Brief Introduction to Magnetoelastic Torque Sensors
Magnetoelastic torque sensors produce signals that are a function of torsional stress, not strain. As a result they are generally much stiffer mechanically than the conventional elastic torque sensors. Additionally, they offer significantly higher frequency response, typically on the order of 2-4kHz. Indeed the determination of surface stress from the measurement of magnetic quantities by magnetoelastic methods provides an inherently non-contacting basis for measuring torque in a more compact construction than those required for either the twist angle or surface strain elastic methods. Magnetoelastic torque sensors measure magnetic quantities related to the surface shear stress by either of two methods:
a. PB Type I - "Permeability Based"
In Type I, the permeability changes in the shaft surface, caused by the stress-induced magnetic anisotropy, affect the permeance of a magnetic flux path which includes a magnetizing source and a pickup (sensing) coil.
b. PB Type II - "Polarized Band"
In Type II, the stress-induced magnetic anisotropy causes a remanently magnetized magnetoelastically active member to generate a measurable magnetic flux.
Traditional, PB Type I "Permeability Based", Magnetoelastic Torque Sensors
As explained above, in a Type I torque sensor the output signal derives from changes in permeabilities of regions on the shaft surface due to the magnetic anisotropy that arises with transmitted torque. Specifically, larger values of permeability occur along the stress induced easy axis and smaller values occur along the hard axis which is perpendicular to the former. Advantages of the Type I permeability-based magnetoelastic torque sensor stem from its naturally wireless transduction and mechanically robust construction. Various specific excitation/pickup constructions can be utilized, but local variations in magnetic properties of typical shaft surfaces limit the attainable accuracy in both the branch and the cross pickup constructions. In order to eliminate many of the rotational issues associated with the cross and branch Type I constructions, another important group of Type I torque sensors was developed utilizing an axisymmetric solenoidal construction. In this design, oppositely directed helical grooves are machined or formed (typically along ±45° angles to the axis) on adjacent circumferential regions of a steel shaft, and solenoidal coils encircling these regions are used for excitation and sensing. The axial permeability of a grooved region increases when the easy axis of the stress-induced magnetic anisotropy occurs in parallel to the line of grooves, whereas it decreases otherwise. This results in different voltages being induced in the sense windings encircling the two regions and this difference provides the measure of the torque. The solenoidal construction for Type I magnetoelastic torque sensors, by way of the axisymmetric structure of the windings, hides local variation of the magnetic properties of the shaft. This is one of the reasons why it is the most common implementation of a Type I sensor.
Despite their various benefits, Type I permeability-based magnetoelastic torque sensors suffer from a number of deficiencies which have limited their proliferation in real-world applications. These deficiencies ultimately stem from the fact that the variable being measured, permeability, does not depend exclusively on the applied torque. In particular, it is important to note the following:
- Permeability is not an intrinsic property of a magnetic material
- Permeability is not a single-valued, structure-sensitive property
- In any one material composition, processed in one controlled manner, permeability will still generally vary over a large range both with temperature and with magnetization
- Permeability will also generally vary with frequency
The end result is that the range of permeability variation with most of these factors, which do come into play in many practical applications, can exceed the changes in permeability that are a function of torque, which is the quantity of interest.
MagCanica's PB Type II, "Polarized Band" Torque Sensor Technology
Schematic diagram of the operating principle
underlying MagCanica's PB Type II Polarized
Band torque sensor system.
MagCanica is a leader in the development of the novel Type II, or polarized band, class of magnetoelastic torque sensors. These sensors provide many of Type I sensors' benefits of noncontact, robust, compact construction, while at the same time overcoming many of their problems.
In MagCanica's PB Type II, or Polarized Band sensors, magnetization of the active region does not occur continuously in service but rather is carried out one single time before the sensor enters service. Sensor operation is based on the reorienting effects of torsional stress on the individual magnetic moments that have been remanently circularly magnetized.
In response to the magnetoelastic energy associated with the biaxial principal stresses by which torque is transmitted along the shaft, each moment will rotate towards the nearest positive principal stress direction and away from the nearest negative principal direction. This reorientation of the originally circular magnetization results in a net axial magnetization component. The divergence of this component at the edges of the polarized bands is the source of a magnetic field in the space around the shaft which can be readily measured with one or more magnetic field sensors.
Type II torque sensors can be constructed either with a thin ring of magnetoelastically active material rigidly attached to the shaft, or by using a portion of the shaft itself as the magnetoelastically active element. Subsequently, when torque is applied, the magnetizations tilt into helical directions, causing magnetic poles to develop at the central domain wall and at the end surfaces. The polarity of the magnetic poles reverses when the applied torque changes its direction, and so does the output signal accordingly. Torque is determined by measuring magnetic flux with one or more magnetic field sensors. Notice that permeability variations do not come into play in the Type II sensor, for the quantity being measured is an externally detectable magnetic field whose intensity is linearly proportional to the shear stress (and by extension, the applied torque).
Schematic diagram of a driveshaft having two polarized bands, surrounded by
an array of magnetic field sensors (FS) to detect the magnetic field under torque.
This concept can be readily applied, for example, to a practical application such as a driveshaft. Polarized bands are applied to a dedicated measurement region on the shaft, and an array of field sensors (FS) is placed in two diametrically opposed pairs around the shaft circumference to detect the field created by the shaft under torque.
Schematic diagram of a driveshaft having two polarized bands, surrounded by an array of magnetic field sensors (FS) to detect the magnetic field under torque.
Benefits of MagCanica's Torque Sensor System
The following is a list of the key characteristics of MagCanica's magnetoelastic, non-contact torque sensor products. These characteristics represent our core advantages that differentiate our products and services from those of our competitors:
- Increased torsional stiffness
- Reduced size and mass
- Non-invasive measurement
- Packaging flexibility
- Reduced complexity
- Naturally wireless mode of measurement
- True non-contact solution
- Outstanding frequency response and dynamic torque measurement (up to 4.5 kHz)
- Scaleability (from the milli Nm range to the Mega Nm range)
- Direct applicability to shafts made of existing automotive and aerospace steel alloys