Technical Field

The invention relates to servo operated can internal spray machines. In particular, the invention relates to a method of operating the spray machines to improve energy efficiency, and a controller and spray machines for implementing the method.

Background During the manufacture of food or beverage cans, it is desirable to spray the whole or a part of the inside of the can with a lacquer to act as a barrier between the metallic can and the contents of the can. This is done by a spray machine in the can production line. Typically, such a machine will run at 300-400 cans per minute. In an exemplary spray machine as shown in Figure 1 , the cans (1 ) are fed into the top of the machine by a trackwork (2), and securely attached to a "spinner pad" (3) using vacuum suction. The spinner pads (3) (usually 6 or 12 to a machine) are arranged around a central turret (4) which indexes (i.e. moves the cans to the processing area at the appropriate times). The spinner pad (3) causes the attached can (1 ) to rotate, e.g. at speeds of around 2200 rev/min.

Each index of the turret (4) moves a rotating can (1 ) in front of a spray gun (5), which is timed to dispense a lacquer for a set time period (typically 100ms). The indexing mechanism must therefore provide a 'dwell' (i.e. a period of time where the can is rotating in place) corresponding to the period in which the lacquer is applied. In general, the indexing mechanism will provide an acceleration phase, a deceleration phase, and a dwell, with the acceleration and deceleration being sufficient to move the cans into the required position. There may also be a period between the acceleration and deceleration where the speed is constant. A spray machine will typically have two or three spray guns, which ensures coverage along the length of the can body.

After passing in front of the spray guns, the can is discharged from the machine, for example via a discharge turret which grips the can body, or out of the bottom of the machine via a further trackwork (6). Most current spraying machines use a mechanical indexing box to rotate the turret. The mechanical indexing box comprises an internal cam which operates to rotate the turret with the desired acceleration profile. The dwell time is therefore a inversely proportional to the rotational speed of the turret, which means that in order to achieve a consistent dwell time the machine must be run at the same speed. In a production line where other machines are able to modulate their speed smoothly, the spray machine must be regularly stopped in order to match the flow of cans from the other machines. This can lead to blockages due to lacquer drying in the spray guns. Recent machines incorporate a servo motor (7) in place of the mechanical indexing box in order to overcome these problems. As the speed of the servo motor can be directly and finely controlled, the dwell time may be maintained whilst still allowing the overall speed of the machine to be changed according to demand. This leads to a smoother flow of cans, and reduces the necessity to stop the machine and risk blocked nozzles. Such a servo-operated spray machine is disclosed in EP 2293881 B1 .

Summary

It is an object of the present invention to increase the energy efficiency of servo operated spray machines.

According to a first aspect of the present invention, there is provided a method of operating a plurality of machines. Each of the machines uses a servo motor and a motor speed controller coupled to the servo motor, and the motor speed controller incorporates a regenerative braking system. The method comprises synchronising the servo motors in order to achieve overlap of the acceleration phases of some machines with the deceleration phases of other machines and providing electrical power from the regenerative braking systems of machines in a deceleration phase to machines in an acceleration phase.

According to a second aspect, there is provided a controller configured to operate a plurality of machines. Each of the machines uses a servo motor and a motor speed controller coupled to the servo motor, and the motor speed controller incorporates a regenerative braking system, with electrical power being redistributed between machines. The controller is configured to synchronise the servo motors to achieve overlap of the acceleration phases of some machines with the deceleration phases of other machines.

According to a third aspect, there is provided a system comprising a controller according to the second aspect, and a plurality of machines each of which comprises a servo motor and a motor speed controller coupled to the servo motor, the motor speed controller incorporating a regenerative braking system, wherein the controller is configured to operate the machines. Further embodiments are defined in the appended claims.

Brief Description of the Drawings

Figure 1 is shows an exemplary servo operated spray machine;

Figure 2 shows acceleration profiles of each of a bank of servo operated spray machines according to an embodiment;

Figure 3 shows a typical velocity and energy profile for a servo operated spray machine;

Figure 4 is a flowchart of a method according to an embodiment; and

Figure 5 is a schematic diagram of system according to an embodiment.

