Observers or sensors for drive controls?

This article shows why open-loop, algorithmic motor control is an accurate, less-costly alternative to sensor-based drive solutions. In combination, the two offer unique redundancy value. Read why.

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&gMotor ControlBy Prof. Dr.-Ing. habil. Klaus Hofer, Bielefeld Technical College


Electrical drives are the tireless driving force behind almost all the movement, dynamics, and convenience found in an industrialized society today. They cover a wide range of applications, from vacuum cleaners to elevators and electric vehicles, all the way to fully automated production lines. With the exception of very-simple applications, electrical drives usually are controlled, regulated and networked by powerful microcontrollers.

The greatest potential for innovation in modern drive engineering lies in the possibility of increasing system intelligence by replacing expensive sensor hardware with inexpensive “observer” software.

The increased intelligence of electrical drives occurs in conjunction with the monolithic fusing of power electronic and microelectronic components at the chip level, making it possible to install complete drive electronics as a power chip in the terminal box or plug connector of the motor.

Drive Engineering Overview
The ability to cover all four quadrants of the speed/torque plane is a prerogative of electrical drives. Controlled electrical drives with their excellent dynamic properties, high flexibility, and efficiency have become an important pillar in the automation industry.

The history of drive engineering began more than 140 years ago, when Werner von Siemens discovered the electrodynamic principle in 1866. It continued with the development of rotary and linear machines as well as transformers at the end of the 19th Century. The history of power converters did not start until the 20th Century with the development of mercury-arc rectifiers. The development of thyristors caused an enormous leap in the innovation of power converters when introduced in 1959. It now was possible to use these controlled-conductivity semiconductor valves to control rectifiers, inverters and converters without problems, and very efficiently. This process was supported by the introduction of integrated circuits such as operational amplifiers and gate modules.

A completely new era started in the mid-1970s when signal processing changed from analog and discrete signal processing to digital technology based on microprocessors. Such digital electrical drives can be used in all areas of advanced automation with connection to a master computer system by parallel or serial interfaces (fieldbuses). The advent of voltage-controlled power transistors, or insulated gate bipolar transistors (IGBTs), in the 1980s cleared the way for practical use of model-based control processes, such as the estimation of actual values with observers or the field-oriented control of three-phase machines with frequency-inverter supply. The power profile of electrical drive engineering increasingly will be governed by highly integrated and multi-functional power electronics in the form of intelligent power chips.

Model-Based Drive Control
The goals of model-oriented control theories concentrate on refining the control process and reducing the sensor workload. Two pioneering developments here were 1) the field-oriented control of three-phase machines by means of a mathematical model, and splitting up the motor current into a flux-forming and a torque-forming component, and 2) EMC control with IxR compensation for DC drives to eliminate the need for a speed sensor.

The replacement of expensive sensor hardware with inexpensive observation software is a milestone in drive engineering because it offers a tremendous potential for growth combined with considerable competitive advantages for users.

Figure 1 below compares the basic control modes in drive engineering. Based on the physical equation of motion, which states a change in speed can only be achieved using acceleration torque, the result is that the familiar cascade structure (nesting) is the optimum control structure for all electrical drives. This means the output of an external closed-loop speed controller indicates the setpoint for any secondary torque controller. As the torque in electrical drives is derived from the motor current, the torque controller will be a current controller that also protects the machine from overcurrents.

Comparing Controllers
The basic modes of drive control involve either (a) closed-loop feedback, (b) state control, and (c) an algorithm-driven binary observer combination.

In a conventional controller design (Figure 1a), the equation systems will be transferred from the time range to the complex range using the La Place transformation. This makes it possible to issue statements concerning the stability of dynamic systems quickly and easily. The systems are, in turn, suitable for designing matching control equipment (P, PI, PID).

The design of state controllers and state observers (Figure 1b) takes place in the state space (time range), which makes it possible to evaluate electrical drives for controllability and observability. Compared with conventional drive controls, this means you don’t need to measure speed when integrating a state observer. A mathematical resource in the state space is the matrix operation and the pole setting.

If you implement cascade control in Figure 1a or state control in Figure 1b into a microprocessor system, both cases require an expensive analog interface (ADC) that has to operate at a higher scanning rate and bit-width due to the severe ripple of the electrical state variables. You eliminate this immense sensor expense completely when the actual value of the electrical state variable is determined by a binary observer (Figure 1c). Non-linear observer algorithms for voltage and current mean values can be derived using the Fourier transformation due to the periodic switching operation of the converter valves.

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