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Today’s industrial hydraulic systems generally require tight control, which translates to fast response and pinpoint repeatability. Electrohydraulic servovalves have served these demanding applications for decades. However, as electric servodrives have become more powerful, hydraulic drives have lost ground in small- to medium-size machines— especially in injection molding and die casting. The reason is simple: poor efficiency of hydraulic servovalve drives. Moreover, electric servodrives are becoming even more powerful, further threatening the application of industrial hydraulic drives.
Conventional hydraulic servovalve drives have been the first choice for precise control of heavy loads for so long because they were the only choice for precisely controlling loads far more massive than those within the capabilities of electromechanical drives. Efficiency wasn’t an issue because hydraulics was the only solution. But electromechanical servodrives have benefitted from advances in materials and technology over the last several years, and the result has been the application of drives in areas that long had been the domain of hydraulics.
Fortunately, hydraulics has not been sitting dead in the water—not by a long shot. Hydraulic servodrives have entered the scene to raise the bar on hydraulic performance. These drives combine the high power capability of hydraulics with the precision and efficiency of electric servodrives. The result is that hydraulics now stands to regain much of the ground that had been lost to electric drives. Considering the many hydraulic servodrives we’ve described recently, it probably would not be a stretch to say that a drive revolution is occurring in industrial hydraulics.
These hydraulic servodrives use a servomotor to drive a hydraulic pump at variable speeds. Controlling rotational speed, torque, and direction of these motors translates to flow and pressure from the pump that rivals that of a servovalve’s output. The big difference is much higher efficiency with the hydraulic servodrive. The biggest difference is during dwell periods. With a hydraulic servovalve drive, the pump consumes significant power just to maintain pressure—even if no work is being performed. With the hydraulic servodrive, the servomotor consumes very little power during dwell periods because it only needs to transmit enough torque (not speed) to maintain pressure. So during dwell periods, the servomotor essentially becomes a torque motor.
Less Energy, Higher Productivity
The latest major player in the hydraulic servodrive market is Voith Turbo H + L Hydraulic GmbH, Rutesheim, Germany. Introduced at the 2015 Hannover Fair, Voith’s servopump adjusts to the power actually needed over varying volume flows and motor speeds. This provides clear advantages over conventional systems, particularly in the part load range, with a uniformly high volume flow that is needed only rarely within an operating cycle. Optimized operation reduces energy consumption by up to 70% and the total cost of ownership (TCO) of the entire hydraulic system by up to 35%. Voith officials say the cost of the servopump is usually amortized within one to two years.
The servopump system consists of three main components: a servo inverter, a synchronous servomotor, and a Voith internal gear pump. The servo inverter analyzes and processes the set point and the actual values of pressure and speed. It controls the highly responsive servomotor, which supplies the required torque to match load pressure. This power is transmitted to the hydraulic system as a function of the pressure or flow through the gear pump. As a result, classic valves are unnecessary, reducing the complexity of the system and the cooling capacity needed. At the same time, the reliability and lifetime increase.
Keys to the performance of the hydraulic servodrive are the low mass moment of inertia of the servomotor and the internal gear pump. This shortens the cycle time of the actuators as much as 50% to increase throughput of the machine. In addition to lower energy consumption, the Voith servopump reduces the noise emissions of the system by up to 20 dBA. The result is lower cost and effort for noise abatement measures. Voith officials say that in many cases, workplace guidelines are met without any additional measures.
The Voith servopump is offered as a turnkey solution. Pressure and flow are individually adjusted to the specific cycle data and synchronized with existing control concepts and systems. The system, therefore, lends itself as a retrofit to existing machines. Typical applications for the servopump include plastics and die-casting machines, machine tools, and presses. Because it is the central component of machines and systems, it continuously measures operating parameters, making it well suited as node for integration within Industry 4.0.
For more information, contact Voith Turbo H + L Hydraulic GmbH, Rutesheim, Germany, via its website at www.voith.com.