Detailed Description

Within a can manufacturing plant, there is a significant desire to reduce power usage, both to save costs and to be more environmentally friendly. Spray machines incorporating a servo motor ("servo operated spray machines") will typically undergo up to 420 acceleration and deceleration cycles a minute. When servo motors decelerate, an electrical current is generated. In known servo operated spray machines, this current is directed through a "braking resistor" and allowed to dissipate as heat. There are commercially available "regenerative braking systems" which convert the current generated by deceleration into usable power which can be used by other devices or stored for later use. However, power storage will generally involve losses, and requires further components to be provided, significantly reducing the potential savings from regenerating braking, and so these systems have not been considered for spray machines. Spray machines are typically placed in banks, generally containing seven to ten spray machines, each of which operates independently. A new way of operating such a bank of spray machines is proposed herein to allow a bank of machines to make better use of power generated from regenerative braking. It is recognised that there is no need for the spray machines to operate in synchronisation with one another. It is therefore proposed to control the bank of machines in such a way that when one machine (A) in the bank is in a decelerating phase, another machine (B) in the bank is in an accelerating phase, and therefore the electricity generated by the machine (A) can be used to reduce the external mains electricity required to operate machine (B). Applying this to all of the machines in a bank of seven to ten machines would allow such an overlap to occur for most of the acceleration phases of the machines, significantly reducing power consumption. Figure 2 illustrates a possible synchronisation scheme for seven machines running at 420 stops per minute, with each machine having a cycle labelled 1 to 7. 420 stops per minute gives a total cycle time of 1/7 of a second, approx. 143ms, which means that each machine has a 100ms dwell, a 21 ms acceleration phase, and a 21 ms deceleration phase. The acceleration phase is shown as +1 on each graph, the deceleration phase as -1 , and the dwell as 0. As can be seen from the graphs, the acceleration and deceleration phases can be matched up so that each acceleration phase corresponds to the deceleration phase of the previous machine, i.e. the acceleration phase of machine 2 occurs simultaneously with the deceleration phase of machine 1 . The synchronisation is not perfect, there is a short period at T=0 where both machines 1 and 7 are accelerating, and only machine 6 is decelerating, but it can be appreciated that there is always an accelerating machine to use the energy generated by a decelerating machine. This ensures that as much energy as possible is moved back into the system without the need for storage. Figure 2 relates to an idealised situation where each machine is operating at the same constant rate, the acceleration and deceleration phases are equal, and there is no period of constant velocity between the acceleration and deceleration phases. This may not be the case, the machines may operate at different rates, and the rate of each machine may change over time, e.g. due to changes in the rates of other machines in the production line. The acceleration and deceleration phases may be varied, provided the minimum dwell time and required stops per minute are achieved, and the motion of the cans is sufficient to move them into the required position. Typically, each machine in the bank will run at the same rate (though this rate may change frequently). A typical velocity profile is shown in Figure 3, with the red line representing velocity and the yellow line representing the power input/output of the servo motor. As will be appreciated, the power input during acceleration and output during deceleration are very similar. The required synchronisation may be achieved, for example, by starting the machines out of phase with each other, so that each machine begins its acceleration phase as the previous machine begins its deceleration phase.

The spray machines are typically fed power via a DC voltage bus, with one bus for each spray machine. To enable power to be transferred between the machines easily, it is proposed that all of the spray machines in a bank be connected to the same DC voltage bus, and that the regenerative braking systems feed power into this bus during the deceleration phase.

While the above description relates to the example of servo operated can internal spray machines, the skilled person will appreciate that a similar principle can be applied to other machines which operate with an acceleration phase and a deceleration phase. A flowchart for the generic method is shown in Figure 4. The machines use a servo motor and a motor speed controller coupled to the servo motor, the motor speed controller incorporating a regenerative braking system, and the servo motors are synchronised in order to achieve overlap of the acceleration phases of some of the machines with the deceleration phases of the other machines S101 . Electrical power is provided from the regenerative braking systems of machines in a deceleration phase to machines in an acceleration phase S102.

The servo motors may be synchronised in order to maximise total overlap between acceleration and deceleration phases of the machines. Of course, this method only applies where there are multiple machines present. This allows the savings of regenerative braking to be maximised, as there are no losses relating to the need to store the energy between a deceleration phase and an acceleration phase. Figure 5 is a schematic diagram of system 100. The system comprises a controller 200 and a plurality of spray machines 300, 301 , 30X. Each machine comprises a servo motor 31 and a motor speed controller 32 coupled to the servo motor, the motor speed controller incorporating a regenerative braking system 33 (shown in machine 300), and the machines are connected in order to allow power to be redistributed between the machines.

The controller may be a single unit, which is connected to each machine, as shown in Figure 5. Alternatively, the controller may be implemented as a distributed unit, with processors in each of the machines cooperating in order to act as the controller.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.