Mitsubishi Heavy Industries Technical Review Vol. 47 No. 2 (June 2010)
15Grid-Powered Electric Transfer Crane

KANJI OBATA*1TOSHIHIKO SAKAMOTO*2NOBUO YOSHIOKA*3NOBORU FUJISAWA*3

HIROFUMI YOSHIKAWA*4

TADAAKI MONZEN*5In response to the growing need for environmental friendliness, Mitsubishi Heavy Industries(MHI) has developed an electric rubber-tired gantry crane (RTG) powered from a standard groundterminal box of a city electrical power grid. This replaces the conventional RTG powered by anengine generator set. We estimate that this new electric RTG reduces both the energy cost and CO2
emissions by approximately 90% compared to our conventional RTGs. Incorporating ourinnovative cable reel control technology, this new electric RTG can travel at 135 m/min, as fast as
a conventional RTG, while stable straight-line travel is guaranteed with an auto-steering control
system. When moving from one operating area to another, the crane disconnects from the groundterminal box and switches to a mobile power pack that it picks up and carries with it. This
technical review describes how these technologies work in our new electric RTGs.

|1. Introduction
In port container terminals, RTGs are used to move intermodal containers from trucks tostacks and back again. An RTG equipped with an engine generator set can travel over the entire
terminal area, changing its tire direction by 90 for lateral and longitudinal movement.
Recent increasing concerns about environmental issues, including reduction in energy
consumption and CO2 production, make powering cranes from the electrical grid essential insteadof relying on engine generator sets. To meet this demand, we developed an electric RTG with acable reel to supply power from a grid terminal box, resulting in a lower energy cost and reduced
CO2 emissions. This crane has a traveling speed equivalent to that of a conventional RTG. It has apower cable with a detachable connector, and a mobile power pack. When operating, the crane isconnected to the electrical grid; however, the crane picks up its mobile power pack and draws
power from it while cross traveling between container blocks. The following sections describe theproduct specifications, environmental performance, and technical features of our new electric RTG.

|2. Electric RTG specifications and environmental performance
2.1Product specifications

Table 1 lists the basic specifications of our electric RTG, and Figure 1 shows the overall
equipment configuration. The cable reel and high-voltage receiving equipment are installed where
the engine generator set was located on a conventional crane. The absence of the engine generator
set eliminates engine maintenance, noise, and exhaust, and thus improves the working
environment. The cable reel rolls in and out automatically at a speed that is synchronized with thecrane’s traveling speed. Because electric RTGs travel over long distances at high speeds without atrack (rail), excessive or insufficient cable tension is a challenge. We solved this by using*1 Deputy General Manager, Industrial Machinery Business, Technology & Solutions Division*2 Manager, Industrial Machinery Business, Technology & Solutions Division*3 Industrial Machinery Business, Technology & Solutions Division*4 Manager, Corporate Planning Department, Presidential Administration Office*5 Manager, Takasago Research & Development Center, Technical Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 2 (June 2010)
16

innovative control techniques.
As a result, our cable reel electric RTG can travel faster than a conventional cable reelmodel. We also introduced a robust auto-steering control system, which permits stable straight-line
travel in spite of variations in cable tension or yard inclination.
RTGs usually operate in designated lanes. However, lane changes are sometimes necessary
to move cranes to the maintenance area or to other busy areas of the facility. When moving to a
new location, the RTG picks up its mobile power pack to obtain electrical power and disconnects
the detachable connector from the ground terminal box before moving away.
Table 1Product specifications

ItemStorage

6 lines
7 lines

Main hoisting
Transverse traveling
Gantry traveling
Rated load
Speed

Specifications
1 over 46 lines
1 over 4
7 lines
23 to 52 m/min
70 m/min
135 m/min
40.6 t1 over 5
1 over 5

Figure 1 Electric RTG configuration

2.2

Environmental performanceElectric RTGs are economical and environmentally friendly. Figure 2 shows an estimate of
the improvement in their economic and environmental performance compared to conventionalmodels. In conventional engine-powered RTGs, the power regenerated while a load is descending
is dissipated as heat in a resistor, and the fuel consumption during standby periods is notinsignificant. With our new electric RTG, on the other hand, the power regenerated as a load isdescending can be used to operate other equipment in the terminal facility, and the energy
consumption during standby is kept to a bare minimum. As a result, both the fuel consumption andCO2 emissions are reduced by an estimated 90%. The cable reel RTG has significant economic andenvironmental advantages compared to a hybrid RTG equipped with an engine and secondary
batteries.

