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The Challenge:
Developing a flexible, rugged and cost effective ‘Digital electro-hydraulic servo controller’ for accurate loading of test specimen, that can replace inflexible, standalone analog servo controller.
The Solution:
Developing a flexible, rugged, cost effective and accurate ‘Digital electro-hydraulic servo controller’ using NI DAQ cards and signal conditioners (SCXI) along with LabVIEW and PID control toolkit.
Introduction:
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 aircraft, 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 requires 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 of 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 since the driving fluid effectively acts as a cooling medium carrying heat away from the actuator and flow control components. Unfortunately hydraulic systems also exhibit several inherent non-linear effects which can complicate the control problem.
The vast majority of electronic closed loop controllers are based on simple analogue circuit designs offering robust, 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 the use of a programmable digital processor allows the same servo controller to be used with a wide range of hydraulic systems.
Benefits of going to PC based digital controller
- 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
- 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
- Reduce 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 dependant on the servo-controller.
System Overall:
Figure 1 shows the overall schematic diagram of the ‘Digital electro-hydraulic servo controller’. Test system consist of signal generating source of test waveforms, the system employs a generator that generates signals with sine wave, triangle wave, square wave and the combination of the above waveforms. Loads being applied to the test specimens are detected by strain gauge type load cells and then transformed into electrical signals in proportion to the loads by means of a signal conditioner (SCXI-1520). The error signal between the above mentioned generated signal and the load signal, by the 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 actuator. Then the piston is driven to provide a force on to a test specimen.
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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 with 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.
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 digital form of PID controller (NI PID Control Toolkit). PID controller uses the “position form” of PID algorithm. PID controller is capable of loading the system defined by the wave form. Figure 2 shows the system response for the ramp loading at different loading rate.
NI PID Control toolkit was the key to implement as well as tune the controller.
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Shunt calibration, offset nulling and programmable excitation and programmable filter features of SCXI-1520 helped us to meet the data acquisition requirement of load cell. We ware able to achieve the accuracy of 2µV for 100 samples average with the SCXI-1520.
Digital Controller Tuning:
Unfortunately, the nonlinear behavior of the system enforces the use of conservative loop gains, gain scheduling feature of the NI PID control toolkit helped us to implement the same. Controller has been tuned at multiple operating points to take care of the system non linearity. Figure 3 shows the controller response before tuning the controller.
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