Hybrid hydraulic and servo drive technology for higher energy efficiency

Publication date:

Aug 15,2016

In recent years, despite the growing use of electronic drive technology, hydraulic drives are still an integral part of many applications. General hydraulic control system requires a high-performance, suitable control scheme, in the past, people generally use exclusive control solutions, but now, Beckhoff provides a PC-based control solution, compared to more powerful features, so that programming and maintenance work easier. Through the use of EtherCAT bus and TwinCAT hydraulic function library, can meet the high performance requirements. TwinCAT hydraulic function library provides the best energy efficiency for the hybrid drive mechanism composed of hydraulic drive shaft and electric shaft.
Hydrostatic drives still have a large market share in the field of mechanical engineering. The high power density of the actuator and the simple and low cost realization of the linear motion make a compact system design possible. Due to the use of hydraulic accumulators and servo valve technology, it is possible to use highly dynamic and highly accurate drives, for example, in injection molding machines. But at the same time, the hydraulic drive shaft also has shortcomings that can not be ignored: low energy efficiency, environmental noise and hydraulic oil pollution. However, hydraulic pressure can not be completely replaced, because it can provide large force and torque for many applications.
From the control technology point of view, the hydraulic drive shaft in the control algorithm and response.
There are special needs in terms of time. The hydraulic shaft is very sensitive to time in the transition from the guided motion to the control pressure curve. Simple hydraulic shafts usually have low natural frequency and high positioning control system performance. This is why controlling the time displacement curve with PID fails. Thus, in the past, hydraulic shafts often required the use of dedicated hardware controllers.
Hydraulic function library: motion control function through software
Beckhoff PC-based controller is divided into two parts: hardware module and software module. The motion control function is realized through the software library without any special hardware. The process data will be read in and output through the electronic bus terminal module. The use of Beckhoff XFC speed control technology will greatly shorten the system response time; if the use of EtherCAT and powerful Intel®CPU, the sampling time of the controller can reach 100 μs.
The TwinCAT hydraulic function library provides the motion control function of the hydraulic shaft; the electric servo shaft can also be selectively controlled. Module design and certification to PLCopen standards. The user interface is functionally identical to Beckhoff and similar products from other manufacturers. The hydraulic function library supports all common sensor types; the non-linear valve characteristic curve is compensated by using a linearization of the automatically generated characteristic curve.
If a simple PID displacement controller finds a special control system characteristic that is independent of displacement, the traditional NC system will calculate a time-dependent displacement curve. The special control system characteristics of the hydraulic drive require that the generated curve takes into account the displacement-dependent trajectory characteristics, the stick-slip effect and the maximum braking/delay time. The hydraulic function library contains function modules for generating progressive curves. Direction-dependent control system characteristics are also taken into account, and acceleration and deceleration ramps are calculated based on curve accuracy standards or minimum cycle times.
Comparison of Trajectory Control and Time Control
The displacement curve calculated by the traditional NC controller is a function of time, but this calculation method is not suitable for the characteristic curve of the hydraulic actuator. In the time control, it can be observed that the object has strong speed fluctuation, overshoot or stick-slip vibration. Economical hydraulic solutions do not use servo valves, but need to generate a displacement-dependent curve. For these trajectories, curve accuracy is less important than high drive speeds and precise target positioning. For displacement-dependent motion control, the displacement control is not performed during the motion; the control variable that determines the generated curve is the remaining distance, not time. Reaching the target position is achieved by braking, for example, after starting a root function.
Hydraulic shafts are commonly used in devices that require switching from speed control to pressure regulation, such as presses, injection molding machines or die casting machines. XFC combines the functions in the hydraulic function library, with the characteristics of short response time to input signals and high time resolution. It provides an ideal solution for the above applications. In this way, the injection movement is guided by the displacement; the subsequent pressure regulation is effected in a closed loop with high resolution according to the specific process.
In press and bending machine applications, multiple hydraulic shafts are required to be driven at the same time; this will force simultaneous movements and pressure increases of non-lateral forces. In practical applications, the use of hydraulic function library can be achieved with less than 10 or up to thousands of tons of driving pressure to drive up to 20 synchronous shaft. Depending on the specific needs, the hydraulic function library can support master/slave axis or virtual spindle mode. To build high closing forces, linear hydraulic actuators must be combined with non-linear gears, such as connecting rods or crank mechanisms; the hydraulic function library can also provide suitable conversion functions for this purpose.
Combined use of hydraulic shaft and electric servo shaft-significantly improve energy efficiency
In order to increase energy efficiency, hybrid drive solutions-using a combination of electric servo drive shafts and hydraulic drive shafts-have gained increasing popularity in practice. Servo motors can also be controlled in a point-to-point manner by using a hydraulic function library. The time control curve can also be generated by the hydraulic function library. At the same time, the hydraulic function library also supports the synchronous movement of the electric shaft.
The mixed use of servo motors and hydraulically positioned positive displacement pumps will further increase the energy efficiency of hydrostatic drives. The trajectory control in the hydraulic function library is related to the moving part, and the use of highly dynamic EtherCAT drives enables new and innovative motion control schemes.
Scope View software provides easy-to-use diagnostics
Scope View software simplifies the implementation and verification of control software through accurate analysis of important signal characteristics. It can be extended in TwinCAT development environments. All input signals as well as internal variables are displayed with different axis resolutions. In order to be able to operate the hydraulic circuit in an optimum manner, readjustment is often necessary. Developers at headquarters must provide support when an emergency requires maintenance services and a large amount of information. Often, dedicated hardware solutions have significant limitations. When using the TwinCAT Scope View software, the measurement results can be saved and sent for fault analysis, and a template with measurement adjustment function can be provided for on-site maintenance.

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