Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-12-22
2003-09-09
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S317000, C310S318000, C318S114000, C318S118000, C318S119000
Reexamination Certificate
active
06617754
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a circuit arrangement for the dynamic control of -ceramic solid-state actuators, such as for example, piezotranslators with energy recovery by means of magnetic intermediate stores as well as a control loop for operating a piezotranslator.
2. Description of the Related Art
Piezotranslators are electrically controllable actuators whose functions can be attributed to the piezoelectric effect. Active sensors, so-called actuators, can perform the most delicate positioning movements with high accuracy from the subnanometer up to the millimeter range.
Under electrical aspects, a piezotranslator represents a capacitor whose charge has a proportional relationship to its expansion. Consequently, piezotranslators take up energy during the expansion process only. The expansion is maintained without further energy supply. Due to the high capacitance of the piezotranslators, however, a high output power of the driver circuit is required in the case of fast positional changes as they occur under dynamic operation conditions. The associated control electronics must therefore have special properties for each application case and has to be optimised in order to ensure the successful employment of a piezotranslator.
In control processes for the dynamic piezocontrol in which the actuating element must follow up fast changes of a reference variable, it is desired that the amplitude of the movement characteristic agrees with the input signal as close as possible. However, such a linear transfer behaviour cannot be ensured for frequencies up to any magnitude, but is limited, on the one hand, by the resonance frequency of the translator or the entire actuating system, respectively, and by the output power of the amplifier, on the other hand.
Due to the hysteresis phenomena of a piezotranslator the absolute expansion of the actuating element can be determined only inaccurately via the applied amplified input voltage of the amplifier. The resulting expansion has an inherent error of up to 10%, both with respect to its absolute value and to its relative movements. In order to eliminate this error, it is known to provide closed control loops, i.e.. a measuring system for the expansion and control electronics which control the operating voltage in accordance with a comparison of the reference variable and the actual value. Closed positioning control loops are therefore provided with external probes in order to be able to determine the position.
Due to the fact that piezotranslators can electrically be described as capacitors, as mentioned above, onto which electric charges must either be applied or from which said charges must be withdrawn for the purpose of a length variation, which for example can be realised by means of a switch, charging or discharging between the piezotranslator, on the one hand, and the charging current circuit, on the other hand, will inevitably result in power losses, if this is done via controllable variable resistors,. e.g. transistors.
From the final report of the joint project “Entwicklung leistungsoptimierter, hybrider Hydraulikkomponenten auf der Basis piezoelektrischer Aktuatoren” (Development of Power-Optimised Hybrid Hydraulic Components on the Basis of Piezoelectric Actuators) of the Institut für Fertigungstechnik und spanende Werkzeugmaschinen (Institute of Manufacturing Engineering and Metal Cutting Machine Tools), Hanover, Germany, November 1996, it is known to design digital control amplifiers for driving piezoelectric actuators, which are provided with a controlled energy recovery capacity. The known final stage preferably includes inductively coupled coils in order to increase the efficiency of the energy recovery.
In the known circuit arrangement two separate magnetic intermediate energy stores are provided, with the energy stores being switched by a controller in a clocked manner, in order to achieve a predetermined output voltage curve of the control circuit. At the piezotranslator a voltage-dependent non-linear charging of the capacitance will result, with the available current decreasing upon reaching the supply voltage. Due to the circuit arrangement as two separate blocking transformers, only one direction each of the output current can be driven.
In order to obtain a desired linear voltage increase at the piezotranslator, the storage volume of each store must be designed extremely large in the state of the art. The reason of this is that the blocking transformer must be dimensioned according to the smallest voltage increase AU at the range limits of the operating voltage. In the middle output range, however, the available storage volume cannot be utilised so that a correspondingly implemented output amplifier does not operate efficiently.
With respect to the control of piezotranslators in actual applications, the actual momentary value of the output voltage supplied to the piezotranslator is determined by means of a control loop. This is to compensate for existing deficiencies of the control circuit or the final stage, respectively, such as its non-linearity, temperature drift, and frequency dependence, as well as any undesired behaviour of the connected piezotranslator because of the already mentioned voltage hysteresis.
In this context, it has been known to apply part of the piezo output voltage as the actual value to the input of an error amplifier, or to use an exact physical measuring system which yields an output voltage as the actual value. The desired reference variable is supplied to the input of the error amplifier, with the output of the amplifier being connected to the control circuit itself.
The above described controller concept achieves satisfactory results with arrangements without energy recovery, which are mostly equipped with a conventional loss-inherent final stage of the type of controlled series resistors. However, problems occur with a corresponding use in energy recovering control circuits. The reason for this is the considerable inductances which are responsible for the desired energy recovery and which are connected in series to the piezotranslator. Together with the piezotranslator which represents a capacitor these form a high-quality series or resonance circuit. Depending on the type or size of the translator and employed inductor, its resonance frequency is mostly within the frequency band in the range from 1 to 5 kHz, which is of interest for the amplifier operation. The series resonance circuit in turn causes a high increase of the amplification in the resonance range, together with an undesired phase shift in the working band of the control loop approaching the critical 180° limit, which affects the compensation. From this, an undesired post-pulse oscillation or self-oscillation results. The desired flat amplitude characteristic of the overall system up to the upper working frequency range is therefore no longer achievable.
Therefore, it is the object of the invention to provide a circuit arrangement for the dynamic control of piezotranslators with energy recovery as well as an improved control loop for the operation of piezotranslators, which allow the almost linear charging of the piezotranslator over the entire voltage range and at the same time the optimisation of the energy recovery with a small installation size of the implemented circuit. Simultaneously, energy storing elements such as capacitors or accumulators must be able to be operated at a maximum piezotranslator supply voltage so that the return currents can be maintained correspondingly low. With respect to the control loop, it is essential to prevent points of resonance in the working and transfer range so that any self-oscillation can effectively be avoided.
SUMMARY OF THE INVENTION
The object of the invention is solved in that for achieving a predetermined linear voltage characteristic at the piezotranslator, the secondary circuit is designed as a half-bridge consisting of the clocked switches at whose output the inductive intermediate store is arranged in series with the piezot
Cuevas Pedro J.
Dougherty Thomas M.
GSG Elektronik GmbH
Kueffner Friedrich
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