Load management for robots in plastics processing

They are familiar to us as yogurt pots on the breakfast table or as red drinking glasses from American college films: thin-walled, plastic cups. Millions and millions of them are used in the food industry. The cups are manufactured using an injection molding process. Here the high volumes and production speeds typically encountered in the food industry mean stringent productivity, efficiency and availability requirements. The retrieval method used to remove the finished plastic cups from the injection molding system is gaged against these criteria.

Ilsemann Automation, a Bremen-based global specialist in robot kinematics, has developed a new XYZ gantry with multi-axis-coordinated servo drives for this task. With the aid of highly dynamic forwards and backwards movements, the drive system of the retrieval robot guarantees a very high speed of production.

The development staff faced one particular challenge: that of load management. They wanted to ensure that the drive technology performs the energy-intensive movements as efficiently as possible. And not only under ideal conditions, but also if the power supply doesn’t remain constant, for example, should the voltage fluctuate or should the power even fail as a result of the supply system collapsing completely.

Ilsemann brought an engineering partner on board for this tricky job: our drive specialists at SEW-EURODRIVE and the Power and Energy Solutions (PES). Not least because, as a global company, we are able to provide support for Ilsemann customers on all continents. The environmental award we received from the state of Baden-Württemberg for our smart performance and energy management may have played a role in us winning the contract too.

The “muscles” of Ilsemann’s gantry system for the food industry comprise the servomotors for the gantry drives and furtherservo drives for rotary, hinged and depositing axes, which are arranged in a DC link connection. A central supply from the MDP92A series handles the supply of power to the servomotors, while seven MOVIDRIVE^®drive controllers – a kind of “nerve system” – are responsible for control.

To achieve the high speed of production needed in plastics processing, the gantry’s servomotors accelerate large masses at great speed – and decelerate them again almost straight away. A considerable amount of energy is required for such high dynamics, but the effect of regeneration during the braking process means that the drive system is able to recuperate a large share of the energy used. This is where our load management solution comes into play. Working with Ilsemann, we integrated an EMF plate-type capacitor from MOVI-DPS^®series into the DC network. From an electronic standpoint, it sits between the supply unit and drive controllers.

Rather than dissipating the released braking energy via resistors, the capacitor module absorbs the energy from the servo drives to return it to the drives during acceleration. This setup delivers two huge benefits, giving Ilsemann an edge over the competition: greater operating reliability and significant cost savings from improved energy efficiency and limiting the load peaks.

During the retrieval process, the handling unit reaches directly into the working chamber of the plastic injection molding machine. It only has an extremely brief time window until the machine closes again: In just 0.7 seconds, the gantry removes the finished cups to then stack them onto an unloading conveyor belt. Should the voltage fluctuate during the retrieval process, there is a great risk of the servo drives not being able to complete the movement in time. And basically, the handling unit collides with the injection molding die. The resulting damage is extremely costly to rectify and the price goes up for every minute of lost production.

But the EMF plate-type capacitor changes all that. It supplies the DC network of the motors from its buffer. So it reliably compensatesfor any supply fluctuations – even complete failure of the power supply. This is all thanks to the capacitance of the storage unit, which is designed so that the handling unit can always complete the current operating cycle. A collision with the expensive injection molding die is ruled out. A big tick for the supply and operating reliability of Ilsemann’s customers.

What’s more, the load management supplied by SEW-EURODRIVE offers significant benefits in terms of power requirements: Measurements confirm that the gantry's energy consumption has been halved. This is down to the efficiency of the setup: the capacitor acts like a battery, which gives the motors a boost when accelerating. And it does this so effectively that no kinetic energy need be dissipated via braking resistors.

However, reduced energy demand and reliable operation are not the only benefits for Ilsemann’s customers. They also stand to gain huge potential savings from restricting load peaks, which the gantry compensates for using a buffer store within the system. This is a huge benefit when you consider grid usage fees. Very high load peaks can send these fees to dizzy heights in a matter of minutes.

Here’s asample calculation for a company, which pays €120 a kilowatt for more than 2500 hours a year: If the amount of power bought in now increases by 100 kilowatts within 15 minutes – due to a load peak for example – they will have to pay €12 000. This problem is effectively eliminated by the SEW-EURODRIVE setup in the Ilsemann gantry.

The supply installation displays another positive effect. Because the buffer deals with the load peaks the gantry would otherwise have to handle, the supply infrastructure simply has to provide a constant supply later on. So it can assume a much more slimlined and a much simpler design.

In the first customer project: Peak load fell with the new Ilsemann gantry from its previous level of 52 kilowatts to just six kilowatts. The connection cross sections for the supply could therefore be reduced from 16 to 2.5 square millimeters. This effect can be seen even more positively in the fact that there is no need for an uninterrupted power supply. All in all, the installation is much simpler, cheaper and more compact.

Coupled with its improved operating reliability and energy efficiency, SEW-EURODRIVE’s energy concept gives Ilsemann Automation a competitive edge, which the Bremen-based company can now benefit from globally in the food industry.