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The Use of Single Turn Absolute Magnetic Sensors in 3D
Reconstruction of Objects
IVAN KREJČÍ, Michal Vopálenský
Department of Electrical Engineering and Computer Science,
College of Polytechnics Jihlava, Jihlava, Czech Republic
Introduction
Usual application of absolute angular position sensors is the determination of a rotating
element position in tasks of the industrial automation, e.g. in the actuating mechanism
position control. They often replace incremental sensors (encoders) because of the actual
absolute position availability, easy data processing and lower price. In principle, they take
advantage of a permanent magnet placed on the rotating part of the object, the position of
which is to be measured. The rotation changes the magnetic field orientation, which is
measured by the magnetic sensor placed close to the magnet. The sensor contains an
integrated array of Hall elements [1], [2] placed symmetrically to the rotating axis. This
arrangement makes possible to detect the angle position within one revolution and offers it as
the unique digital code. High angular resolution and easy data processing led to the idea to use
the sensors for position digitization of micrometric screws controlling the markers placed on
important points of a stereoscopic image of an object, the 3D reconstruction of which has to
be done. The developed device senses the rotary position of four setting screw micrometers
for table position adjustment. The mechanical resolution is 4mm in the range of 75 mm.
The mechanical and electrical design of measuring device taking advantage of the ams A.G.,
integrated circuit, the 12-bitmagnetic rotary sensor AS5045, as well as the signal processing
and achieved results are discussed.
The sensor
The sensor construction can be divided between the mechanical and electro-magnetic
performances which determine the sensor accuracy and repeatability of its position reading.
As to the mechanical arrangement, the system consists of two basic parts, the stator and rotor.
The stator consists of an aluminium shield, the function of which is to carry the PCB with the
sensor electronic circuitry and to serve as the bearing of the rotor. The rotor is created by a
brass shaft equipped with the case of the permanent magnet placed in the axis of the
equipment on one side of the shaft. There are two conditions that determine the best sensor
performance, placing the magnet and sensing element in the axis of rotation and minimizing
radial clearance of the shaft. Both conditions are achieved by the precision mechanical design,
namely by the long leading of the shaft in the slide bearing. The mechanical performance is
shown in the picture Fig. 1.
Fig.1. The mechanical construction of the sensor. Mention the permanent magnet placed in
the internal rotor part.
The electrical construction involves the circuits of the magnetic sensor itself, and of the
communication with a host system. The SSI link taking advantage of the RS 422 logical
levels is used for the data transfer from the sensor to the host. The host computer operates as
the master unit of that communication, and therefore it is the source of the serial clocks.
Besides, the EMI preventing circuits are integrated to the electronic equipment. All circuits
are placed on a PCB, the diameter of which is 25 mm. The PCB is mounted to the cavity in
the stator so that the sensing element is placed close to the permanent magnet. The sensors
shafts are connected with micrometric screws with known lead so that it is possible to
transform the change of the angle position to the linear position change.
The host computer unit
The host computer unit collects data from four magnetic sensors the output data of which
represent coordinates of a selected point in the stereoscopic snapshot of a reconstructed space,
transfers them to the string of ASCII characters and sends them via the standard serial link RS
232 to a PC. To do it correctly, its firmware is capable of the determination of both snapshots
reference (zero) points. Measured positions are then related to these references. The unit is
handled by two pushbuttons which make possible to change the machine status or check the
data. All information dealing with the system status and actual pushbutton function are
displayed on a two line LCD. The unit is controlled by the Texas Instruments Stellaris
LM4F120 microcontroller. The system is supplied from an AC/DC adapter the output voltage
of which is 5V / 1A. The unit performance is shown in the picture Fig. 2.
Fig.2. The host computer unit overall view.
Conclusions
The equipment makes possible to digitize the positions of important points in the stereoscopic
snapshots during 3D reconstruction of objects. This method is often used in the non – invasive
archaeological mapping of inaccessible rooms or cavities. In these cases, the stereoscopic
photographs are provided by a special probe equipped with a stereoscopic camera. The
snapshots are made from well defined positions so that the internal objects dimensions could
be determined observing the change of their parallax.
The parallax measurement requires the accurate and sensitive position measurement of
analysed object displayed in the stereoscopic picture. From this point of view, the used
magnetic sensors with well defined and stable magnet-to-sensing element geometry offer the
problem solution, because their 12 bit resolution makes possible to resolve 4mm in the range
of 75 mm.
References
[1] AS5045, 12 Bit Programmable Magnetic Rotary Encoder, Datasheet v. 1.7, p. 12,
austriamicrosystems A.G., Schloss Permstätten, Austria,
[2] Motion Control, Catalogue of the Baumer Electric A.G. p. 2.06, Baumer Electric A.G.,
Frauenfeld, Switzerland, Dec. 2004.