Optimization ofa Low-Cost Position Sensor fora Permanent Magnet Linear by brm24619


									   Optimization of a Low-Cost Position Sensor for a
          Permanent Magnet Linear Drive
                              Ralf Wegener1 , Sebastian Gruber2 , Kilian N¨ tzold2 , Stefan Soter1
                  1 Institute   of Electrical Drives and Mechatronics, University of Dortmund, Germany
                                http://eam.e-technik.uni-dortmund.de; Fax: +49 231 755 7374;
                                     ralf.wegener@uni-dortmund.de; stefan.soter@ieee.org
                        2 Electrical    Machines and Drives Group, University of Wuppertal, Germany

Abstract— This paper deals with a custom made low cost sensor for measu-        any linear positioning sensor, but the usage of a standard sensor
ring the position of a permanent magnet linear motor. The sensor based on       with accuracies from 50µm up to 10µm is very expensive. In this
two hall sensor elements provides an adequate accuracy for lower ranking
control loop of the position if an upper control loop is needed. The achieved   case a linear sensor with an accuracy of 200µm to 1mm is ab-
accuracy is around 0.2mm.                                                       solutely sufficient. A positioning sensor with this accuracy can
Two different sensor layouts are presented and the optimization steps are       be designed by two low-cost hall sensor elements, some surface
described to develop this kind of sensor. The electromagnetic interferences
are eliminated by the design of the sensor element.
                                                                                mounted devices and a small pcb. The permanent magnets ha-
The proper operation of the sensor is proven by measurements with a linear      ve to provide a sinusoidal field outside the winding area of the
drive in comparison to a commercial high precision linear sensor.               machine to control the linear drive. In most cases the sinusoidal
Keywords— Position Sensor, Permanent Magnet, Linear Drive
                                                                                field can be detected by placing the small positioning sensor in
                                                                                an optimal distance. The sensor can be mounted nearby the ma-
Topic— Motor Drives & Motion Control — Advanced Sensor Concepts for
                                                                                gnets and the windings and needs no extra space like the rod of
Motor Drives
                                                                                a conventional linear sensor.
                            I. M OTIVATION
                                                                                                    II. C ONSTRUCTED S ENSOR
In some application fields linear drives are not used for precise
                                                                                It is necessary to measure the magnetic field with two hall sen-
positioning but to regulate pressure, flow or force of an indu-
                                                                                sors which are mounted in a distance of the half effective length
strial process, for example in squeezing machines. The position
                                                                                of one magnet (see figure 1). This produces two sinusoidal ap-
                                                                                proximations of the magnetic field with a phase shift of 90◦ as
                                                                                shown in figure 2. The signals provides the possibility to deter-
                                                                                mine the direction of the movement.

                                                                                  Fig. 2. Functional description of the detected sinusoidal approximations.

                                                                                An integrated sensor element is used for the measurement which
                                                                                is very easy to assemble (see figure 3). The output of the hall-
                                                                                sensor element is amplified already inside the package of the
Fig. 1. Characteristic of the magnetic field based on the permanent magnets on
    the rotor, measured in the optimal distance.                                sensor. The current source is included, too. Only the two mea-
                                                                                sured signals from the A- and B-Trace have to be adapted to the
                                                                                inputs of the converter. This is done by two operational ampli-
is only needed for lower control loops and the force is the hig-                fiers (A and B) with a differential output. This is used to elimi-
her ranking closed-loop control. In this special application it is              nate electromagnetic interferences on the wires. In addition two
hardly possible to control the linear drive sensorless, without
other operational amplifiers correct the offset of the sensors. A       in green, is approximated with a sinusoidal signal (blue). The
final operational amplifier provides a constant voltage for the          difference between these signals is shown in red.
common mode input of the operational amplifiers A and B. The            To prove the operation of the sensor in cooperation with the clo-
voltage corresponds with half of the power supply of the sensor.       sed loop control of the linear machine, the small sensor shown
The complete circuit is shown in figure 3.                              in the right part of figure 10 is mounted in the machine.
                                                                       The resulted position signal is compared to the reference signal
                                                                       of an optical linear sensor with an accuracy of 10µm shown in
                                                                       figure 5. It shows the two measured signals printed in blue and
                                                                       green and the error between them is printed in red. The absolute
                                                                       error of the position is very low during the whole measurement

            Fig. 3. Functional description of the built sensor.        Fig. 5. Position signals of the system measured with the hall sensor and an
                                                                           optical reference sensor in µm.

                                                                                           IV. O PTIMIZATION OF THE S ENSOR
The principal operation of the sensor is proven by the simple
measurement of the magnetic field relative to the position of           As already shown in [2] the sinusoidal signals are overlaid by
the armature shown in figure 4. The measured signal, printed            a significant noise twice over a full rotation of the electric field
                                                                       (see figure 6).

