Power Management & Industrial Electronics
Measurement techniques
With motors consuming upwards of 40 per cent of worldwide energy, measurement techniques for industrial motion control are becoming increasingly important
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ndustrial motion control covers a breadth of applications, ranging from inverter based fan or pump control, to factory automation with more complex AC drive control, to advanced automation applications such as robotics with sophisticated servo control. These systems require sensing and feedback of a number of variables such as motor winding current or voltage, DC link current or voltage, rotor position and speed. The selection of variables and the required measurement precision depends on the end application demands, the system architecture, target system cost, or system complexity amongst other considerations such as value-add features e.g. condition monitoring. With motors reportedly consuming 40% of worldwide energy (Frost & Sullivan), international regulations have increased the focus on
simple control systems like pumps, fans and compressors etc. that can be implemented without precision feedback and just use a simple microprocessor. As the system complexity increases, moving towards the higher end of the spectrum, complex control systems require precision feedback and fast communications interfaces. Examples of these would be sensored or sensorless vector control of induction motors or permanent magnet motors, and high power industrial drives designed for efficiency - shown in Figure 1 as large pumps/fans/compressors.
Drive architectures There are many challenges associated with designing systems to address the variety of applications within the industrial motion control space. A generic
measurement is not trivial. For example, recovering small signals or transmitting digital signals within such a noisy environment is challenging, while isolating an analogue signal is an even bigger challenge. In many cases, signal isolation circuits introduce phase delays that limit system dynamic performance. Phase current sensing is particularly
application also impacts the demand on the ADC functionality. For example a higher channel count ADC may be required for multi axis control.
Current and voltage sensors The most commonly used current sensors in motor control are shunt resistors, Hall effect (HE) sensors and current
Figure 2: Generic motor control signal chain Figure 1: Industrial drive application spectrum
system efficiency across the entire range of Industrial motion applications increasing the importance of these variables, especially current and voltage.
Application spectrum Motor control applications can range from simple inverters to complex servo drives, but all include motor control systems with a power stage, a processor that drives a pulse-width modulator (PWM) block with differing levels of sensing and feedback. A simplified view of the application spectrum is shown in Figure 1, illustrating systems with increasing complexity as one moves from left to right across this spectrum, from
motor control signal chain is shown in Figure 2.
Of critical concern are isolation
requirements and these usually have a significant influence on the resulting circuit topology and architecture. There are two key factors to consider; why to isolate and where to isolate?
Current measurement techniques Signal chain implementations to sense current and voltage differ by sensor choice, galvanic isolation requirements, ADC choice and system integration in addition to system power and ground partitioning as already outlined. Signal conditioning to realise a high fidelity
32 September 2014 Components in Electronics
challenging, as this node is connected to the same circuit node as the gate driver output within the heart of the power stage (inverter block) and therefore experiences the same demands in terms of isolation voltages and switching transients. Determining the measurement signal chain (technique, signal conditioning and ADC) to be implemented within a motor control system relies on three key factors. 1. The point or node in the system, as this determines what needs to be measured.
2. The power level of the motor and
resulting sensor choice - one that is inherently isolated or not. The sensor choice has significant influence over the ADC choice, both in converter architecture, functionality and analogue input range.
3. The end application. This can drive the need for high resolution, accuracy or speed within the sensing signal chain. Implementing sensorless control over a wide speed range for example, demands more measurements, taken more often and of higher accuracy. The end
Measurement locations and topologies Quite apart from the sensor type, there are several motor current measurement nodes to choose from. The average DC link current can be used for control purposes but motor winding current is used as the primary feedback variable in more advanced drives. Direct in-phase winding current measurement is the ideal and used in high performance systems. However, the winding current can be measured indirectly using a shunt in each
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transformers (CT). While shunt resistors don’t provide isolation and incur losses at higher currents they are the most linear of all the sensors, the lowest cost and suitable for both AC and DC measurements. CTs and HE sensors provide inherent isolation allowing them to serve high current systems, but they are higher cost and result in a less accurate solution than that which can be achieved through shunt resistor usage, either due to the sensor itself having poorer initial accuracy or worse accuracy over temperature.
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