147 CHAPTER 6 EMI EMC MEASUREMENTS AND STANDARDS FOR TRACKED VEHICLES (MIL APPLICATION) 6.1 INTRODUCTION The electrical and electronic devices, circuits and systems are capable of emitting the electromagnetic energy either intentionally or unintentionally. They can cause interference with the neighboring systems. The tests that are essential to measure the electromagnetic interference levels and compatibility are 1. Conducted emission (CE) 2. Susceptibility to conducted emission (CS) 3. Radiated emission (RE) 4. Susceptibility to Radiated emission (Rs) They cover both broadband and narrow band emission. The narrow band test deals with continuous wave (CW) mode of emission and interference. Broadband emission involves EMI due to external cause such as electrostatic discharges, switching surges etc. The automotive vehicle requirements are stringent (Don 1987) for the field condition for various applications.
148 6.2 MIL STD 461D /462 Military standards 461 and 462 are the most comprehensive set of standards for the electromagnetic compatibility. MIL STD 462 specifies test methodologies and detailed procedures for compliance with MIL STD 461. MIL STD 461 gives the performance specification limits for the electrical, electronic and electromechanical equipments and sub systems. Presently these documents were revised and published as MIL STD 461 D and MIL STD 462 D in Jan 1993.Recently MIL 461E for design &testing has also been formulated for above purpose. The EMI control levels stipulated in MIL STD 461D apply to sub system level hardware to ensure EMC in when integrated in a larger system. MIL STD 462D lays down permissible levels for conducted emissions, susceptibility and immunity to conducted emissions, radiated emissions and susceptibility and immunity radiated emissions. 6.3 CONDUCTED INTERFERENCE LIMITS The basic purpose of the lower frequency portion of the conduction emission limit is to ensure that connection of equipment under test (EUT) to the mains power supply does not corrupt the power quality (or introduce distortions in the voltage waveforms) on the power mains beyond allowable limits.
149 The objective of imposing limits on the conducted emission in higher frequency ranges is to protect the receivers (which are connected to antenna terminals) against degradation caused by radiated interference from power cables associated with the EUT. CS 109 is to ensure that equipment performance is not affected by the magnetic fields caused by any current flowing in the platform Structure. CS 114/116/115 is to ensure immunity of the equipment for any current and voltage waveform (including transients) or electromagnetic fields, which may be generated on the platform. 6.4 MILITARY STANDARS FOR TRACKED VEHICLES Military standards 461C/462 part 4 is applicable for a ground based automobile subsystems, and MIL STD 461C/462 part 8 is applicable for special vehicle and engine driven equipment. 6.5 METHODOLOGY AND TESTING OVERVIEW The measurements were carried out in a RF chamber to evaluate the performance of the vehicle as a complete system. The RE-02 tests were carried out within the vehicle. The essential automotive systems were kept on in all tests during these measurements in the chamber. Similarly CE-03 test were carried out on the vehicle after switching on all the EMI sources. The measurements were carried out in broad band and narrow band to have insight into the noise levels. The stipulated procedure as laid down in the specification was followed during the entire test.
150 The RE-02 radiated interference measurements were carried out on a automotive chassis in the anechoic chamber. This test method requires the use of E field antenna located 1 m away from the source. The measurements can also be made with 11 cm Loop antenna which measures H field from that E field can be computed for the system at 1 m distance. Figures 6.1to Figure 6.6 indicate the actual plot level obtained in radiated emission (RE 02) & conducted emission (CE03) EMI during testing on advanced digital tracked vehicle. These are actual measurements made onboard the vehicle. The same results have been tabulated and compiled at Tables 6.1 to Table 6.6. The designer has to investigate the cause of deviation for specified condition and carry out hardening to reduce the level to the limits. The measurements were taken from remotely from safety point of view for vertically polarised wave as well as for the horizontally polarised waves. The idea is to test the efficacy of EMI counter measures taken by the designer. The vehicle is an improved version of analog relay based logic control system which replaced by a digital bus controller. The digital bus has a basic frequency of operation (in MHz range) that has contributed to the higher levels of EMI as evident from the graphs. This system is a dual processor redundant, hence more reliable from operational point of view.this problem can be circumvented by keeping the bus in silent operation mode during deployment of any critical weapons onboard the vehicle.
