Abstract – High resolution optical encoders are used in industrial servo drives with position and/or speed control for providing the respective feedback signals. In applica- tions with lower accuracy requirements resolvers are used as feedback sensors with respect to lower cost. As a resolver is nothing else than an electrical AC machine of special design, there is the basic idea to operate the servo motor itself like a resolver – in parallel to its original drive operation, of course. High frequency resolver sig- nals are injected to the servo motor without disturbing its original purpose (i. e. the production of torque). The reaction of the servo motor on the high frequency signals can be detected by the current sensors, which are availa- ble with respect to the current control loop anyway. A lot of schemes and concepts for encoderless AC drive control providing good operation during standstill are already published. Acceptance by industry, however, is not very extended. The main reasons for hesitations in industry are
• enhanced processing performance being neces- sary for the controller or signal processor • additional sensors or hardware • parameters to be adjusted with respect to enco- derless control
A high frequency component is superposed to the cur- rent control output signal resulting in a high frequency current response, which can be used for detecting the orientation of the rotor. The position dependent high frequency currents can be measured with the standard current sensors available in industrial drives anyway. There is no need for any additional hardware being not available in standard industrial drives. The estimated rotor position is adjusted by a tracking controller, which works without impact of any machine or drive parameter. Therefore it is possible to detect even small anisotropies as typical for synchronous machines with surface mounted permanent magnets. The software effort is comparable to the software for a rotor model of a standard field oriented control for induction machines. Standard microcontrollers used in servo drives with field
oriented control are sufficient to perform the sensorless control scheme. The experimental results presented in this contribution show the encoderless control scheme having no limita- tions with respect to a minimal speed; the drive is able to provide full torque in encoderless operation even at standstill. The control behaviour of position and speed control is equal to a servo drive with resolver feedback. Recent developments show that synchronous machines with conservative magnetic design show better perfor- mance in encoderless control than "modern" motor de- signs. This contribution shows some interesting results obtained from synchronous machines with permanent magnet excitation as well as from synchronous reluctance machines. Future developments and possibilities will be discussed.
I INTRODUCTION uring the last decades a variety of encoderless control schemes for AC machines have been investigated and published. Some of these – especially when based on funda- mental models of AC machines – are already successfully used in industrial drives. In low speed operation or standstill, however, fundamental models show limitations with respect to encoderless speed or position control. Consequently algo- rithms have been investigated detecting the reaction of the AC machine on additional high frequency signals.
AC machines usually show a characteristic sinusoidal local distribution of flux density resulting in space dependent saturation effects in the stator as well as in the rotor. For that reason there is at least a saturation dependent magnetic ani- sotropy, which is coupled to the rotor position in the case of synchronous machines. The AC machines respond to a high frequency voltage signal injected to the AC motor by a high frequency current in addition to the basic stator currents. The high frequency current component is influenced by the mag- netic anisotropy resulting in a position dependent carrier modulation. As the theoretical background of HF injection encoderless control is presented in the literature (e. g. [2], [3], [11], [12], [15]), this is not shown in detail here.
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2010, 12th International Conference on Optimization of Electrical and Electronic Equipment, OPTIM 2010
A synchronous machine responds to a rotating high fre- quency voltage phasor (vector) uc ideally by an elliptically rotating current ic (Fig. 1).
α
jβ
d
ωc
u = ue j cc ωt c
Anisotropie
α
jβ
d
ωc
i = ie j cc ωt c
(a) rotating voltage phasor uc (b) elliptic current response ic
Fig. 1: high frequency voltage phasor uc and elliptic current response ic with respect to the magnetic anisotropy
II ALTERNATING CARRIER SIGNALS Besides encoderless control schemes using rotating high frequency signals there are also algorithms based on alternat- ing carrier signals for detecting magnetic anisotropies of AC mach