Starting-process controller for starting a piezomotor

The invention relates to a starting-process controller

• having a voltage-controlled oscillator (VCO), a power output stage, and a resonance converter, wherein
• the voltage-controlled oscillator (VCO) generates the control signals required for the power output stage,
• the resonance converter converts the stepped output voltage from the power output stage into a sinusoidal voltage at its output,
• the piezomotor is driven by the sinusoidal voltage from the resonance converter,
• the motor current that flows when the piezomotor is driven is measured and compared with the phase of the drive voltage in a phase comparator,
• the output signal from the phase comparator is a measure for the phase difference at the time between current and voltage,
• a phase-locked loop filter smoothes the phase-difference signal,
• the smoothed signal controls the oscillator (VCO).

Known from DE 199 42 269 is an electronic drive for a piezomotor (e.g. a micropush motor). The piezomotor is connected to a phase-regulated a.c. voltage. During operation, the current drawn by the piezomotor is measured by means of a diode. The phase angle of the current is detected by comparison with the voltage fed to the motor. A peculiarity of the piezomotor is that the current through the motor, and hence the power drawn by it, decreases under load. This contrasts with electromagnetic drive systems where the current increases under load.

This peculiarity of the piezomotor is attributable to a rise in the internal resistances of the system.

Hence, when a piezomotor and its drive system are being designed, it has to be borne in mind that the current, or rather the applied motor voltage, has to be corrected when operating under load and the motor power adjusted to the load in this way.

Another known effect is that, if there is a changing, i.e. variable load, then this will change the resonant frequency of the motor at the same time. This, once again, causes the active power drawn by, and the efficiency of, the motor to decrease. The two effects described reinforce one another such that the motor may possibly come to a halt. At the same time, the phase-regulating system goes to a self-locked state, from which it generally does not recover. An automatic restart is no longer possible. The reason for this tip-over or stalling effect is that the oscillator is taken from the capacitive range of operation through its resonance and into the inductive range, which causes a phase rotation.

It is an object of the invention to ensure a stable and reliable starting under different loads.

This object is achieved in accordance with the invention by two variant embodiments that are defined in claims 1 and 10 and that can be used separately but may also be combined with one another.

The first variant embodiment, which is defined in claim 1, is characterized by a start-assisting switching element that fixes the output voltage of the phase-locked loop filter at start-up and thus applies a constant voltage to the input of the voltage-controlled oscillator (VCO).

The introduction of this start-assisting switching element has the advantage that the motor frequency supplied by the oscillator (VCO) is set to a safe operating frequency. Without the said start-assisting switching element and the effect described for it, the control frequency would be moved through its control range by the phase-regulating system too quickly when starting under load and would take the control loop to the self-locked state before the motor could start the load moving.

Other advantageous modifications of the first embodiment can be seen from the subclaims dependent on claim 1. These modifications relate to the design of the start-assisting element that switches a switching element, and to the connecting-in period that is suitable.

The second variant embodiment, which is defined in claim 10, is characterized by an adjustable time-delay element by which the phase angle between the voltage applied to the motor and the motor current is changed in start-up operation from an initially large starting angle for a safe and reliable breakaway towards a smaller angle at the operating point, so that start-up will be completed safely and reliably irrespective of the loading condition.

The curve followed by the change in phase-angle can be freely preset. It depends on the load and on the resonant frequency required for optimal efficiency at rated speed. It must be set in such a way that the power drawn by the motor remains in the capacitive range and hence the value for the resonant frequency is not exceeded.