Description of the background art Conventional serving carts used on aircraft by e. g. flight attendants are manually driven back and forth in the aisles between the rows of seats. The carts are equipped with swivel casters enabling the carts to be steered. Since these carts and their load are very heavy (fully loaded carts weigh 60-90 kg), flight atten- dants cannot start serving passengers until after an aircraft has taken off and must be concluded before the aircraft descends. Thus, the aircraft must be virtually hori- zontal. However, the wheels have brakes used to prevent inadvertent movement while passengers are being served. If the carts were to start roiling when the air- craft is in a nose-up or nose-down attitude, servers trying to stop the moving cart would be subjected to a heavy load.

Service to train passengers operates in the corresponding manner, i. e. a serving cart is rolled between seats throughout the length of the train. A major problem here is presented by the train's acceleration and braking. They can cause a serving cart to start rolling during service and crash into seats and/or doors on the train.

Purpose of the invention The purpose of the present invention is to achieve a serving cart and a module for same, said invention solving the aforesaid problems and making it pos- sible to move a serving cart backwards and forwards and turn same, even when a aircraft is tilted or when a train applies the brakes.

A purpose of the invention is also to make possible gentle braking of a moving serving cart as a complement to the existing foot-operated parking brake and to propose advantageous steering means for such a serving cart.

The purpose of the invention is additionally to propose a parking brake for a serving cart by complementing the mechanical brake with an electrical brake that

gradually increases its retardation until it is fully exerted after a specific period of time.

In a broader perspective, the purpose of the invention is to improve the work environment for servers, e. g. flight attendants, when serving airline passen- gers in passenger aircraft and improve the work environment for cabin personnel in general.

Summary of the invention The aforesaid purposes are achieved with the present invention, as de- fined in the independent claims, thereby eliminating the said disadvantages. Ap- propriate embodiments of the invention are set for in the dependent claims.

The purpose of the invention is achieved when a cart is equipped with one or more electrical drive motors operated from the upper part of the cart when one or more operating means on the cart's worktop is/are arranged to cause move- ment back and forth when the self-centring operating means is pushed forward or back. The braking function is activated when the operating means is in the neutral position.

A steering device, with which either of the servo motors is made to drive one or both of the cart's wheels or with which two motors driving the cart's wheels are made to operate at different speeds, is arranged next to the operating means, making it easy to steer the cart. The steering function is such that a joystick-like control in a neutral middle position causes the wheels to steer straight ahead.

When moved to self-centring lateral positions, it steers the wheels in the respec- tive direction. The cart is also equipped with an operating lever which detaches the drive motor from the drive mechanism e. g. if a fault should occur. The cart can then be used in the conventional way without motor drive. An alternative design of the steering function entails having the servo motors contribute to turning the cart when a steering device is pushed in some direction.

In a further advantageous embodiment, the operating and steering de- vices are combined, e. g. like a joystick. Driving and steering are then simultane- ous.

All controls are proportional. This means that the more a control moves, the faster the speed or tighter the turn respectively. This proportionality is achieved with a strain gauge or digital sensors that are affected by movement of a control

up to 3 mm in any direction. In addition, the operating unit can be equipped with a device for switching between two speeds.

The invention also relates to a drive module devised for connection to an existing serving cart. The module is provided with a connection adapter for a serv- ing cart of a particular make and a particular design. The module is equipped with steering means suitable for mounting on the serving cart's worktop. Cabling from the module to the steering means is routed in external channels or mounted inter- nally in the cart.

In one advantageous embodiment of the invention, the steering means on the cart's worktop communicates with the drive module via wireless radio commu- nications.

Rechargeable accumulators, preferably NiCd or NiMHi, power the electri- cal motor. Charging can be performed from a conventional power outlet in the ve- hicle during transport or from an external terminal. In an alternative embodiment, the source of power is rechargeable batteries that can be detached from the drive module and charged separately.

Brief description of the drawings The invention will now be described in greater detail with reference to em- bodiments and with reference to the accompanying drawings, in which Figure 1 is a schematic depiction of the serving cart according to the present invention.

Figure 2 is a schematic depiction of a module according to the present inven- tion.

Figure 3 is a block diagram of the drive and steering devices.

Figure 4 is a schematic depiction of a combined steering and operating means.

Figure 5 is a schematic depiction of an alternative steering and operating means.

Figure 6 is a schematic depiction of another alternative steering and operating means.

Figure 7 is a diagram showing how resistance in the stator winding varies over time.

Description of the invention Figure 1 shows a serving cart 10 according to the invention. It consists of a chassis section 11, a drive unit 12, an adapter unit 13 and a steering/operating means 14. The chassis section 11 is equipped with drawers 15 and tray holders 16. The drive unit 12 is equipped with two sets of wheels. One set serves as the drive wheels 17 and one set consists of swivel casters 18. The drive unit is also equipped with a mechanical brake 19 with a foot-operated brake pedal and a foot- operated brake release pedal. The cart also has a worktop 20 arranged with raised edging 21. Two steering means/control means 14 are arranged on the worktop 20.

They operate the functions FORWARD, REVERSE, STOP (electrical braking) and LEFT, RIGHT. The cart is also equipped with an electrical disengagement device 22, which in one position allows electrical drive of the cart and in the other position disconnects the electrical drive, enabling the cart to be used like a conventional serving cart. The disengagement device operates by mechanically disconnecting the drive wheels from the motor unit and by disconnecting the electronics.

