Project Title for EMIassessmentOutdoorMobile Robotic Assistants Areaswhere possible EMI issues exists: ContentsBelowdiscuss the electromagnetic interference aspects and areas where possibleelectromagnetic interference issues may exist.
The suggestions and improvementsrelated to electromagnetic interference protection, reduction and avoidance isstated. NearField CouplingWhentwo conductors such as the copper traces on the printed circuit board areplaced close to each other, electrical energy can be diverted from oneconductor to another due to electric and magnetic field coupling. Moreover,crosstalk can exist between any two traces on a PCB and is a function of mutualinductance and mutual capacitance proportional to the distance between the twotraces, the edge rate, and the impedance of the traces. This happens naturallyand it is not what we wanted. Therefore, a few ways to reduce near fieldcoupling noise and crosstalk can be used.
One of the ways is by reducing therate of change of input signal for digital pulse or frequency for sinusoidalsignal using pulse shaping circuits. The trace separation can be increasedwhile the dielectric thickness can be reduced to improve the isolation betweentraces. While for digital systems as in what we used for the project, crosstalkcaused by mutual inductance is typically larger than the crosstalk caused bymutual capacitance. Mutual inductance can be reduced by increasing the spacingbetween the two traces or by reducing the distance from the ground plane.DecouplingCapacitorAnynoise on the power supply tends to alter the functionality of a device underoperation.
Generally, noise coupled on the power supply is of a high frequency,thus a bypass capacitor or decoupling capacitor is required to filter out thisnoise. A decoupling capacitor provides a low impedance path for high-frequencycurrent on the power plane to ground. The path followed by the current as ittravels toward ground forms a ground loop. This path should be kept to aminimum possible level by placing a decoupling capacitor very close to theintegrated circuit. A large ground loop increases the radiation and can act asa potential source of electromagnetic compatibility failure and therefore itshould be avoided in our project. The reactance of an ideal capacitorapproaches to zero with increasing frequency. However, ideal capacitor does notexist in real world.
In addition, the lead and the integrated circuit packageadd inductance as well. Multiple capacitors with low equivalent seriesinductance should be used to improve the decoupling effect. Capacitivecoupling pathThisis the coupling of an alternating signal through a dielectric medium to routeit from one function to another, while blocking the flow of direct current. Acapacitive path can be intentional or unintentional. When filtering analternating signal, the modification of a signal depends on the circuittopology should be considered as whether or not the capacitance is in serieswith or parallel to the circuit, the reactance to the signal from thecapacitance is 1/2 pifC and the impedance to the harmonic components of thesignalInductivecoupling pathInductivecoupling path is the blocking of an alternating signal caused by the presenceof an opposing magnetic field in an electrical conductor, while passing theflow of direct current.
An inductive path also have to types which isintentional or unintentional. When filtering an alternating signal, we considerthat the modification of a signal depends on the circuit topology, such as whetheror not the inductor is in series with or parallel to the circuit, the reactanceto the signal from the inductance is 2 pifL, and the impedance to the harmoniccomponents of the signalResistivecoupling pathResistancescan be introduced to modify the amplitude and routing of any signal. Aresistive path can be can be separate into intentional or unintentional. Anypractical or physical connection between nodes or components involves aresistive coupling path, even the copper traces on the PCB and the terminationsof individual lumped component elements can be resistive coupling paths.
Thedesign team must determine the impact these resistance coupling paths have onEMC objectives. Resistors in bias lines from a common power source formpotential coupling paths for EMI. In such cases, the supply lines may need tobe decoupled.Differentialand Common Mode SignalsFordifferential mode radiation, the single trace on the PCB behaves as a smallloop antenna.Whilefor common mode radiation, the ground plane acts as a dipole antenna which isextended on the plane and occurs parallel to the single trace.
