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Automation of a Clamp Mechanism for EMC Testing

Andrew Nafalski and Ozdemir GolUniversity of South Australia, Mawson Lakes 5095, Australia

Abstract— This paper reports on the development of an automated absorbing clamp mech-anism with video monitoring and position control to conduct the measurement process entirelyfrom outside the screened room where the clamp is used for EMC tests. The system is im-plemented with a minimal change to the EMC measurement environment within the screenedroom.

1. INTRODUCTION

The issue of electromagnetic compatibility (EMC) has been of growing concern throughout the lastcentury. Measurement and quantification of electromagnetic radiation has thus been the objectof intense attention. Comite International Special des Perturbations Radioelectriques (CISPR)standards stipulate the use of an absorbing clamp for the measurement of radio disturbance powerin the radiation frequency range of 30MHz–1 GHz [1, 2]. An absorbing clamp consists of a calibratedferrite-core current transformer and two sets of ferrite rings [3]. One set of ferrite rings surroundsthe supply cable from the equipment under test (EUT) and acts as an absorber of energy and animpedance stabiliser to isolate the EUT from the external power source, illustrated in Fig. 1. Thesecond set of ferrite rings is contained within the clamp body. This set surrounds the lead fromthe transformer to the electromagnetic interference (EMI) meter to minimise standing waves. Theabsorbing clamp moves along the track with the mains cable of the equipment under test (EUT)running through it as shown in Fig. 2.

Figure 1: Open absorbing clamp showing ferrite rings.

Track

Mains cable Ferrite clamp

Test receiver

EUT

Absorbing clamp

Power terminal

Figure 2: Measurement setup with an absorbing clamp.

Manual operation of the setup is cumbersome and time consuming; it requires setting up andrecording the clamp position, leaving the room, closing the door, taking measurements and repeat-ing the process for the next clamp position. Consequently, an automated system to control the

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clamp movement and its positioning has been designed to be controlled from outside the screenedroom. The system includes a monitoring camera inside the room. The main challenge was to designand implement a system which operates without affecting the operation of other devices, at thesame time being immune to the external electromagnetic environment [4, 5].

2. EMI MITIGATION

The proposed mechanised clamp mechanism is based on the use of an air motor to avoid elec-tromagnetic interference. The movement of the absorbing clamp along the track is monitored bymeans of an off-the-shelf closed circuit television (CCTV) camera (Fig. 3), mounted in the cornerof the screened room.

Figure 3: Bullet size Sharp 1/4 CCD CCTV security color camera.

It was necessary to confirm whether the camera requires screening. The first radiated emissiontests of the unshielded camera powered by 12V power supply connected to the mains (Fig. 4)made it very clear that some remedial EMC action is definitely needed as EMI was higher than40 dBµV/m at certain frequencies as measured by a vertical bi-conical antenna [6].

Figure 4: Radiated emissions from camera measured in screened room without shielding at frequencies from30MHz to 300 MHz.

The emissions exceeded those stipulated by CISPR standard [1] and a process of their reductionhas been implemented starting with placing the camera together with a rechargeable battery ina die cast aluminium box (Fig. 5). Several further EMI mitigating steps were implemented andemission tests were conducted at each step. Final emission test was conducted with the camerasealed in the aluminium box, connected to the wall terminal of the screened room using a quadshielded RG6 coaxial cable, with the lens aperture provided with a metal mesh. To emulate theworst case scenario the box was placed in the middle of the screened room and connected tothe wall terminal using a 5 m cable. Attenuating ferrite rings were placed around the cable. Theradiation emission test of the camera under the conditions defined above confirms that its emissions

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are at the level of the ambient noise level and that the camera system complies with the CISPRnorm (Fig. 6). Measurements repeated for frequencies between 300 MHz and 1GHz also confirmedcamera’s compliance with the relevant standard [1].

Figure 5: Camera, battery and bulkhead connector inside aluminum box, prior to sealing.

Figure 6: Radiated emissions from camera in aluminum enclosure with a shielded aperture, connected usinga 5 m RG6 quad shielded coaxial cable with ferrite rings, measured at frequencies from 30MHz to 300MHz.

3. CLAMP MOVEMENT

The next stage of the project was to develop the motorized clamp mechanism illustrated in Fig. 7.The clamp moves along the 6 m track using a pulley system driven by an air motor. A standardoff-the-shelf hand-held air drill was used to drive the clamp. The cable connecting the absorbingclamp was suspended from a rail on the ceiling, to ensure that it does not become entangled in themechanism. Air for the actuator was supplied via non-metallic pipes connected to an air compressoroutside the screened room.