Figure 2 shows the drive unit 12 equipped with a left motor 23 and a right motor 24. The motors 23,24 are connected to the drive wheels 17 by means of a left gearbox 25 and a right gearbox 26. Worm gear transmissions are preferable in order to achieve powerful braking when the motors operate as generators. In addi- tion, rechargeable accumulators 27,28 are located in the drive unit 12 together with an electronics unit 29 connected by an electronics cable 30 to the steering means/operating means 14. The accumulators are charged when the drive unit is manually connected to a source of power or automatically when the serving cart is parked in its storage rack in the vehicle. Charging is monitored in the conventional way. The electronics unit 29 controls all of the cart's electrical and electromechani- cal functions. The figure also shows an adapter 31 arranged for connection to the drive unit to allow the drive unit to be mounted on an existing serving cart after the wheels have been removed. The adapter is obviously not needed when new serv- ing carts are manufactured according to the invention. The drive unit is further equipped with safety skirts 32, suggested with the dashed lines in the figure, which shut off the power from the accumulators and activate the electrical brake if the cart hits any obstacle. Moreover, electrical connection means 33 are arranged near the accumulators for connecting the accumulators to a recharging power source. The connection means can also be devised as two electrically conductive

contact areas on the drive unit 12. Collector shoes are employed for connecting these contact areas to a recharge terminal.

An additional embodiment of the version according to figure 2 is achieved when the drive unit 2, its bottom plate and its adapter 31 are horizontally and tele- scopically adjustable to fit a long serving cart. The weight of the driving unit's com- ponents is then distributed over the telescopically moveable parts. Here, the tele- scopic parts can be locked in at least an interior and an exterior position.

All electrical parts in the drive unit are encapsulated units, preferably ar- ranged in three blocks. The electronics unit with its circuit board and relays are in a first block, the motor, gearbox and disengagement device are in a second block and the power source's rechargeable accumulators are in a third block. Electrical cabling for signal and power transmission runs between each block. Each block has waterproof encapsulation to allow the blocks to be hosed off when the cart is cleaned. Moreover, a protective hood with holes shields the three encapsulated blocks from mechanical impact. The hood is also equipped with a thin, wear-resis- tant PEHD (high-density polyethylene) skids whose purpose is to enable the cart to slide over elevated areas such as thresholds.

Figure 3 shows how different parts in block form are interconnected in one possible configuration according to the invention. The steering means and operat- ing means 14 are remote connected to the electronics unit 29 controlling the mo- tors 23,24 on the basis of signals from the electronics unit. The motors drive the wheels via the gearboxes 25,26. The disengagement device 22 is located be- tween the gearboxes and the electronics unit. The disengagement device is also arranged to disconnect the motors electrically if the cart should hit some obstacle acting on the safety skirts 32. Moreover, one or more sources of power in the form of rechargeable accumulators 27,28 are connected to the electronics unit 29 and supply voltage to the motors 23,24. Dashed lines in the figure designate connec- tion of the accumulators to the recharge unit and connection of the gearboxes to the drive wheels respectively.

Figure 4 shows a first embodiment of a steering means/operating means in the form of a self-centring joystick 41 that rebounds to a neutral position. Its movement is proportional and controls the functions FORWARD, REVERSE, STOP (electrical braking), RIGHT and LEFT. This joystick 41 is connected to the electronics units by the electronics cable 30.

Figure 5 shows a second embodiment of a steering means/operating means devised as a bridge 51 whose entire length can be resiliently tipped for- ward and back for driving forward and in reverse. Pivoting, articulated spring-back steering arms 52,53 are connected to the bridge 51. Pressure on the left steering arm 52 causes the cart to steer to the left, whereas pressure on the right steering arms 53 causes the cart to steer to the right.

Figure 6 shows an alternative steering means/operating means similar to the one shown in fig. 5, i. e. devised as a bridge 61 permanently mounted on the worktop 20. On the said bridge 61 a rotating handle is arranged as a steering means/operating means 62. The cart is propelled forward when the means is ro- tated forward, and the cart is propelled to the rear when the means is rotated to the rear. This steering means/operating means is also proportional, i. e. the greater the rotation the higher the speed. This steering means/operating means is also equipped with steering arms 52,53.

The steering means according to fig. 5 and fig. 6 are doubled and operate in a'master-slave'relationship in which the first steering means activated is the master, and the other is completely inactive. When the first steering means acti- vated is released, thereby activating the other steering means, the latter becomes the master and the first is deactivated etc.

The mechanical brake 19 serves as a complement to the electrical brake.

The electrical brake is fully controlled by the steering means/operating means in such a way that when the means is released and returns to the normal position, the electric motors are short-circuited, i. e. level of resistance through the stator winding is minimised corresponding to the level of resistance in the stator winding.

Figure 7 is a diagram showing the way the electrical brake works. At time t=0, i. e. when braking commences, resistance through the motor's stator winding is about 60 Q, whereupon this resistance declines to a minimum value in the stator winding at time t=4 s (seconds). The stator winding is therefore short-circuited at time t=4 s, thereby maximising the effect of electrical braking. This short-circuiting can be achieved by charging a capacitor, which ultimately acts on a transistor or a relay to short-circuit the generator.

In one advantageous embodiment of the invention, two drive motors are arranged, each driving one wheel. Each of these two drive motors is connected to gearbox for reducing the speed to a value suitable for propelling a serving cart.

The gearbox greatly contributes to cart braking. In addition, each motor is equip- ped with an electrical braking circuit as described above. There is no need for a mechanical brake in this embodiment. The electrical brake is sufficient to prevent the cart from rolling.