Fromthe radiated emission formula due to differential and common mode current, Itis shown that by reducing current, ID, and loop area, A, fordifferential emission and current, IC, and line length, L, forcommon mode emission, the electromagnetic filed can be minimized. A common modechoke can also be used as it will present high impedance to common mode currentwhile having minimal impact on differential current. For a cylindricalconductor such as cables or wires used in the project, a common choke as simpleas a hollow cylindrical ferrite bead can be used. RadiationForradiation due to current loop, some method can be used to reduce if noteliminate it. First of all is to reduce the current loop area to be as small aspossible. It can be done by using thin dielectric for strip lines and placingreturn paths close to incident paths. The current should also be minimized tobe compatible with reliable operation. The leads carrying synchronous periodicwaveforms such as clock, must be minimized in terms of length and number.
Lastly, a dedicated return path must be used for clock leads. While forradiation due to electric dipole, the current should be forced to return viathe path closest to the incident path to minimized common mode currents and indirectlyreduce the electric field sources. For interconnections with high frequencysignals, shielded cables or balanced twisted-pairs should be used. A filtercapacitor can also be applied into the PCB on low frequency lead that act asradiating structure. Last but not least, the wire length should be minimized toreduce the number of dipoles while the signal voltage should also be minimizedto a value where it is still compatible with reliable operation. DigitalCircuits Whendealing with digital circuits in the robot, extra attention must be given toclocks and other high-speed signals. Traces connecting such signals should bekept as short as possible and be adjacent to the ground plane to keep radiationand crosstalk under control.
With such signals, we should avoid using vias orrouting traces on the PCB edge or near connectors. These signals must also bekept away from the power plane since they are capable of inducing noise on thepower plane as well. While routing traces for an oscillator, apart from groundno other trace should run in parallel or below the oscillator or its traces.
The crystal should also be kept close to the appropriate chips. Moreover, groundtraces carrying return current should be kept close to the trace carrying itsassociated signal to keep the current loop as short as possible. Tracescarrying differential signals should run close to each other to mosteffectively use the advantage of magnetic field cancellation. MotorSuppressionDCmotor will produce spark noise after working for a period of time.
The way tosolve is to fix a small value capacitor across the brushes preventing the noisegetting out of the motor as shown in the figure below.Thecapacitor should be in minimum possible value for example 10nF to eliminate thecommutation noise as a large value of capacitance will most likely make thecircuit to become short circuit thus creating a large inrush current throughthe controller and create higher heat dissipation at the output transistors orcauses the motor to be faulty if it the controller has a current limitprotection system.MiscellaneousImprovements GroundingAlow inductance ground system is the most vital element when designing a PCB forminimizing EMC. Maximizing the ground area on a PCB reduces the inductance ofground in the system, which in turn reduces electromagnetic emissions andcrosstalk.
A recommended design approach is to have a full ground plane as itprovides the lowest impedance as the current returns back to its source.However, a ground plane requires a dedicated PCB layer which may not befeasible for two-layer PCBs. In such case, we can use ground grids. Theinductance of ground in this case will depend on the spacing between the grids.
The way a signal returns to system ground is also very important because when asignal takes a longer path, it creates a ground loop which forms an antenna andradiates energy. Thus, every trace carrying current back to the source shouldfollow the shortest path and must go directly to the ground plane. Connectingall the individual grounds and then connecting them to the ground plane is notadvisable because it not only increases the size of current loop but alsoincreases the probability of ground bouncing. Especially for the case where weused Bluetooth wireless communication where its frequency range is about 2.4Ghztypically which is a lot more than 10Mhz. Thus using a multipoint ground ismore suitable for our project. ShieldingShieldingis a mechanical approach to reduce electromagnetic compatibility.
Metallicpackages such as conductive or magnetic materials are used to preventelectromagnetic interference from escaping the system. A shield may be usedeither to cover the whole system or a part of it, depending upon therequirements. A shield is like a closed conductive container connected toground which effectively reduces the size of loop antennas by absorbing andreflecting a part of their radiation. In this way, a shield also acts as apartition between two regions of space by attenuating the radiated EM energyfrom one region to another. A shield reduces the EMI by attenuating both the E-Fieldand H-field component of radiating wave. Moreover, for inductive and capacitivecoupling, shielding can be used to eliminate magnetic coupling between twowires.