151 Figure 6.1 RE-02 Radiated emission Spectrum (NB) - Horizontal Figure 6.2 RE-02 Radiated emission Spectrum (BB) - Horizontal
152 Table 6.1 Radiated Emission RE-02 Broadband inside Vehicle (Vertical polarization) RE-02 Limit db/ V/m/M Hz Measured db/ V/m/M Hz Error or difference db 1 MHz 80 60-20 10 MHz 68 45-21 25 MHz 65 75 +10 100 MHz 60 60 0 400 MHz 38 48 +10 Table 6.2 RE-02 Radiated emission Spectrum Narrowband (vertical polarization ) inside vehicle RE-02 Limit Measured Error or db/ V/m db/ V/m difference 0.15 MHz 30 15-15 5 MHz 16 10-6 15 MHz 21 20-1 99 MHz 30 32 +1 Table 6.3 RE-02 Radiated emission spectrum inside the vehicle (horizontal polarisation) indicating the RE limits as measured values (Broad band) RE-02 Limit db/ V/m/ MHz Measured db/ V/m / MHz Error or difference db 1 MHz 80 65-15 10 MHz 70 55-15 33 MHz 62 38-24 80 MHz 58 56-2 200 MHz 55 65 +10
153 Figure 6.3 CE03 Conducted emission Spectrum (NB)- Vertical Figure 6.4 CE 03 Conducted emission Spectrum (BB)- Vertical
154 Figure 6.5 RE-02 Radiated Emission Spectrum in Drivers compartment (NB) Figure 6.6 RE-02 Radiated Emission Spectrum in Drivers compartment (BB)
155 Table 6.4 RE-02 radiated Emission limits (horizontal polarization) within vehicle (Narrow band) MIL Limit Measured Error or difference db/ V/m db/ V/m 0.15 MHz 30 20-10 1 MHz 27 7-20 10 MHz 22 22 0 15 MHz 22 35 12 65 MHz 25 15-10 100 MHz 30 35 +5 Table 6.5 CE-03 Conducted emission measured within the vehicle indicating limits as per MIL STD and error (Narrow band) MIL Limit db/ V/m Measured db/ V/m Error or difference 0.1 MHz 55 72 +17 0.5 MHz 32 31 +1 1 MHz 27 10-17 10 MHz 15 26 +11 70 MHz 15 10-5 Table 6.6 Conducted emission CE-03 measured with the vehicle indicating limits as per MIL STD and error (Broadband) MIL Limit db/ V/m/MH z Measured db/ V/m/MH z Error or difference 0.1 MHz 97 125 +28 0.25 MHz 84 84 0 1 MHz 62 47-15 10 MHz 20 <10 <-10 30 MHz 20 <5 <-15
156 6.6 EMI FIELD LEVELS AT ANECHOIC CHAMBER The attenuation of the fields in the presence of the tracked vehicle mass is interesting case study was taken up to enable us to know field levels in close proximity after estimation of attenuation levels due to the vehicle. The normalised levels obtained during the course of the above evaluation are shown in Tables 6.7, Table 6.8, Table 6.9 and graphically in the Figures.6.7, Figure 6.8, Figure 6.9 resp. Table 6.7 Magnetic field attenuation level at Driver s Console (Vertical& Horizontal ) (MHz ) Attenuation levels(db) Horizontal Vertical 0 65 68 10 87 72 20 83 78 50 76 69 75 85 70 100 73 73 125 65 69 150 90 83 200 95 73
157 100 80 60 40 20 0 frequency (MHz) Horizontally polarise d wave Vertically polarised wave Figure 6.7 Magnetic field attenuation level at Drivers Console for Horizontally polarized wave and vertically polarized wave Table 6.8 Magnetic field attenuation by the vehicle (Vertical & Horizontal ) MHz Attenuation levels (db) magnetic fields Horizontal Vertical 0 5 18 10 9 21 20 3 28 50 10 33 75 8 17 100 3 1 125 18-5 150 11-11 200 5 10
158 40 30 20 10 0-10 -20 frequency (MHz) Horizontally polarised wave vertically polarised wave Figure 6.8 characteristics of magnetic field attenuation by the vehicle for the horizontally and vertically polarized waves Table 6.9 Magnetic field strengths at about 1.5 m above the ground level (Vertical& Horizontal ) Attenuation levels (db) magnetic fields Horizontal Vertical 0 65 68 10 87 72 20 83 78 50 76 69 75 85 70 100 73 73 125 65 69 150 101 83 200 95 73
159 125 105 85 65 45 25 frequency MHz horizontally polarised wave vertically polarised wave Figure 6.9 Magnetic field strengths at about 1.5 m above the ground level 6.7 INFERENCE BASED ON VEHICLE TESTING EMI spectrum clearly indicate that the vehicle meets the stipulated requirements after adopting the proper EMI hardening methods such as proper grounding, use of EMI gaskets etc. The higher levels of EMI at above certain frequencies indicate the need to adopt mitigating techniques to counter the same. Tables 6.10 to Table 6.14 indicate EMI standards for military application. The EMI radiated measured pattern also shows peaks associated with various inherent emitters such as oscillators, Flat panel displays, power switching devices etc. However amplitude, bandwidth has not resulted in major problem onboard the tracked vehicle. The vehicle is made of stainless steel that gives good shielding against low impedance magnetic fields. This also performs best in combination with advanced ceramics that counters high impedance electric fields and plane waves as well in the spectrum. In fact some of the vehicle use advanced techniques to counter the fields to prevent the safety of friendly troops from unwanted explosive triggering.
