Digital Electro-Hydraulic Servo Controller

Author(s):
Mondeep Duarah - Captronic Systems, PVT. LTD.
Vijay Jayabalan - Captronic Systems, PVT. LTD

Industry:
Electromechanics/ Electrotechnics, Machines/Mechanics, Industrial Controls/ Devices/ Systems

Products:
LabVIEW, PXI/CompactPCI

The Challenge:
Developing a flexible, rugged, and cost effective digital electro-hydraulic servo controller for accurate loading of test specimen that replaces inflexible, standalone analog servo controllers.

The Solution:
Using NI data acquisition cards and signal conditioners (SCXI) with NI LabVIEW software and the NI LabVIEW PID Control Toolkit to develop the digital electro-hydraulic servo controller.

The range of applications for electro-hydraulic servo systems is diverse. It includes manufacturing systems, materials test machines, active suspension systems, mining machinery, fatigue testing, flight simulation, paper machines, ships and electromagnetic marine engineering, injection moulding machines, robotics, and steel and aluminum mill equipment. Hydraulic systems are also common in aircrafts, where their high power-to-weight ratio and precise control makes them an ideal choice for use in flight trajectory control.

Although electrical motors are sometimes used in many of these applications, motion control systems require either very high force or wide bandwidth and are often addressed more efficiently with electro-hydraulic rather than electromagnetic means. In general, applications with bandwidths greater than about 20 Hz or control power greater than about 15 kW can be regarded as suitable for servo-hydraulic techniques.

 

Apart from the ability to deliver higher forces at fast speeds, servo-hydraulic systems offer several other benefits over their electrical counterparts. For example, hydraulic systems are mechanically “stiffer,” resulting in higher machine frame resonant frequencies for a given power level, higher loop gain, and improved dynamic performance. They also have the important benefit of being self-cooled, because the driving fluid effectively acts as a cooling medium by carrying heat away from the actuator and flow control components. Unfortunately, hydraulic systems also exhibit several inherent, non-linear effects that can complicate the control problem.

 

The vast majority of electronic, closed-loop controllers are based on simple analogue circuit designs offering durable, low-cost implementations of the well-known PID control strategy. This approach works well in systems with simple topology and limited bandwidth. However, the growing use of complex control strategies, coupled with the need to support enhanced features such as data logging and digital communications, has led to increased interest in the use of digital processors for control of hydraulic servo-systems. Nowhere is this more apparent than in the field of mechanical test equipment, where using a programmable digital processor means the same servo controller can be used with a wide range of hydraulic systems.

 

Some of the benefits of going to a PC-based digital controller include:

•         Immunity from errors arising from component tolerance, thermal drift, and aging

•         Improved noise immunity

•         Ability to modify and store control parameters

•         Ability to easily implement digital communications

•         System fault monitoring and diagnostic capabilities

•         Data logging capability

•         Ability to perform automated calibration

•         Ability to perform diagnostic monitoring, including frequency spectrum analysis to identify mechanical vibrations and predict failure modes

•         Efficiently implement high-order digital filters including sharp cut-off notch filters to remove energy that would otherwise excite resonant modes and possibly lead to instability

•         Reduction in system cost by taking advantage of a rich integrated peripheral set to minimize component count and board size

 

The performance of a high-quality hydraulic actuator is very dependent on the servo controller.

 

System Overall

The test system consists of a signal generating source of test waveforms; the system employs a generator that generates signals with sine waves, triangle waves, square waves, or a combination of the three waveforms. Loads being applied to the test specimens are detected by strain gage-type load cells and then transformed into electrical signals in proportion to the loads by means of a signal conditioner such as the NI SCXI-1520 universal bridge input module. The error signal between the generated signal and the load signal, by way of comparison, is amplified and then put into a servo valve as an input. The servo valve opens and closes in proportion to the quantities of input signal so that high pressure oil flow rates from the pump unit are controlled and then sent on to the cylinder of an actuator. Then the piston is driven to provide a force on to a test specimen.

 

Electro-hydraulic actuators are widely used in industrial applications. They can generate very high forces, exhibit rapid responses, and have a high power-to-weight ratio compared to their electrical counterparts. However, it is well-known that they exhibit a significant nonlinear behavior which makes the controller design a challenging task. Nonlinear flow/pressure characteristics, variations in the trapped fluid volume due to piston motion, and fluid compressibility are major sources of nonlinearity in the actuation system. There can be other factors such as transmission nonlinearities, flow forces and their effects on the spool position, and friction, all contributing to this nonlinear behavior.

An implemented digital servo controller uses the traditional approach of local linearization of the nonlinear dynamics about a nominal operating point.

 

Closed Loop PID Control

Control is achieved by using a digital form of a PID controller, such as the NI PID Control Toolkit. The PID controller uses the “position form” of the PID algorithm. PID controller is capable of loading the system defined by the wave form.  The PID Control toolkit was the key to implement and tune the controller.

 

Shunt calibration, offset nulling and programmable excitation, and the programmable filter features of SCXI-1520 helped us to meet the data acquisition requirement of load cell. With the SCXI-1520 we were able to achieve an average accuracy of 2 µV for 100 samples.

 

Digital Controller Tuning

Unfortunately, the nonlinear behavior of the system requires the use of conservative loop gains, and the gain scheduling feature of the PID Control Toolkit helped us to implement the same. The controller has been tuned at multiple operating points to take care of the system’s nonlinearity.

 

We used the PID Autotuning Wizard to tune the servo controller. The relay tuning method in the PID Autotuning Wizard offers the advantage of not bringing the system to marginal stability during the tuning process, which avoids accidents and damage to the test component.

 

The entire controller was designed, developed, and tuned within eleven man-days, thanks to LabVIEW and NI hardware.

 

The digital servo controller is highly flexible, rugged, and cost effective. We have replaced the existing analog controller with the NI based digital controller.

For more information, contact:

 

 

Jijay Jayabalan

 

Captronic Systems, PVT. LTD.

 

#3 Victorian Meadows, Airport-Varthur Rd.

 

Marathahalli P.O, Bangalore, India 560 037

 

Tel: +91-80-4037-3947

 

Fax: +91-80-4037-3839

 

E-mail: vijay@captronicsystems.com

 

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