The principle of shielding is to generate an opposite current flowing inthe shield and produce a magnetic field to cancel the original magnetic fieldfrom the active wire. It is aware that the shield must be grounded recommendedfrom the near end and far end for it to take effect especially when using highfrequency signals. The figure below shows an example of ground grid layoutwhich can be used in the project.CablesMostelectromagnetic compatibility related problems are caused by cables carryingdigital signals that effectively act as an efficient antenna. Ideally, thecurrent entering a cable leaves it at the other end. In reality, parasiticcapacitance and inductance emit radiation. Using a twisted pair cable helpskeep coupling to a low level by cancelling any induced magnetic fields.
When aribbon cable is used, multiple ground return paths must be provided. ComponentSegregationForan electromagnetic compatibility free design, components on the PCB need to begrouped according to their functionality, such as analog, digital, power supplysections, low-speed circuits, high-speed circuits, and so on. The tracks foreach group should stay in their designated area. For a signal to flow from onesubsystem to another, a filter should be used at subsystem boundaries.
ClockTerminationTracescarrying clock signals from a source to a device must have matchingterminations because whenever there is an impedance mismatch, a part of thesignal gets reflected. If proper care is not provided to handle this reflectedsignal, larges amount of energy will be radiated. There are multiple forms of effectivetermination, including source termination, end termination, AC termination,etc.Orientationand placementThestrategic placement and orientation of SMT and leaded capacitors, inductors andresistors can significantly reduce EMI generation.
Some components have highermagnetic fields in one orientation versus another. In general, SMT and leadedcomponents should be mounted to achieve the lowest possible profile on the PCBto minimize magnetic fields. Therefore rectangular-shaped SMT components shouldbe orientated with longest side parallel to PCB and leaded components should bepositioned as close to the PCB as possible with minimum lead length. Whenconnecting one terminal of an SMT or leaded component to ground, the componentshould be placed on the PCB so the ground connection is made to the lowestimpedance ground point in that area of the PCB. DiscontinuitiesStubsshould be kept as short as possible and frequency-based rules are often appliedto limit their length. We should limit the maximum length of stubs to fractionsof wavelengths, thereby minimizing the potential for EMI radiation from onesection of the PCB to another. Consider how multiple components are connectedto a conductive path. Each of these connections represents a discontinuitywhich can cause signals to be reflected into unknown paths and causeinterference.
It is preferable to join all the components to the conductor at asingle location (star connection).FerritesAddinga ferrite is effectively putting a resistor and inductor in the path of theinterference signal thus reducing high frequency currents travelling along thewires it is attached to. In order to reduce the amount of unprotected wire, theposition of the ferrite is placed as close as possible to the device beingprotected. The effectiveness of the ferrite at low frequency can be increasedby increasing the number of turns the wire goes through it.
The figure belowshows an example of proper usage of ferrites. The ferrite bead should be placedclosest to the noise source.Avoid90° anglesToreduce EMI it’s important to avoid 90° angles for traces, vias, and othercomponents because right angles cause radiation.
Here, capacitance increases inin that corner and characteristic impedance changes causing reflection, whichin turn causes electromagnetic interference. To avoid 90° angles, traces shouldbe routed at least on two 45° angles to the corners.LumpedelementsResistors,capacitors, inductors and transformers are physical entities often referred toas lumped elements. Lumped elements possess parasitic features which canunknowingly couple EMI signals. While it is generally known that positions andorientations of lumped elements influence specific coupling phenomena, it maybe less evident to consider the impact on EMC of side-effects due to parasiticelements of any component. It is often beneficial to research and possiblycharacterize lumped elements in advance if this information is not alreadyincluded in the designers’ component database.
Such characterization includesthe component behavior at the design frequency, temperature ranges, operationalvoltage and current, as well as other component behavior and harmonicfrequencies.