By Thomas Polzer, Development Engineer, Physik Instrumente (PI) GmbH
Direct-drive ultrasonic piezo motors enable ultra-low response times and highly precise positioning without influence from strong magnetic fields. This paper examines one such positioning system – PILine® from Physik Instrumente – for use in strong magnetic fields, or in environments where the drive needs to behave neutrally in a magnetic sense.
When a material is introduced into a magnetic field, it starts to interact physically. These interactions can influence the desired magnetic field to an unacceptable extent or can also disturb the functioning of technical systems in the magnetic field. If these interactions are adverse, so-called non-magnetic drives are used.
In nature, there are various forms of magnetism that cause different degrees of physical interaction.
Differentiation Between Diamagnetic, Paramagnetic and Ferromagnetic Materials
Materials can be divided into three groups with different magnetic properties. Here, the interactions within a body that are caused by the external stimulus play an important role. To differentiate, we may consider the mathematical dependency of the magnetic field.
B = μ0 ∗ μr ∗ H [ T ] with μ0 = 4 ∗ pi ∗ 10−7 V∗s A∗m
The magnetic flux density B (or, in everyday language, magnetic field) is linked with the magnetic stimulus H via the permeability. The permeability is made up of the natural constants of the permeability μ0 and the material-based relative permeability μr. In everyday life, the magnitude of the magnetic flux density ranges from the strength of the earth's magnetic field with 20-70 μT to magnetic resonance scanners with 7 T.
Diamagnetic Materials
Diamagnetic materials form a field that opposes the external stimulus. This is because the moving electrons in the material are deflected by the external field, causing the buildup of a field that opposes the external field. The following applies for the relative permeability < 1. The diamagnetic effect is very minor, disappears when the external field is switched off, and occurs for all materials. It often gets superimposed by the paramagnetic or ferromagnetic effect. An example of the diamagnetic effect is the levitation of pyrolytic graphite above permanent magnets.
Paramagnetic Materials
In the case of paramagnetic materials, microscopic dipoles in the material line up in the external magnetic field, leading to an increase of the field in the material. Hence, paramagnetic materials are drawn into a field. In this case, μr > 1 applies to the relative permeability. On removing the external field, the effect also disappears.
Ferromagnetic Materials
Ferromagnetic materials behave in a similar way to paramagnetic materials; the difference being that when the external field is removed, a so-called "remanent polarization" remains, and the actual material creates a magnetic field. In this case, the relative permeability μr >> 1 can reach values of up to 10^6. Whereas forces can only be proven using complex tests for diamagnetic and paramagnetic materials, these are far stronger for ferromagnetic materials. Permanent magnets made of neodymium iron boron are an impressive example of highly magnetic interaction forces.
Therefore, in positioning systems that are not meant to influence the magnetic field only diamagnetic and paramagnetic materials should be used. In other words, materials with a relative permeability of approximately 1. In this case, the influence of the internal magnetic fields from the respective components on the external magnetic fields is very low, meaning that the influence on the application or on the positioning system is insignificant. Hence, if possible, for non-magnetic applications the use of (strong) ferromagnetic materials should be avoided.
Operating Principle of PILine Positioning Systems
PILine® positioning systems, developed by Physik Instrumente (PI), are based on a direct drive with ultrasonic piezo motors. This enables very low response times and exact positioning. Due to the special operating principle of these positioners, they are self-locking even when switched off. The drives do not influence magnetic fields, nor do magnetic fields affect their function. At the core of every PILine system is a piezoelectric actuator that is preloaded against a runner using a coupling element (Figure 1).
Figure 1: Schematic design of a PILine motor: A piezoelectric actuator is preloaded against a runner. The actuator is stimulated to oscillate.These oscillations are converted into a forward motion that is transmitted to the runner via a coupling element (Image courtesy PI).
When a voltage is applied, the inverse piezoelectric effect causes the ceramic to deform. The deformation takes place at crystalline level within the material and is hence not affected by magnetism. This is why these motors do not interact with magnetic fields, and why they can be used in the following applications:
- Operation in strong magnetic fields;
- Use in a sensitive environment without influencing existing magnetic fields.
By integrating sensor technology, a PILine system can make controlled movements across wide travel ranges in the millimeter range, with resolutions in the nanometer range, and a speed of up to 200 mm/s. When at rest, or powered down, the drives are self-locking. Linear and rotational systems are possible.
Selecting Materials – Typically Used Materials
The knowledge of usable materials and their relative permeability is essential for setting up a non-magnetic system that does not influence the magnetic field of a sensitive environment. Whereas plastics and ceramics are usually diamagnetic, metals may have paramagnetic or ferromagnetic properties. The following table lists some of the materials which, for example, can be used in non-magnetic stages with the PILine drive.
Table 1: Material values for relative permeability μr. (Table courtesy PI)
Cold forming causes austenitic corrosion-resistant steels to form friction or deformation martensite, meaning that materials with a low relative permeability in their original state subsequently have a much higher μr. A material that very clearly shows this behavior is 1.4305 (grade 303 stainless steel). Hence, the relative permeability may be between 1.05 and 3.42, depending on the extent of deformation. This particularly needs to be considered when using austenitic corrosion-resistant standard parts as, in most cases, there is no information available on the degree of cold deformation.