                                                                                                                                               Trace A

                                                                                                                                               Trace B









                                                                                         Fig. 6. Measured sensor signals with noise overlay.
     Fig. 4. Magnetic field of the permanent magnets of the armature.
This is caused by a high current through the coil nearest to the                       mory and a serial interface for programming the EEPROM. This
sensor. In order to provide a suitable position signal these si-                       enables the calibration without changing the resistor ratio on the
gnals can be filtered to reduce the interferences.                                      pcb. The parameters of the sensor can be changed even if it is
The optimization of the sensor is done by minimizing noise be-                         mounted in the linear machine without dismounting.
fore overlaying the sinusoidal signals. This is realized in three                      In conjunction with the new hall sensor the minimization of the
steps. First of all a box of a copper coated pcb is used to shield                     pcb is the last optimization step. Therefore the devices are pla-
the whole sensor. Furthermore a twisted pair cable replaces the                        ced on a 4 layer pcb. In comparison with a 2 layer pcb the 4
normal shielded cable and is contacting the sensor with the con-                       layer pcb is just a little bit more expensive but it has the advan-
verter.                                                                                tage of better EMC-compliances. Due to this property an expen-
Figure 7 shows the measurement of the sinusoidal signals which                         sive shielded box around the sensor is not necessary any longer.
are recorded with the shielded sensor. It is obvious that the inter-                   Consequently the 4 layer pcb is economical reasonable.
ferences of the signals can be reduced. In comparison to figure                         Figure 9 presents the new functional description of the sensor.
6 the amplitude of the overlaid noise is much lower, but the qua-                      Three of the five operational amplifiers could be abandoned. The
lity of the signals is not exact enough to run the linear machine                      function of the sensor is still the same as before. The reduction
without any vibrations.

                                                                             Trace A

                                                                             Trace B









                 Fig. 7. Measured sensor signals with the shielded Sensor.
                                                                                                     Fig. 9. Functional description of the new sensor.

In the second optimization step the analog circuit is reduced be-
cause analog parts are vulnerable to interferences. Therefore the                      of components causes a downsizing of the new pcb. The dimen-
used hall sensor is replaced by a programmable one. In this con-                       sion is scaled down to 40% of the old one. Figure 10 shows the
text the above mentioned analog devices for the calibration and                        two sensors in comparison.
offset correction of the sensor can be left out. The block diagram
of one new hall sensor is presented in figure 8.

                       Fig. 8. Block diagram of the hall sensor [1].

An advantage of the sensor is the internal digital signal proces-                            Fig. 10. Both Versions of the built linear position sensor (in cm)
sing which enhanced the sensor accuracy and reduces the influ-
ence of manipulations by analog offsets, temperature shifts, and
mechanical stress. In addition the sensor has an EEPROM me-
                          V. M EASUREMENTS                                                          Conference of the IEEE Volume 2, Issue , 2-6 Nov. 2003 Page(s): 1337 -
                                                                                                    1342 Vol.2
The optimized sensor gives the measurement results shown in                                     [9] Baris Ozturk, S.; Akin, B.; Toliyat, H.A.; Ashrafzadeh, F. Low-cost di-
figure 11. The position plotted in green cannot de distinguished                                     rect torque control of permanent magnet synchronous motor using Hall-
                                                                                                    effect sensors Applied Power Electronics Conference and Exposition, 2006.
                                                                                                    APEC06. 19-23 March 2006
Position [incr]                                                                   Error in % 
  8,0E+04                                                                            10
                                                                     Difference      8


  4 0E+04



  0,0E+00                                                                            0






 ‐8,0E+04                                                                            ‐10

Fig. 11. Position signal of the optimized sensor in comparison to the high pre-
     cision Sensor.

from the reference position (blue) measured with a high precisi-
on optical position sensor. In difference to figure 5 the position
error is magnified to a percent scale and plotted in red. The po-
sition error of this particular sensor had been reduced to 1.6%
which is sufficient for the described application.

                           VI. C ONCLUSION
The measurement results of the custom made position sensor
shown in this paper have demonstrated the possibility of a low
cost sensor with sufficient accuracy to provide the lower ranking
position control loop. This sensor can be mounted inside of a
permanent linear drive without extra space consumption. The
costs of this sensor compared to a commercial optical sensor are
very low.

                              R EFERENCES
[1] Micronas GmbH Data Sheet: HAL 815 Programmable Linear Hall Effect
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[2] Wegener, R.; Senicar, F.; Junge, C.; Soter, S. Low Cost Position Sensor
    for Permanent Magnet Linear Drive, International Conference on Power
    Electronics and Drive Systems (PEDS 2007)
[3] Senicar, F. Entwicklung einer Hall-Sensor basierten Lageerfassung f¨ r  u
    einen Linearmotor mit permanenterregtem Sekund¨ rteil, Studienarbeit am
    LS EAM, University of Dortmund, 2006
[4] Morimoto, S.; Sanada, M.; Takeda, Y.           High-performance current-
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[7] Yong Ho Yoon; Mu Sun Woo; Seung Jun Lee; Chung Yuen Won; You
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