Magnetic Sensors for Motion Control

The challenges associated with improving performance and reducing system costs in a motion control system have led the market towards the trend for magnetic devices. The current transducer is used to measure the current without interrupting the circuit, thus making the measurement very safe.

Along with the current detection, there are several control techniques or algorithms used to control the motor. The control circuits in various industrial devices must operate as efficiently as possible, and in total safety. Higher efficiency translates into enormous potential as a result of greater power and reduced heat dissipation, which can eliminate the need for heat sinks in the engine. The current transducer uses the magnetic field to determine the current of the conductor. The current sensor accurately detects the current in the motor to offer perfect control of synchronization and orientation.

CURRENT SENSORS

A power supply and drive system require solutions to monitor and control its operation. In the past, this function was performed by electromechanical devices. Replacing these mechanical devices with their electronic equivalents has allowed the monitoring system to become more versatile.

In the design of the current transducer, the input current is isolated from the output current. As in the industrial environment, many interferences cause the measurement signals to be inaccurate, the use of the current transducer can eliminate these interferences. Moreover, in the case of high voltage or current, it can enter protection mode, interrupting the conversion process, and thus providing isolation between the input and output.

Current sensors can have two leading technologies: open-loop or closed-loop. The originally developed current transducer is the Open Loop Hall Effect. The current transducer consists of three parts: a magnetic circuit, a Hall cell, and an amplifier. The closed-loop differs from the open loop by adding a secondary winding to the output. The advantages of the closed loop are a virtual absence of parasitic currents, immune to variations in gain as a function of temperature and higher bandwidth. Closed-Loop Flux gate measurement technology essentially removes the Hall effect with a Flux Gate detector. The latter is essentially a winding located in the empty space of a magnetic circuit.

Hall effect sensors are devices activated by the presence of an external magnetic field. The Hall effect is essentially a manifestation of the Lorentz force acting on each electric charge that moves in a magnetic field B. When they are subject to this magnetic field, they respond with an output voltage that is proportional to the strength of the field. Since the output voltage is in the order of mV, the Hall effect sensor is combined with other electronic components such as amplifiers, to improve sensitivity and hysteresis.

The ACS724 current sensor IC from Allegro is an accurate solution for the detection of AC or DC current in industrial, automotive, commercial, and communication systems. The device consists of a linear Hall sensor circuit with a copper conduction path located near the mold surface. The applied current flowing generates a magnetic field that is detected by the Hall sensor and converted into a proportional voltage supplied by the BiCMOS. The current is perceived differentially to repel the fields in standard mode, improving accuracy in magnetically noisy environments (figure 1).

The Danisense current sensor technology is based on a closed loop system, powered by flux gate just as the magnetic field detector. Danisense’s DS200 works with currents up to 200 A at a ratio of 1:500, while the DS600 can be used up to 600 A (figure 2). National Instrument used Danisense current sensor solutions in tests with its RM-26999 4-channel power measurements conditioner. Additional Danisense current transducers including DS50UB-10V, DS200UB-10V, DS600UB-10V, and DS2000UB-10V are characterized by high accuracy, high bandwidth, and low phase shift: these are fundamental requirements for accurate power measurement.

LEM provides a new family of LZSR printed circuit board (PCB) transducers for the non-intrusive and isolated measurement of DC, AC, and impulsive currents, from 100A to 200A nominal. The family consists of various models for application where a low offset drift is required.

Figure 1: block diagram of the ACS724 [Source: Allegro]
Figure 2: Danisense’s latest solution was implemented in the CERN accelerator [Source: CERN / Danisense]

CURRENT SENSING MODE

There are mostly three ways to measure the current in a motor control system: measurements on the high-side, low-side, and in-line measurements. When a pulse width modulated signal (PWM) is used to drive the motor, in-line measurements are more challenging to implement to obtain accurate values; this is primarily due to common mode transients (dV / dt). In an inverter system with control feedback, phase measurements are essential for a correct estimate of the current.

In a three-phase motor, a series of polyphase PWM signals guide the load through the creation of magnetic fields (figure 3). A brushless motor is generally more efficient than its brushed DC counterpart, mainly due to the absence of brushes. Furthermore, the engine is switched electrically rather than mechanically. However, the advantages described here have a negative aspect which is represented by a more complicated engine that requires electronic control to reach the maximum efficiency.

Figure 3: Simplified Block Diagram of a Three-Phase Motor [Source: Texas Instruments]

In low-side current sensing, an amplifier is added to gate-driven FETs with a shunt resistance positioned in line with each switching pin (Figure 4). The common mode voltage at the shunt is about zero; therefore, the robustness of the amplifiers is not an essential factor to keep in mind. However, this solution places a further resistance between the load and the load ground path, compromising the ability to detect faults in the load of short circuit.

Figure 4: Low-side measure of Current Sensing [Source: Texas Instruments]

For the detection of the high side current, a shunt is inserted in the line immediately after the supply voltage of the FET matrix. With this method, the system can detect ground faults. In any case, the common mode voltage experienced by this amplifier is approximately that of the supply voltage (Vsupply) and requires a more robust differential amplifier which is able to meet this requirement.

Please visit Power Electronics News for the complete article

Maurizio Di Paolo Emilio is power electronics editor and European correspondent at AspenCore and editor of Power Electronics News.