Recent years have continued to exhibit a pronounced trend from open-loop to closed-loop controlled hydraulic drives. Servovalves provide high rigidity to ensure positioning accuracy under load changes, while making use of their outstanding acceleration capacity. Servovalves exert the greatest influence on dynamic characteristics of closed-loop hydraulic systems. Servovalves must meet growing demands to maintain the competitiveness of electrohydraulic drives in meeting increasingly stringent performance requirements.
Continuous development must be incorporated into the design of servovalves to achieve these objectives. The most important criteria are:
- achieving a high degree of accuracy in the manufacturing of valve spool and sleeve to achieve symmetric, linear volume flow,
- reducing the mass of moving parts,
- reducing frictional, static pressure, flow, and other interference forces that exert a negative influence on the dynamic performance of the spool,
- reducing pilot control volumes for pilot-operated valves, and
- improving the actuating dynamics of the electromechanical transformer. This is an area that can benefit from applying piezoelectric technology.
Basics of piezo actuators
The piezoelectric effect describes the charge transfer of ferroelectric materials when subjected to mechanical stress. Reversing this results in a dimensional change in a ceramic material when an electric field is applied. The change of the piezo element's length can be used as an actuator.
In fluid power, two types of piezo actuators are used. A piezo stack translator contains thin layers of piezoelectric material that alternate with layers of electrode material to produce a stack. The total stroke of the piezo stack presents the sum expansion of each layer element. Piezo stacks are used in both low-voltage actuators (up to 200 V, with strokes to 50 µm) and high-voltage actuators (greater than 1000 V with strokes to 100 µm).
However, considerably larger strokes — to 1 mm — can be achieved with piezoelectric bending elements. These are only suitable for use as pilot control elements in hydraulic applications because of the low forces they generate, which amount to about 10 N. Unlike all other electromechanical transducers, piezoelectric actuators do not require any electrical power to maintain a position, making them suitable for low-power valve drive systems.
Of course, some disadvantages to piezo actuators inhibit their use — especially for fluid power applications. The biggest limitation is limited stroke, which necessitates the use of a travel amplification system for directly controlled valves. This decreases dynamics, and due to the vastly different thermal expansion coefficient between a piezo actuator and its housing, actuator systems must incorporate temperature compensation.
Also, unlike the proportional magnets used in servovalves, the characteristic force-stroke curve of a piezo element makes it incapable of building up a constant force throughout its entire working stroke. Furthermore, the piezo element must be preloaded to generate tension.
Poor durability from brittle ceramics and poor electrical insulation plagued piezo actuators of the past. Although this problem has been solved with suitable housing designs, the high prices of the piezos and control units still inhibit their application.
Direct-operated piezo valves
A research project supported by the German research association DFG is developing a hydraulic servovalve that accommodates short strokes and has dynamic performance three times better than that of a conventional servovalve. This is achieved by using a highly dynamic piezo element to drive the valve sleeve and spool in the conventional manner, shown below. As a result, two active actuating elements can be adjusted relative to one another, which changes the cross section fluid flows through.
The challenge of this concept is to find a way of integrating a second actuator (and the a displacement measuring system) into what is already a compact valve package. New challenges are also emerging with respect to the servocontrol system because technology has not yet reached the frequency range targeted — 1-1.5 kHz.
Pilot-operated piezo valves
IFAS at Aachen University, also in Germany, is working on developing a pilot-operated servovalve with two piezo stack actuators for each of two pilot control chambers in the valve's main stage (see illustration). The full-bridge connection of the valve spool allows taking full advantage of the maximum power amplification of a rheostatic control concept. Furthermore, transmission ratio of the valve's four individual control edges can be adjusted electronically.
The compression volume flow through the pilot control chambers is much higher than the adjustment volume flow of the main stage in the targeted frequency range. As a result, particular attention must be given to minimizing the installation space of the pilot control chambers.
Another concern is the pressure-compensated construction of the piezo-operated actuating resistors. The piezo stroke is short to begin with, and it is reduced further by static pressure forces in the pilot control chambers. This means opening and closing of the variable orifices cannot be guaranteed, even by observing tight manufacturing tolerances. The resistors are a form of seat-type elements to ensure leak-free sealing of the control chambers.
One additional challenge facing researchers is the choice of suitable power electronics to drive the piezo stack. Although the piezos require virtually no electrical energy in the neutral position (apart from a small amount of leakage current), highly dynamic operation of the servovalve gives rise to energy values in the kilowatt range. This demands high power of the required output stage. Again, this increases price of the actuators. Using switched-mode digital output stages as in the automotive industry offers one possible solution in this respect.
The future of piezo technology
Researchers have been studying the piezo effect for servovalves for more than 15 years. So why has this technology not asserted itself in practical applications? It is important to realize that conventional servovalves with electromechanical transformers needed several decades to establish themselves as well as they have today.
Up to now, a lack of reliability, high prices, and small useful stroke have limited the use of piezo actuators. However, tremendous progress has been made in these areas over the last five years. For example, 109 load cycles can now be achieved without any problem, and the useful stroke has been virtually doubled in the last 15 years. High costs stem from the small numbers produced and unrefined manufacturing processes.
This material is adapted from a paper given at the ICFP 2005 conference held in Hangzhou, China (sklofp.zju.edu.cn/icfp) by Frank Bauer, Ph.D, and Hubertus Murrenhoff, Ph. D. Both are with the Institute for Fluid Power Drives and Controls (IFAS), Aachen University RWTH, Aachen, Germany. Contact them via e-mail at Frank.Bauer@ifas.rwthaachen.de, or Hubertus.Murrenhoff@ifas.rwthaachen.de