Electrical Noise . . . Sources, Symptoms and …

1 A52 Control SystemsMultiple devices on the same circuit should begrounded together at a single , power supplies and programmablecontrollers often have DC common tied to Earth(AC power ground). As a rule, it is preferable tohave indexer signal ground or DC common floatingwith respect to Earth. This prevents noisyequipment that is grounded to Earth from sendingnoise into the indexer. The Earth groundconnection should be made at one point only asdiscussed in Ground Loops on p. many cases, optical isolation may be required tocompletely eliminate Electrical contact between theindexer and a noisy environment. Solid-state relaysprovide this Conducted NoiseExternally conducted Noise is similar to power linenoise, but the disturbances are created on signaland ground wires connected to the indexer. Thiskind of Noise can get onto logic circuit ground orinto the processor power supply and scramble theprogram.

2 The problem here is that controlequipment often shares a common DC ground thatmay run to several devices, such as a DC powersupply, programmable controller, remote switchesand the like. When some noisy device, particularly arelay or solenoid, is on the DC ground, it may causedisturbances within the solution for DC mechanical relays andsolenoids involves connecting a diode backwardsacross the coil to clamp the induced voltage kick that the coil will produce. The diode should be ratedat 4 times the coil voltage and 10 times the coilcurrent. Using solid-state relays eliminates thiseffect Coil Suppression MethodsElectrical Noise ..Sources, Symptoms and SolutionsNoise related difficulties can range in severity fromminor positioning errors to damaged equipmentfrom runaway motors crashing blindly through limitswitches. In microprocessor controlled equipment,the processor is constantly retrieving instructionsfrom memory in a controlled sequence.

3 If anelectrical disturbance occurs, it could cause theprocessor to misinterpret an instruction, or accessthe wrong data. This is likely to be catastrophic tothe program, requiring a processor reset. MostCompumotor indexers are designed with awatchdog timer that shuts down the system if theprogram is interrupted. This prevents the morecatastrophic of NoiseBeing invisible, Electrical Noise can be verymysterious, but it invariably comes from thefollowing sources: power line disturbances Externally conducted Noise Transmitted Noise Ground loopsSome common Electrical devices generateelectrical Noise . Coil driven devices: conducted and power linenoise SCR-fired heaters: transmitted and power linenoise Motors and motor drives: transmitted and powerline Noise Welders (electric): transmitted and power linenoisePower line disturbances are usually easy to solvedue to the wide availability of line filtering equipmentfor the industry.

4 Only the most severe situations callfor an isolation transformer. Line filtering equipmentis required when other devices connected to thelocal power line are switching large amounts ofcurrent, especially if the switching takes place athigh frequency. Corcom and Teal are twomanufacturers of suitable power line , any device having coils is likely to upset theline when it is switched off. Surge suppressors suchas MOVs (General Electric) can limit this kind ofnoise. A series RC network across the coil is alsoeffective, (resistance; 500 to 1,000 , capacitance; to F). Coil-driven devices (inductive loads)include relays, solenoids, contactors, clutches,brakes and motor Typical RC NetworkAC or DCRI nductive LoadCDCD iodeAC or DCVaristor (MOV)A53 AEngineering ReferenceControl SystemsTransmitted NoiseTransmitted Noise is picked up by externalconnections to the indexer, and in severe cases,can attack an indexer with no external indexer enclosure will typically shield theelectronics from this, but openings in the enclosurefor connection and front panel controls may leak.

5 As with all Electrical equipment, the indexer chassisshould be scrupulously connected to Earth tominimize this high current contacts open, they draw anarc, producing a burst of broad spectrum radiofrequency Noise that can be picked up on anindexer limit switch or other wiring. High current andhigh voltage wires have an Electrical field aroundthem, and may induce Noise on signal wiring(especially when they are tied in the same wiringbundle or conduit).When this kind of problem occurs, considershielding signal cables or isolating the signals. Aproper shield surrounds the signal wires to interceptelectrical fields, but this shield must be tied to Earthto drain the induced voltages. At the very least,wires should be run in twisted pairs to limit straightline antenna Compumotor cables have shields tied toEarth, but in some cases the shields must begrounded at installation time.

6 Installing the indexerin a NEMA Electrical enclosure ensures protectionfrom this kind of Noise , unless Noise -producingequipment is also mounted inside the external to the enclosure must the worst Noise problems, in environmentsnear 600 amp welders and 25kW transmitters, havebeen solved using enclosures, conduit, opticalisolation, and single-point ground LoopsGround Loops create the most mysterious noiseproblems. They seem to occur most often insystems where a control computer is usingRS-232C communication. Garbled transmissionand intermittent operation Symptoms are problem occurs in systems where multipleEarth ground connections exist, particularly whenthese connections are far a Model 500 is controlling an axis, and thelimit switches use an external power supply. TheModel 500 is controlled by a computer in anotherroom. If the power supply Common is connected toEarth, ground loop problems may occur (mostcomputers have their RS-232C signal common tiedto Earth).

7 The loop starts at the Model 500 s limitswitch ground, goes to Earth through the powersupply to Earth at the computer. From there, theloop returns to the Model 500 through RS-232 Csignal ground. If a voltage potential exists betweenpower supply Earth and remote computer Earth,ground, current will flow through the RS-232 Cground creating unpredictable way to test for and ultimately eliminate a groundloop is to lift or cheat Earth ground connections inthe system until the Symptoms NoiseThe best time to handle Electrical Noise problems isbefore they occur. When a motion system is in thedesign process, the designer should consider thefollowing system wiring guidelines (listed by order ofimportance).1. Put surge suppression components on allelectrical coils: resistor/capacitor filters, MOVs,Zener and clamping Shield all remote connections and use twistedpairs. Shields should be tied to Earth at Put all microelectronic components in anenclosure.

8 Keep noisy devices outside. Monitorinternal Ground signal common wiring at one this ground from Earth if Tie all mechanical grounds to Earth at one chassis and motor grounds to the frame,frame to Isolate remote signals. Solid-state relays or optoisolators are Filter the power line. Use common RF filters(isolation transformer for worst-case situations).A Noise problem must be identified before it can besolved. The obvious way to approach a problemsituation is to eliminate potential Noise sources untilthe Symptoms disappear, as in the case of groundloops. When this is not practical, use the aboveguidelines to troubleshoot the about the equipment referred to may beobtained by calling the numbers listed below. Corcom line filters (312) 680-7400 Opto-22 optically isolated relays (714) 891-5861 Crydom optically isolated relays (213) 322-4987 Potter Brumfield optically isolated relays(812) 386-1000 General Electric MOVs (315) 456-3266 Teal power line isolation filters (800) 888-8325