Table 2: Material values for relative permeability μr for different degrees of deformation of 1.4305. (Table courtesy PI)
Magnetic Measuring of Stages
The Research Center Jülich (Forschungszentrum Jülich) has been using rotation stages equipped with PILine piezo motors for high magnetic field applications.
For the exact characterization of dipole magnets, it was necessary to determine the influence of the positioner on the subsequent characterization of the magnets. To do this, they measured both a U-624.03 PILine standard positioner and a special version based on the U-624, optimized for non-magnetic environments. The U-624.03 rotation stage has a surface of 30 x 30 mm² and a height of 12 mm.
Figure 2: Model U-624.03 PILine standard positioner. (Image courtesy PI)
Measurement Setup
The magnetic measuring of the positioners took place in a measuring chamber at the Forschungszentrum Jülich and is illustrated in the following figure. An external permanent magnetic field of 123 mT was created that was aligned along the positive x axis. The field component thus induced in the Y direction was scanned in the area illustrated in green at approximately 1 mm above the rotor surface of the PILine positioner. The cable for the rotation stages was always placed to the right-hand side.
Figure 3: Measuring setup for magnetic measurement of rotation stages (Image courtesy PI)
Measurement Results
In a first step, the U-624.03 standard rotation stage was measured in the external magnetic field of 123 mT containing ferromagnetic components. Hence, it was possible to measure the values of the field component in Y at approximately 1 mm above the rotor level; the values ranged between -800 and 800 μT. The following figure shows the results of this measurement. Here, the external contour is shown as a square, the positioner's rotor as a circle, and the cable as a line pointing to the right.
Figure 4: U-624.03 standard rotation stage without modified material selection (Image courtesy PI)
Two poles are formed, visible here as a red and a blue area, in which the field lines are vertical to the image plane and run into or out of it. The standard rotation stage has a ball bearing made of chromium steel near the rotor surface that causes the high measurement values.
For the second U-624.03, all ferromagnetic components were replaced by suitable replacement components with low relative permeabilities. Here, relevant modifications are the all-ceramic ball bearings, numerous changes of components on the PCB and screws made of titanium.
The previously described measurements in the magnetic field were also made with this modified positioner.
Figure 5: U-624 modified, measurement 1 (Image courtesy PI)
During the first measurement, even when displayed in the already reduced measuring range of -200 to 265 μT (Figure 5), no obvious influences on the components used in the positioner are visible. Here the magnetic field appears almost homogenous. Therefore, it is necessary to make a second measurement with a further reduced measuring range (-25 to +106 μT) to be able to record and illustrate the significantly lower field values (Figure 6).
Figure 6: U-624 modified, measurement 2, high resolution (Image courtesy PI)
The maximum stray field originating from the positioner that is made visible by this measurement is in the bottom right corner and has a value of 106 μT. In this position of the rotation stage the sensor head, which is necessary for closed-loop operation of the positioner, is installed below the rotor.
Due to the described modifications, it was possible to reduce the measured magnetic field value by ~87 % compared with the U-624.03 PILine standard rotation stage, and at 106 μT, this is in a very low range. Hence, the modified positioner is suitable for many non-magnetic applications
Function Tests in External Magnetic Fields
Function tests in high magnetic fields were further important investigations conducted at the Forschungszentrum Jülich. As well as the minor influence on the magnetic fields to be measured, the positioners in the planned application must also be able to move a measuring probe in the high magnetic field in order to exactly characterize the dipole magnets. In this case, both U-624.03 PILine rotation stages were also tested, with and without modifications. A PI C-867.1U controller with PIMikroMove® software were used for control.
Measurement Setup
The positioners were screwed onto a plastic block, and installed into the gap of a dipole magnet with a maximum static field of 1.8 T. The direction of the magnetic field was vertical (Figure 7) or aligned parallel to the axis of rotation, and the positioners were in the middle of the dipole field.
Figure 7: Large dipole magnet with U-624.03. Controlled movements with an angular velocity of 360 °/s were made in the travel range of 0° to 360°.The magnetic field was gradually increased to 1.8 T, and the measurements were repeated accordingly. (Image courtesy PI)
Measurement Results
It was possible to reference and position both the standard U-624.03 positioner and the modified version without errors up to the maximum magnetic field of 1.8 T. In the process, the function was always correct; both with the perpendicular or parallel alignment of the field to the axis of rotation. Rotation speeds of up to 720 °/s at 1.8 T were also tested successfully.
Summary
PILine drives work due to the inverse piezoelectric effect. The deformation takes place on a crystalline level and is hence not affected by magnetism.
A technical system that is brought into a magnetic field interacts with it. This can influence the magnetic field significantly, or, on the other hand, destroy the function of the technical system. Hence, when using closed-loop positioners in magnetic fields, the demands on the selection of materials and their non-magnetic properties are very high.
It is possible to use the relative magnetic permeability μr as a measure of the material-specific property to interact with the magnetic field. Systems to be used in the environments described should consist of materials that are close to a μr value of 1.
The magnetic measurement proved the excellent non-magnetic properties of a modified rotation stage based on the U-624 with a PILine drive. The investigated positioning system could also be operated smoothly and reliably in high magnetic fields of up to 1.8 T.
About the Author
Thomas Polzer is a development engineer for piezo drives and systems at Physik Instrumente (PI) GmbH.