General Drive Control
Below you will find an overview of our competences in the field of drive control.
In addition to rotor position feedback, further in-depth knowledge is required for efficient and dynamic control of synchronous machines. As experts in the field of motor control, we have extensive know-how and many years of experience in the development of sensorless control processes.
Depending on the particular application, various requirements are imposed on a drive. While for small, cost-effective systems the computational efficiency is a major objective, in high power traction drives for instance maximum torque and energy efficiency are paramount. With our long-standing expertise in electrical drive technology across various applications, we assist you to make the most of your system within its limitations.
Our competences cover two main subjects:
Drive control is necessary as soon as the drive is to follow a given setpoint signal, such as rotational speed or torque. The quality of control determines the extent to which the machine is able to follow the setpoint value or how well it reacts to disturbance variables.
However, the quality of control is influenced by:
- Required control bandwidth/dynamics
- Accuracy of sensors (current and position)
- Delays in the feedback
- Limitations of the system (e.g. current and voltage)
- Accuracy of the system model
Apart form achieving the best possible control performance, many applications also demand high energy efficiency of the drive.
We always keep an eye on our goal of ensuring the most efficient and efficient regulation possible, and use our expertise in a targeted manner:
For many applications a fast and robust current regulation is important. Especially within the classic cascade-based control structure, where the current control is located in the most inner loop, the performance of all outer cascades depends on the current regulation. The challenges in the application of a fast current regulation lie in the correct knowledge of the system and in dealing with computational dead times.
Due to magnetic saturation, the inductance values of the machine change depending on the operating point. Many strategies for the control of AC machines are designed for linear behavior between flux and current, which applies realatively well to certain types of machines. However, for machines with a strong saturation this results in considerable disadvantages. For those machine types the control structure therefore has to be fundamentally reworked in order to achieve optimal dynamics, stability and stability.
In order to operate AC machines dynamically in the entire speed range (and beyond) it is not sufficient to use MTPA control (maximum torque per amp). Especially in the middle and higher speed range, it is necessary to make use of the remaining voltage reserve. By using intelligent setpoint trajectories, stable and optimal operation at the voltage limit can be achieved. The figure shows an example of the possible expansion of the power range when using an intelligent strategy to select points offside the MTPA. However, this only occurs when required during power peaks, since the optimum efficiency of the MTPA is used for stationary operation.
Good current measurement is essential for the use of sensorless methods. For this reason, we have built up a great expertise in this field. However, also current control benefits from a good current measurement so that the performance of the whole system can be improved significantly. We advise you individually with regard to the various current measurement strategies in order to find the best compromise between cost and current measurement quality.
The knowledge of current and rotor position are the basis for the application of field-oriented control. While current measurement takes place directly in power electronics, a position sensor must be attached to the shaft of the machine for position detection. It is also possible to use sensorless methods as an alternative to the application of a position sensor, which determine the rotor position based on the measured current and the applied voltage.
In addition to our actual product, the software position sensor dynAIMx®, we also offer other services related to the issue of sensorless control. These services generally aim at the ability to use our library dynAIMx® in the best possible way. Moreover, the knowledge gained enables you to better understand and apply the potentials and limitations of sensorless rotor position identification.
We offer advice concerning the sensorless detection of the rotor position in the following key issues:
In the case of sensorless control, the motor itself is used as a sensor for the identification of the rotor position in addition to current sensors. For this reason, not every motor is equally suitable for this technology. Especially designs with concentrated winding topology place high demands on sensorless control.
By using sensorless control, the current measurement becomes the source of information for the rotor position identification and its signal quality is crucial for the properties of the rotor position signal. In this case, the current measurement does not necessarily have to be expensive since an improvement can also be achieved by software-based approaches. Depending on the power class and the price segment, there are a number of traditional and new approaches that should ideally be discussed during the hardware conception phase.
The rotor position information within the induced voltage (EMF) becomes too weak at low and zero speed. Hence, at such operating points the rotor position is determined by measuring the phase inductances using an high frequency voltage excitation. This creates an audible noise, which may not always be acceptable for sensitive applications. Also in this case concepts can be selected which reduce the noise to an acceptable level or completely cancel it by means of a frequency shift into the non-audible range.