Ready for recuperation of braking energy, use as a short-term UPS, for controlled load peak reduction and fieldbus communication: active energy management devices from the Pxt family.
Pictures: Koch
Active energy management
Far more than just greater energy efficiency
At Interpack 2023, it was one of the stars of Gerhard Schubert's stand - a slimline cobot called tog.519. With a record-breaking 90 picks per minute, it reaches precisely into chaotic piles of products and places each item exactly where it is supposed to be. The active energy management system PxtFX from Michael Koch GmbH supports it in several ways.
It stands there elegantly, a silver-black column, around two metres high on its pallet-sized base with recesses for a pallet truck, topped off with a kind of roof with an integrated LED strip. The Scara arm, the first axis in the column, protrudes two thirds of the way up. For Achraf Ben Salem, Head of Cobot Development at Schubert and his team, the Cobot tog.519 is the first official product for Schubert. "I had a lot of freedom during development, was able to employ good people and utilise our creativity," says Ben Salem. And the result is impressive.
Cobot rethought
Every detail of the Schubert cobot was rethought from scratch, concepts were developed, discarded, added to, rewritten and rethought again. For example, the cobot became mobile - with all the necessary consequences. Or: at the core of its software is a unique AI that can handle products of all kinds, shapes and colours. The mechanics are extremely resilient, accelerations of up to 7 g can be achieved cyclically and the cobot remains stable. The movement radius of around 760 millimetres is traversed quickly and the speeds of the gripper axes are adapted to those of the arm. The image processing is fast and the process speed of the entire system is breathtaking. Everything is optimised for 90 picks per minute. On the other hand, operation is simple and format changes can be carried out in next to no time without any programming effort - just hold a product in front of the cobot.
Requirements for the drive technology
Ben Salem's team broke new ground in the conception and design of the electric drives, simply because the familiar and existing technology from the Schubert drawer was not up to the requirements of the compact and fast cobot. The two main axles are equipped with 400 volt drives, the three secondary axles with 48 volt drives. The two large drives in particular are subjected to high loads due to the extremely fast start-stop cycles, which is why the motors are also very special customised versions. When searching for a suitable servo inverter, the team had to take into account the very limited installation space in the cobot's column. A compact servo inverter with an efficient heat dissipation concept was used. In the prototype phase, the cobot was operated with braking resistors, but this led to very high power losses due to the short cycles and high braking power. A different solution had to be found; after all, Schubert is particularly committed to sustainability under the "Mission Blue" banner. In the search for alternatives for the braking resistors, the capacitor modules of the servo inverter were ruled out for space reasons alone. Mains feedback was rejected for several reasons. The decision was made in favour of the PxtFX active energy management device from Michael Koch. In its smallest configuration, the device fits the application and the entire mechanical configuration perfectly. Connected directly to the DC link of the servo inverter, the PxtFX actively transfers the braking energy to its electrolytic capacitor at lightning speed. During acceleration, it replenishes the buffered energy so quickly that the DC link voltage only drops when the buffer is empty and energy is needed from the mains. Well over one kilojoule of kinetic energy is released per pick-and-place cycle, i.e. two accelerations and two decelerations, which the active energy management device collects and returns to the system. At 90 cycles per minute, the energy saving is significant, which is then clearly noticeable in the CO₂ balance and also in the electricity bill. In addition, the device operates with very low losses with this electronically controlled clean-out of electrical energy, so that in combination with the possible elimination of braking resistors, the environment is largely spared the usual heat development. This is a major advantage of the Cobot in the food, pharmaceutical and cosmetics industries, among others. The PxtFX is installed in the Cobot. In addition to the low heat generation, there are no hot air flows or surfaces, so there is no risk of burns.
Why is the tog so fast?
The high speeds and the many cycles are ultimately only achievable on a permanent basis because of the active energy management device. If the energy were to come from the mains for each cycle and the braking energy were to be dissipated via a braking resistor, the voltage surges in the servo inverter's DC link would be so severe that the service life of the DC link capacity would be jeopardised after a short time. It is also likely that the overall thermal load on the inverter would be far too high, meaning that the speed and therefore the number of cycles would have to be set significantly lower from the outset. With active energy management, the voltage curve in the servo inverter is greatly stabilised, and these reserves ensure that it is not the drive electronics but the motors or even the robot's mechanics that become the limiting factors in the cobot system. The active energy management device is also very compact in this application because it only requires a single large electrolytic capacitor for storage. In contrast to capacitors that have to compensate for a ripple, the storage capacity of the capacitor in the device can be utilised almost completely: a range of 30 to 450 volts is possible. Around 100 million cycles are specified as the target value, which can be considered a good value for mechanical engineering applications. The capacitor was developed specifically for this requirement in very close co-operation between Koch and the manufacturer. The "active" in the energy management device means an electronic separation between the DC link voltage and the capacitor voltage. The device's electronics ensure a smooth transition between the two voltage levels in the drive and storage, each of which can be configured to meet the requirements of the cobot application. For the high power requirements of the cobot, 30 volts was not enough, as the formula voltage times current applies to the power on both sides of the PxtFX electronics.
Uncomplicated and reliable
"It really is that simple," says Ben Salem, referring to the use of the PxtFX. "We sent the performance data of our cobot to Koch and the delivered device fulfils all the promises: Screw it in, plug it in - and it works." The members of the development team didn't really believe this, as they had to do extra rounds with almost all the other elements. The experience gained so far during extensive testing is also very promising. So far, the possible evaluation of the internal capacity monitoring of the energy management system has therefore been omitted. The Pxt product family offers this function, which determines changes to the storage capacity during operation and issues warning messages when preset values are reached in the interests of preventive maintenance. Experience to date in the Cobot has shown that, despite the high loads, no signs of ageing of the memory occur. The specified operating hours are reliably exceeded in practical use. This corresponds to the results of the simulation calculations that Koch carries out for specific applications with the help of tools developed in-house based on the development findings and well over ten years of field experience. The influences of the loads and environmental conditions not only on the memory, but also on the entire electronics are included in the simulation calculation.
UL and CSA the icing on the cake
"Reliability is a crucial criterion for our tog.519," explains Ben Salem, referring to the complete system. "The user must be able to rely on all hardware and software components equally." Quality and durability are therefore very important decision criteria for him when selecting components. "It's also very helpful if one component supports another!" adds Ben Salem. Just as the PxtFX does for the servo inverter by stabilising the DC link. Incidentally, it also does this imperceptibly during voltage fluctuations or brief interruptions in the grid, as long as there is still enough energy in its memory. In Schubert's Cobot, this UPS function is not the primary purpose of the device, but it does help to overcome the odd minor fault. The Schubert team is very positive about the UL and CSA certification. The PxtFX device, which is technically easy to use, once again confirms its simplicity for those involved in export processing and standardisation testing. (hw)