160 Table 6.10 EMI - EMC applicable test for Part 4 MIL STD 461C/462 EMISSION CONDUCTED RADIATED CE-01 CE-03 RE-02 30 Hz-15 khz 14 khz-50 MHz 14 khz-10 GHz for NB and 14 khz-1 GHz for BB Conducted emission from power leads Conducted emission from power leads Radiated emission from system as a whole Table 6.11 Specifications for susceptibility immunity to conducted emission under MIL STD 461B SUSCEPTIBILITY CONDUCTED RADIATED CS 01 CS 02 CS 06 RS 03 30 Hz-50 khz 50 khz-400 MHz 100 V, 10 s, 10pps with rise 10 V/m, 14 khz-1 GHz CW RF wave is super imposed on DC powerline CW RF wave is super imposed on DC powerline time of 1 s pulse Pulse is superimposed on DC power line System is subjected to RF field The Military Standards for tracked vehicle is described in Appendix 1 and automotive standards on EMI EMC in Appendix 3.
161 Table 6.12 Specification for susceptibility immunity to conducted emission under MIL STD 461D Specification range Applicability CS 101 30Hz to 50kHz (if the EUT is DC operated); second harmonic of EUT power supply frequency to 50kHz (if EUT is AC operated) CS 103 (inter modulation antenna port) CS 104 (un desired signals at antenna port) CS 105 (Cross modulation at antenna port) CS 109 (structure current ) 15kHz to 10GHz 30HZ to 20 GHZ 30HZ to 20 GHZ Equipment and subsystem input power leads (AC and DC), but not returns Receiver front ends, such as communication receivers, RF amplifiers, transceivers, radar receivers, and electronic warfare receivers. Front end of receivers that normally process amplitude modulated RF signal. 60 Hz to 100 Hz Specialized requirement for equipment and subsystems whose operating frequency range is 100kHZ or less, and whose operating sensitivity CS114 CS 115 (impulse excitation) CS 116 (Damped sinusoidal transients ) 10 khz to 30 MHz for all; 30 MHz to 400 MHz (for specific systems) Interconnecting cables, power cables Aircraft and space system interconnecting cables, including power cables 10 khz to 100 MHz Interconnecting cables, including power cables and individual power leads.
162 Table 6.13 Limits for radiated emission under MIL STD 461D Specification RE 101 (magnetic field) RE 102 (electric field) RE 103 (antenna spurious and harmonic outputs) range 30 Hz to 100 khz 10 khz to 18 GHz 10 khz to 40 GHz Applicability Equipment and subsystem enclosures, and all interconnecting cables (specific exclusions exist) Equipment and subsystem enclosures, and all interconnecting cables (specific exclusions exist) This test is an alternate for CE 106 Table 6.14 Specification for susceptibility / immunity to radiated emissions under MIL-STD-461D Specification RS101(magnetic field) RS 103 (electric field) RS 105 (Transient Electromagnetic Pulses) range 30 Hz to 100 khz 10 khz to 40 GHz Applicability Equipment and subsystem enclosures, and all interconnecting cables (specific exclusions exist) Equipment and subsystem enclosures, and all interconnecting cables (specific exclusions exist) Equipment and subsystem enclosures, when the equipment or subsystem is located outside a shielded facility.
163 6.8 CONCLUSION The MIL STD 461C and MIL STD 462 are the military standards for electromagnetic compatibility. The vehicle was tested for performance after EMI hardening. The radiated measurements RE-02 and conducted measurements CE-03 were carried out. The tabulated results indicate that the EMI levels are well within the laid specification for tracked vehicle and does not cause any problem to the neighbouring system. The conducted emission levels (about 35dBs) are seen to be higher near 0.01 MHz to 1 MHz region due to the digital bus adopted for control of the vehicle. Similarly radiated emission levels are higher in the region of 100 MHz to 1GHz by about 25dBs attribute extensive cable module level EMI. The level of EMI is though higher for the vehicles from operational point of view is still a bargain against the old cumbersome relay based hardwired technology. However this has been reduced by 1. Use of double shields to protect the data cables and signal cables attenuation levels up to 80dB is achieved in practice. 2. Use of multipoint grounding with advanced ring bus architecture is adopted in the vehicle. 3. High density connectors for signals such as MIL38999 series IV and power connectors for distribution within vehicle is adopted to reduce radiated emission from cables. 4. The lines for the signals and power are separated from each other to reduce coupling effects etc. Thus the vehicle EMI levels after the modifications falls well within the limits at a cost..