The mechanical setup of the drive system is shown in Fig. 8 [7]. The air drill drives the mainshaft that in turn through a pulley system moves the clamp. One of the geared down DC motors-linear actuators controls the On/off function of the droll, the other the direction of the clampmovement.

As the two DC motors are supplied and controlled electrically, a number of measures needed tobe implemented to reduce EMI to the level compliant with the CISPR standard. These included:decoupling of the motors using a 1µF monolithic capacitors across each of the motor terminals,use of twisted pair cables throughout and application of ferrite beads on cables near the motorterminals.

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Absorbing clamp

Air

operated motor Track Conveyor belt

Rail to suspend cable

Air

out Air in

To

receiver

Figure 7: The conceptual setup of the motorised absorbing clamp mechanism.

Main shaft

Optical encoder On/off input

Direction input

Figure 8: The mechanical setup for the motor and encoder assembly.

4. POSITION MEASUREMENT

For position measurement of the clamp mechanism the rotary optical decoder Bourns ENA1J-B28-L00064 was used, principle of operation of which is shown in Fig. 9. Its output was connected to acustom designed and constructed decoder (Fig. 10) that in turn communicated via RS-232 interfacewith a control program written in National Instruments LabVIEW 8.2. The same program alsocontrolled the motor control unit (custom built PCB). The rotary incremental optical encoder wastested for emission EMI [1] and easily passed the tests in the frequency range 30 MHz–1 GHz [8].

Light Source

Collimating Lens

Light Detector

Incrementally Coded Plate

Encoder Shaft Shaft Bearing

Figure 9: Operation of the optical encoder.

ccV G A

Int0

B

I/0 Pin

Rotary Encoder

Decoder

ccV

GND

Rx

RS-232

0.1 µF

5V

Figure 10: Rotary encoder interface.

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5. CONCLUSIONS

The paper describes successful projects leading to a low-cost automation and monitoring of EMCtesting in a screened room. Design objectives were achieved by a careful consideration of EMCprinciples at the design, prototyping, re-designing and testing stages.

ACKNOWLEDGMENT

The authors gratefully acknowledge the contribution of Mr. Chris Preece, General Manager ofWooddale EMC Consultants Pty. Ltd., Adelaide, Australia, to the technical aspect of the projectspresented in the paper.

REFERENCES

1. AS/NZS 1052:1992 IEC/CISPR 16:1987, “CISPR specification for radio interference measuringapparatus and measurement methods,” Australian Standard/New Zealand Standard, 1992.

2. IEC/CISPR 16-1-3-Ed.2.0, “Specification for radio disturbance and immunity measuring ap-paratus and methods - Part 1–3: Radio disturbance and immunity measuring apparatus -Ancillary equipment - Disturbance power,” 2004.

3. Rhode & Schwarz Absorbing Clamp MDS-21/22, “Ferrite clamp EZ-24,” Rohde & Schwarz,2005.

4. Townsend, D. A., T. J. F. Pavlasek, and B. N. Segal, “Breaking all the rules: Challenging theengineering and regulatory precepts of electromagnetic compatibility,” IEEE Transactions onMagnetics, 194, 1995.

5. Pratt, G. E., “A methodology for low cost electromagnetic compatibility testing at the proto-type stage of development,” IEEE International Symposium on Electromagnetic Compatibility,285, 1995.

6. Preece, C., S. Shingadia, S. K. Tiong, L. Y. Yun, A. Nafalski, and O. Gol, “Motorised absorb-ing clamp mechanism,” Digests of Asia-Pacific Symposium on Applied Electromagnetics andMechanics (APSAEM2006), 123, Sydney, Australia, 2006.

7. Ng, C. H. and T. N. Nguyen, “Remote sensing and automated positioning of an absorberclamp,” Final Year BEng Project 2007n, School of Electrical and Information Engineering,University of South Australia, 2007.

8. Ng, C. H., A. Nafalski, and O. Gol, “Contactless position measurement for EMC apparatus,”Proceedings of the 17th Technical Seminar on Operation of Electrical Machines and Drives,Research and Development Centre of Electrical Machines KOMEL, Rytro, Poland, May 28–30,2008 (submitted for publication).