100 cm, but during normal operation the maximum allowed skew is 50 cm. A preview of gantry drive parameters, controller design and set-up values are taken from reference (Mitrovic et al., 2010).

At the beginning, we analyzed the behavior of gantry drives without a skew controller and the main results are shown in Fig.23a).

In this case, the load-sharing controllers for the fixed and free gantry leg are applied. Three working sections are noticeable: crane acceleration, steady state operation, and crane deceleration. The encoder measures the motor speed, while torque is estimated from the frequency converters. The measured data are collected in PLC SCADA system. The observed variables are master motor (IM1) speed *n*1, speed difference *n*1-*n*2 between the master motor (IM1) on the fixed leg and the master motor (IM2) on the free leg, torque differences between motors on the same leg (IM1-IM3, IM2-IM4) and the value of actual skew (*s*). In this case, as the skew is not controlled, it can be seen the increase of the value. During the crane skew, motors (IM1 and IM3) on the fixed leg are more loaded than the motors (IM2 and IM4) on the free leg. In addition to that, the effects of the load-sharing controller can be noticed because the motors on the same leg share loads approximately, i.e. torque difference oscillates about zero value.

The next experiment was performed including the skew controller and under the similar operational regimes as in the previous case: acceleration, steady state operation and deceleration. The experimental results are shown in Fig.23b). During the crane

The limited number of the necessary input data for the calculation and design of the skew controller allows quick adjustment of parameters and the choice of EDC components, and

In the analyzed example, the loads of fixed and free legs are different, partly because of asymmetry of gantry, but mostly because of the trolley moving along the gantry. The calculated critical skew of gantry structure is 100 cm, but during normal operation the maximum allowed skew is 50 cm. A preview of gantry drive parameters, controller design

At the beginning, we analyzed the behavior of gantry drives without a skew controller and

In this case, the load-sharing controllers for the fixed and free gantry leg are applied. Three working sections are noticeable: crane acceleration, steady state operation, and crane deceleration. The encoder measures the motor speed, while torque is estimated from the frequency converters. The measured data are collected in PLC SCADA system. The observed variables are master motor (IM1) speed *n*1, speed difference *n*1-*n*2 between the master motor (IM1) on the fixed leg and the master motor (IM2) on the free leg, torque differences between motors on the same leg (IM1-IM3, IM2-IM4) and the value of actual skew (*s*). In this case, as the skew is not controlled, it can be seen the increase of the value. During the crane skew, motors (IM1 and IM3) on the fixed leg are more loaded than the motors (IM2 and IM4) on the free leg. In addition to that, the effects of the load-sharing controller can be noticed because the motors on the same leg share loads approximately, i.e. torque difference

The next experiment was performed including the skew controller and under the similar operational regimes as in the previous case: acceleration, steady state operation and deceleration. The experimental results are shown in Fig.23b). During the crane

Fig. 22. The principle block diagram of skew controller.

the proposed algorithm makes it suitable for industrial applications.

and set-up values are taken from reference (Mitrovic et al., 2010).

the main results are shown in Fig.23a).

oscillates about zero value.

acceleration/deceleration, due to different loads between the fixed and free leg temporarily skew can be observed. The skew controller action eliminates this start-up disturbance in a few seconds. Simultaneously, with the action of a skew regulator, load-sharing controllers provide motor loading in proportion to their rated power. At constant speed operation, the trolley moves between the fixed and free leg, which causes additional differences in loads, but the proposed controller successfully compensates for these disturbances. In the case of crane deceleration, it can be seen that the characteristic case of the free leg stopping is postponed in order to complete the elimination of skew and for the fine position adjustment.

Fig. 23. Behavior of gantry drives: a) without skew controller; b) with skew controller.
