Especially for gantry systems, rapidly accelerating heavy loads can shoot the electrical power demand of frequency drives through the roof. At the same, time while braking, regenerative energy is being put back into the drive system. For Ilsemann Automation, the challenge was to make the most effective use of the braking energy and not dissipate it in braking resistors. In its retrieval robots for plastic processors, the company uses a drive solution from SEW EURODRIVE that minimizes energy consumption while at the same time boosting the operational reliability of the system.
Thin-walled plastic cups are widely used in the food industry. Given the millions of pieces involved, the injection molding technology demands are incredibly high in terms of productivity, efficiency, and availability. These requirements also apply to the retrieval technology that removes the finished plastic cups from the injection molding system. For this task, Ilsemann Automation uses an XYZ gantry with multi-axis-coordinated servo drives. An MDP92A-series central supply unit from SEW EURODRIVE provides power to the gantry drives and other rotary, transfer, hinged, and depositing axes in the DC link connection.
In a new development, the Bremen-based company integrates a double-layer capacitor for buffering the energy released in the power supply. The MOVI DPS-series storage unit is placed between the supply unit and the seven MOVIDRIVE drive frequency inverters. This construction has three key advantages — operational reliability, energy efficiency, and the limitation of peak loads.
Ilsemann Automation is a global provider of handling systems for injection molding technology. Its new power supply system’s key unique selling point is its robustness in voltage fluctuations. Thanks to the temporary storage, the systems can be used in countries with limited grid quality without any additional protective measures.
Compensating for supply fluctuations is important because the retrieval system is at work near the injection molding machines. In this, it must be ensured that the plastic cups are removed in exactly the correct time window to stack them on a conveyor belt. The limited-time for this is linked to the production speed of the injection molding system. The retrieval must happen within a 0.7-second window. This speed can only be achieved with highly dynamic forward and backward movements. If supply fluctuations occurred, the risk of a collision between the tool and the handling unit would increase because the required movement ramps can no longer be realized.
“We must ensure that our system will not collide with the expensive injection molding tools, even if there is a power failure,” says Gerhard Kropp, Design Manager for Electrical Engineering at Ilsemann Automation.
The robot kinematics experts from Bremen, Germany, therefore got together with the SEW EURODRIVE team in a joint engineering project to find a way of achieving greater supply reliability. The development goal led to the indirect supply of the multi-axis system from an EMF plate capacitor. This feeds the DC link of all the drives via its storage buffer with the requisite level of reliability and safely equalizes any potential supply fluctuations all the way to complete failure. The unit’s capacity is sized in such a way that the handling unit can safely complete the work cycle that has begun before the gantry shuts itself down in a controlled fashion. This eliminates any potential collisions with the open injection molding tool.
Along with increased operational reliability, the unit integrated into the DC link connection also offers further benefits such as the increased energy efficiency and the reduction of load peaks. Both of these factors are directly linked to the fact that the energy currents in the gantry are harmonized, and above all, retained. Especially in highly dynamic gantries, accelerations and decelerations happen successively within a short time frame. The capacitors in the Ilsemann Automation handling units collect the energy released by the motors while braking and make it available to the drives again when they accelerate. The capacitor module acts as a short-term battery with a booster function during acceleration. Ideally, this construction works so effectively that no kinetic energy needs to be dissipated via braking resistors.
Measurements have shown that the energy solution was able to halve the gantry’s energy consumption. Regenerative units that feed back into the power grid are much less suited to this application because they cannot reach anywhere near the same level of efficiency. A calculation example from Germany provides a further argument in favor of buffer storage — the current generated under braking is not fed cheaply back into the grid, only for the system to draw energy for acceleration from the local power supply company at a more expensive rate.
Keeping the braking energy in temporary storage positively affects the grid usage fees and annual service costs end users pay to the local utility company for its services. It should be considered that the cost of extraordinary load peaks can become enormous after just a few minutes, as the power supply costs are calculated annually. Measurements are taken over 15 minutes. Here is a calculation example for a company with its own medium-voltage supply, over 2,500 hours of power usage a year, and costs of 120 euros per kW. If load peaks drive the planned usage up by 100 kW within 15 minutes, this will lead to costs of 12,000 euros. Smoothing out these load peaks is vitally important in the context of energy management.
This effect is supported by the slimmer design of the supply installation that can be achieved because the storage handles the peak loads of the Ilsemann gantry. The supply infrastructure, therefore, only has to provide more continuous power. In one of the first gantries configured with SEW EURODRIVE, the peak load was cut from the 70 Hp typical of such an application to just 8 Hp. The cable cross-sections were also cut accordingly — from 16 mm2 to 2.5 mm2. By removing the need for an uninterruptible power supply, saving space, and making installation easier overall, this also cuts the installation costs.
With its retrieval gantry, Ilsemann Automation shows how easily a storage-based DC link connection can be implemented in a multi-axis-coordinated drive application. Since the braking energy remains in the system, achieving effective load management is also relatively straightforward. The advantages include lower input power, greater operational reliability during supply fluctuations, and more efficient use of electrical energy overall.
Filed Under: Drives (servo) + amplifiers