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Continuous Real Time Field Control in Magnetic Measurement Systems

 

Introduction

In the R&D and production of magnetic materials, one of the most important goals is to have strict control over the field-related properties of magnetic materials. Sometimes a change of a few percent in the coercive field of a material will make the material useless. For some materials, the coercivity is measured in mOe.

 

To get the best measurements on this type of materials, having a system with real-time field control is highly beneficial. This is the reason why most companies and research labs working on materials with critical field requirements rely on MicroSense magnetic measurement systems with Real Time Field Control for both research measurement systems and production quality control systems. MicroSense VSM systems offer field noise that is at least 10 times lower than our most common competitors. The field resolution is 50 times better than some Helmholtz coil based VSM systems and 60 times better than some super conducting magnet based VSMs.

 

Field Control

What is Real Time Field Control and what makes it so special? Most magnetic measurement systems use an electro- magnet to apply fields. This electromagnet typically consists of a pair of coils, a yoke and a pair of poles. Both the yoke and the poles are made of magnet iron or a magnetic alloy that inherently behaves non-linearly. The current vs. field curve is not a straight line and it shows hysteresis. What may be even worse is that the field at a certain current is not constant; it changes with time as domain walls in the pole tips move and eddy currents die out in the first few seconds and minutes after changing the magnet current. So, to correct for this, a field control system is needed. To explain the advantages of the real-time field control, it helps to first understand how systems traditionally tried to solve this problem.

 

The traditional, non real-time, part-time field control

Traditionally, measurement systems such as VSMs often used an indirect form of field control. When the user (or the program that runs the magnetic measurement) wants to set a field, the sub-routine that controls the field will first program a current (which may have been obtained from a lookup or calibration table or such) from the magnet power supply. After this, an iterative process starts of taking a field readings from the Gaussmeter and adjusting the magnet current until the field subroutine is satisfied with the achieved field and allows the user or the measurement to continue.

This was typically a slow process because of the lag in the communication between the Gaussmeter and the response of the power supply. What is even worse is that once the field subroutine is satisfied and allows the software to take its measurement reading, the field is still not constant. It may still change by several Oersted, due to slow magnetization or demagnetization processes taking place in the pole tips and magnet yoke. By the time the software takes it reading, the field may be different from what it was intended to be and a signal reading will be taken at a field that differs from the field that is reported to the user. This can lead to significant measurement inaccuracies. This problem is potentially even worse when remanence measurements are done, where it is very important that the remanence reading is taken at exactly zero field. If this type of field control sets the field to zero and then stops controlling the field, the field will slowly overshoot beyond zero and reduce the accuracy of the remanence measurement.

 

DSP based systems

Nowadays many systems have moved the field control from the PC to a DSP inside the field control unit. This is much faster and full time (it doesn’t stop controlling the field after the target field has been reached) but it is still an iterative process with some lag in the response and for example 30 updates per second. Possibly more problematic is that in order to use a DSP, the field readings and current setting at some point must be digitized. If one would want a system that regulates the field with a resolution of 1 mOe with a maximum field of 20 kOe, one would need a true resolution of 24 bits in the AD conversion and in the DA conversion. While 24 bit AD converters exist, these converters are typically not linear enough to accomplish true 24 bitlinearity. Furthermore in this type of processing a large amount of noise is introduced. While most of our competitors do not specify the field noise in their systems, based on other information they provide we know that the minimum field noise in their systems is at least 0.5 Oe peak to peak, some 20 times higher than what is offered in MicroSense systems.

 

Fulltime, real-time field control

 

The field control that is used in MicroSense measurement systems works completely differently from what is described above. The field control in MicroSense systems is not managed by a subroutine that resides in the PC running the measurements or in the software of a DSP. Instead, a dedicated piece of hardware has been developed that has only one task: controlling the field. This field control is part of an electronic control loop that continuously adjusts the field to maintain the field to the set point. Because this is dedicated analog hardware, the field control is fulltime. This means that if the remanence of the pole tip slowly changes, the field control unit immediately detects it and continuously makes minute changes in the magnet current to correct for the change. As a result, the field is absolutely steady. Because the field values are never digitized, no quantization noise and similar phenomena are introduced in the process, the field noise in the MicroSense system is extremely low. The noise in the MicroSense VSM systems is essentially equal to the noise in the hall probe measuring the magnetic field.

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