06-07-2012, 02:12 PM
UNIVERSAL CURRENT SENSOR
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ABSTRACT
The measurement of electric current strength is not always easy especially
when the measured signal requires further electronic conditioning. Simply connecting an ammeter to an electrical circuit and reading out the value is no longer enough. The current signal must be fed into a computer in which sensors convert current into a proportional voltage with minimal influence on the measured circuit. The basic sensor requirements are galvanic isolation and a high bandwidth, usually from DC up to at least 100 kHz. Conventional current measurement systems therefore tend to be physically large and technically complex
Early Solutions
The oldest technique is to measure the voltage drop across a resistor placed in the current path. To minimize energy losses the resistor is kept very small, so the measured voltage must be highly amplified. The amplifier’s offset voltage must be as small as possible and its supply voltage must be at the potential of the circuit, often 110 V mains (230 V in Europe) with high parasitic peaks from which its output must be isolated. This requirement increases overall system cost.
principle is the transformer. Its construction is much simpler widespread, but it doesn’t allow the measurement of DC signals. Isolation between primary and secondary sides is implicitly given. A problem is the limited frequency range
Hall sensors also measure the magnetic field surrounding the conductor but, unlike current transformers, they also sense DC currents. A circular core of soft magnetic material is placed around the conductor to concentrate the field. The Hall element, which is placed in a small air gap, delivers a voltage that is proportional to the measured current. This sensor also offers a galvanic isolation.
The very small output voltage of the Hall element must be highly amplified, and the sensitivity is temperature dependent and requires adequate compensation. There is an inevitable offset, i.e., a small DC voltage at zero current; the offset amplitude and temperature coefficient are subject to significant fluctuations. The smaller the current to be measured, the higher the offset-induced relative error. Also of note is sensitivity to short current peaks in the circuit: according to the hysteresis properties of the core material, these peaks can cause a static magnetization in the core that results in a permanent remanence, and finally to an offset alteration of the Hall element.
Figure 2. Magnetoresistive field sensors are usually configured as a half or full bridge. The barber poles are positioned such that in the presence of a magnetic field the value of the first resistor increases and that of the second decreases.
of Hall effect current sensor are open loop and closed loop. In the former, the amplified output signal of the Hall The element is types directly used as the measurement value. The linearity depends on two that of the magnetic core. Offset and drift are determined by the Hall element and the amplifier. The price of these sensors is low, but so is their sensitivity.
Closed-loop Hall sensors are much more precise. The Hall voltage is first highly amplified, and the amplifier’s output current then flows through a compensation coil on the magnetic core (see Figure 1). It generates a magnetization whose amplitude is the same but whose direction is opposite to that of the primary current conductor. The result is that the magnetic flux in the core is compensated to zero.
(The principle is similar to that of an op amp in inverter mode, for
The following figure shows the closed loop type hall sensor
Magneto resistive sensor
Practical magnetic field sensors based on the magnetoresistive effect (see “The Magnetoresistive Effect”) are easily fabricated by means of thin film technologies with widths and lengths in the micrometer range. They have been in production for years in many different executions [1,2,3,4]. To reduce the temperature dependence, they are usually configured as a half or a full bridge. In one arm of the bridge, the barber poles are placed in opposite directions above the two magnetoresistors, so that in the presence of a magnetic field the value of the first resistor increases and the value of the second decreases (see Figure 2).
For best performance, these sensors must have a very good linearity between the meas uredquantity (magnetic field) and the output signal. Even when improved by the barber poles, the linearity of magnetoresistive (MR) sensors is not very high, so the compensation principle used on Hall sensors is also applied here. An electrically isolated aluminum compensation conductor is integrated on the same substrate above the permalloy resistors (see Figures 3 and 4). The current flowing through this conductor generates a magnetic field that exactly compensates that of the conductor to be measured. In this way the MR elements always work at the same operating point; their nonlinearity therefore becomes irrelevant. The temperature dependence is also almost completely eliminated. The current in the compensation conductor is strictly proportional to the measured amplitude of the field; the voltage drop across a resistor forms the electrical output signal