Figure 2 Environmental performanceMitsubishi Heavy Industries Technical Review Vol. 47 No. 2 (June 2010)17|3. High-speed traveling with tension control
Because the cable reel RTG travels long distances at high speeds without a track, it requires
sharp acceleration and deceleration, or auto-steering controlled transient acceleration anddeceleration. With conventional technology, such operation results in excessively high or low cabletension. To solve this problem, we developed a new type of cable reel control system using ourmachine control technology.
In a conventional cable reel crane, the rotational speed of the cable reel is synchronized withthe crane traveling speed. For our new crane, we adopted a torque control system that applies anoptimum torque to the reel in the roll-in direction. This is to control the cable tension, not the motor
torque; the applied torque varies according to the rolled diameter of the cable to maintain theoptimum constant tension.
The traveling acceleration is measured and used to correct the torque reference value to copewith fluctuations in the cable tension. This system also corrects for the cable reel rotational
resistance corresponding to cable roll-in and -out directions. As a result, our electric RTG cantravel faster than conventional cable reel RTGs, at a speed of 135 m/min (10 s acceleration and 6 sdeceleration), equivalent to the speed of conventional engine-powered RTGs. Figure 3 shows thefluctuation in cable tension of a conventional reel control system compared to that of our newsystem. Our new RTG crane exerts stable tension control without any excessive cable tension.

Figure 3

Stable cable tension control|4. Straight-line traveling performance
The RTG is equipped with two auto-steering sensors, as shown in Figure 4. By detecting
deviations from the magnetic guideline buried in the yard, the auto-steering control systemprovides stable motion along the guideline. For example, when the crane deviates from theguideline, the control system causes the tires on one side to run faster than those on the other side
to correct.
Recovering from a major auto-steering deviation requires a long time. To reduce the chanceof such a deviation, we reduced the influence of cable tension and yard inclination by calculating
the directional deviation angle during straight-line traveling and taking this angle intoconsideration. Figure 5 shows the measurement result of deviations when traveling at themaximum speed of 135 m/min. This shows how the crane is capable of stable straight-linemovement.

Figure 4 Auto-steering control

Figure 5 Stable straight-line motion

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 2 (June 2010)18|5. Lane changes using mobile power pack
During normal container handling operations, the electric RTG is powered from a terminalbox installed at the end of each lane as shown in Figure 6, and the crane travels up and down thelane. On occasion, however, the RTG needs to move to another lane or to the service area formaintenance. To cater to such contingencies, we equipped the power feed cable with a detachable
connector (Figure 7) and provided a mobile power pack (Figure 8) that the RTG can pick up anduse as a power source when traveling outside a lane. Figure 9 shows how the crane switches fromone source of power to the other.

Figure 6

Lane change traveling

Figure 8

Mobile power packFigure 7

Figure 9

Terminal box

Switching between power sources|6. Conclusion
We have developed an innovative cable reel control system to reduce the fluctuation intension in our new environmentally friendly and economical electric RTG, and included a newauto-steering control system to ensure robust performance. We have solved the problems
associated with lane changes by using a power cable with detachable connector, and a mobile
power pack for power while disconnected from the power grid.
We also offer a remote controlled automated RTG. With this model, transloading between
the stack and trucks is controlled from the administration building. Fully automatic operation of theRTG will be possible when an automated guided vehicle system is introduced to replace chassis
trucks.
We are developing hybrid RTGs that use both an engine generator set and secondary
batteries. We are working on a complete line of automated electric RTGs for better environmental
performance and energy efficiency in container terminals.
We are grateful for the support and cooperation of the Tobishima Container Berth Co., Ltd.