Automation of a Clamp Mechanism for EMC Testing

Transcription

1 PIERS ONLINE, VOL. 4, NO. 5, Automation of a Clamp Mechanism for EMC Testing Andrew Nafalski and Özdemir Göl University of South Australia, Mawson Lakes 5095, Australia Abstract This paper reports on the development of an automated absorbing clamp mechanism with video monitoring and position control to conduct the measurement process entirely from outside the screened room where the clamp is used for EMC tests. The system is implemented with a minimal change to the EMC measurement environment within the screened room. 1. INTRODUCTION The issue of electromagnetic compatibility (EMC) has been of growing concern throughout the last century. Measurement and quantification of electromagnetic radiation has thus been the object of intense attention. Comité International Spécial des Perturbations Radioélectriques (CISPR) standards stipulate the use of an absorbing clamp for the measurement of radio disturbance power in the radiation frequency range of 30 MHz 1 GHz [1, 2]. An absorbing clamp consists of a calibrated ferrite-core current transformer and two sets of ferrite rings [3]. One set of ferrite rings surrounds the supply cable from the equipment under test (EUT) and acts as an absorber of energy and an impedance stabiliser to isolate the EUT from the external power source, illustrated in Fig. 1. The second set of ferrite rings is contained within the clamp body. This set surrounds the lead from the transformer to the electromagnetic interference (EMI) meter to minimise standing waves. The absorbing 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. Ferrite clamp Absorbing clamp Mains cable Power terminal Track EUT Test receiver Figure 2: Measurement setup with an absorbing clamp. Manual operation of the setup is cumbersome and time consuming; it requires setting up and recording the clamp position, leaving the room, closing the door, taking measurements and repeating the process for the next clamp position. Consequently, an automated system to control the

2 PIERS ONLINE, VOL. 4, NO. 5, clamp movement and its positioning has been designed to be controlled from outside the screened room. The system includes a monitoring camera inside the room. The main challenge was to design and implement a system which operates without affecting the operation of other devices, at the same 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 electromagnetic interference. The movement of the absorbing clamp along the track is monitored by means of an off-the-shelf closed circuit television (CCTV) camera (Fig. 3), mounted in the corner of 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 emission tests of the unshielded camera powered by 12 V power supply connected to the mains (Fig. 4) made it very clear that some remedial EMC action is definitely needed as EMI was higher than 40 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 from 30 MHz to 300 MHz. The emissions exceeded those stipulated by CISPR standard [1] and a process of their reduction has been implemented starting with placing the camera together with a rechargeable battery in a die cast aluminium box (Fig. 5). Several further EMI mitigating steps were implemented and emission tests were conducted at each step. Final emission test was conducted with the camera sealed in the aluminium box, connected to the wall terminal of the screened room using a quad shielded RG6 coaxial cable, with the lens aperture provided with a metal mesh. To emulate the worst case scenario the box was placed in the middle of the screened room and connected to the wall terminal using a 5 m cable. Attenuating ferrite rings were placed around the cable. The radiation emission test of the camera under the conditions defined above confirms that its emissions

3 PIERS ONLINE, VOL. 4, NO. 5, are at the level of the ambient noise level and that the camera system complies with the CISPR norm (Fig. 6). Measurements repeated for frequencies between 300 MHz and 1 GHz also confirmed camera 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 using a 5 m RG6 quad shielded coaxial cable with ferrite rings, measured at frequencies from 30 MHz to 300 MHz. 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 standard off-the-shelf hand-held air drill was used to drive the clamp. The cable connecting the absorbing clamp was suspended from a rail on the ceiling, to ensure that it does not become entangled in the mechanism. Air for the actuator was supplied via non-metallic pipes connected to an air compressor outside the screened room. The mechanical setup of the drive system is shown in Fig. 8 [7]. The air drill drives the main shaft that in turn through a pulley system moves the clamp. One of the geared down DC motorslinear actuators controls the On/off function of the droll, the other the direction of the clamp movement. As the two DC motors are supplied and controlled electrically, a number of measures needed to be implemented to reduce EMI to the level compliant with the CISPR standard. These included: decoupling of the motors using a 1 µf monolithic capacitors across each of the motor terminals, use of twisted pair cables throughout and application of ferrite beads on cables near the motor terminals.

4 PIERS ONLINE, VOL. 4, NO. 5, Rail to suspend cable To receiver Absorbing clamp Conveyor belt Track Air operated motor Air in Air out 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 a custom designed and constructed decoder (Fig. 10) that in turn communicated via RS-232 interface with a control program written in National Instruments LabVIEW 8.2. The same program also controlled the motor control unit (custom built PCB). The rotary incremental optical encoder was tested for emission EMI [1] and easily passed the tests in the frequency range 30 MHz 1 GHz [8]. 5V Decoder Incrementally Coded Plate Light Source Rotary Encoder V cc Rx RS-232 Collimating Lens V ccg A B Shaft Bearing Encoder Shaft Light Detector 0.1 µf Int0 I/0 Pin GND Figure 9: Operation of the optical encoder. Figure 10: Rotary encoder interface.

5 PIERS ONLINE, VOL. 4, NO. 5, CONCLUSIONS The paper describes successful projects leading to a low-cost automation and monitoring of EMC testing in a screened room. Design objectives were achieved by a careful consideration of EMC principles at the design, prototyping, re-designing and testing stages. ACKNOWLEDGMENT The authors gratefully acknowledge the contribution of Mr. Chris Preece, General Manager of Wooddale EMC Consultants Pty. Ltd., Adelaide, Australia, to the technical aspect of the projects presented in the paper. REFERENCES 1. AS/NZS 1052:1992 IEC/CISPR 16:1987, CISPR specification for radio interference measuring apparatus and measurement methods, Australian Standard/New Zealand Standard, IEC/CISPR Ed.2.0, Specification for radio disturbance and immunity measuring apparatus and methods - Part 1 3: Radio disturbance and immunity measuring apparatus - Ancillary equipment - Disturbance power, Rhode & Schwarz Absorbing Clamp MDS-21/22, Ferrite clamp EZ-24, Rohde & Schwarz, Townsend, D. A., T. J. F. Pavlasek, and B. N. Segal, Breaking all the rules: Challenging the engineering and regulatory precepts of electromagnetic compatibility, IEEE Transactions on Magnetics, 194, Pratt, G. E., A methodology for low cost electromagnetic compatibility testing at the prototype stage of development, IEEE International Symposium on Electromagnetic Compatibility, 285, Preece, C., S. Shingadia, S. K. Tiong, L. Y. Yun, A. Nafalski, and Ö. Göl, Motorised absorbing clamp mechanism, Digests of Asia-Pacific Symposium on Applied Electromagnetics and Mechanics (APSAEM2006), 123, Sydney, Australia, Ng, C. H. and T. N. Nguyen, Remote sensing and automated positioning of an absorber clamp, Final Year BEng Project 2007n, School of Electrical and Information Engineering, University of South Australia, Ng, C. H., A. Nafalski, and Ö. Göl, 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).