Secondary controlled drives: dynamic, efficient, precise and versatile

Drives with secondary control are making inroads into yet another field.

Wind force 8. The ship is pitching and wallowing, its bow and stern lifting and dropping back into the waves. However, work on the crane does not come to a halt. In spite of the rough seas, it continues to lower heavy loads to the seabed, both accurately and smoothly. This is made possible by Rexroth’s proactive wave compensation system that can offset up to 95 percent of wave action.

1. Configuration

Secondary-control drives are operated in high-pressure networks. They consist of at least one hydraulic pump (the primary unit), a hydraulic accumulator, and a using unit (the secondary unit).

The purpose of the hydraulic accumulator is to store the energy fed to the hydraulic network by the secondary unit whenever it is reversed, e.g. in the pumping mode. When operating under load, this accumulated energy is used to compensate for consumption peaks. The charge status of the hydraulic accumulator – in conjunction with the pressure-controlled primary unit and the operational status of the secondary unit – determines system pressure.

While conventional drives work with an interface via volumetric flow, secondary-control drives are linked to the specified operating pressure. Volumetric displacement is, therefore, not related to a precisely defined rotation speed but rather to a certain torque.

If an automated process, or the operator, prescribes the rotation speed required for a certain action, the using unit automatically determines – within milliseconds – the torque required to ensure that this rotation speed is maintained at the operating pressure currently available.

2. Unique features

• Even if a number of using units with varying loads are connected to the system, secondary-control drives eliminate the necessity to throttle down energy-carrying circuits, as is the case in conventional, open systems. The power drain on or energy return to the supply network is controlled to match needs by adapting the machine’s volume displacement to the loading situation. This is why, even in open systems, any number of units functioning as motors or pumps can be connected in a parallel arrangement, without impairing the system’s excellent energy balance.

• Even long distances between secondary and primary units, having to be covered using long hydraulic lines, have absolutely no negative effect on the performance of secondary-control drives.

• The application of hydraulic accumulators makes it possible to store huge amounts of energy which can be charged or discharged for several seconds in the MW range.

3. Application examples

4. Benefits at a glance

• Dynamic and precise: The dynamics of this drive meet the most stringent demands with respect to positioning, rotation speed and torque precision.

• Efficiency: Hydraulic energy can be transformed into mechanical energy and vice versa with the greatest possible degree of efficiency.

• Energy-saving: Braking energy is utilized and stored in accumulators without intermediate valves.

• Versatile: Setting up four-quadrant operation (forward acceleration – forward braking; reverse acceleration – reverse braking) is trouble-free, even in open circuits.

5. Fields of application

6. The principle of energy recovery