1、Application ReportSLLA143-May 20031RS-485 for Digital Motor Control Applications1Clark KinnairdHPLInterfaceABSTRACTThis application report focuses on the benefits of using RS-485 signaling for motor controland motion control applications.This technology has several benefits for these applicationsin
2、terms of noise immunity,wide common-mode voltage range,adequate data rate,andmultipoint capability.Other applications also use RS-485 signaling to take advantage of thesesame benefits.Applications such as process control networks,industrial automation,remoteterminals,building automation and security
3、 systems apply RS-485 extensively to solve theirrequirements for robust data transmission over relatively long distances.Contents1Introduction3.1.1 Motor Devices3.1.2 Feedback4.1.3 Controllers4.1.4 Data Transmission5.1.5 Basic Topology5.2Data Transmission Concerns and How RS-485 Addresses Each6.2.1
4、Environment7.2.1.1EMI/Noise Immunity7.2.1.2Ground Potentials/Common Mode8.2.1.3Electrostatic Discharge9.2.1.4General Ruggedness10.2.2 Speed10.2.2.1Feedback Loop Delay10.2.2.2Propagation Delay(Cable Transmission Delay,Transceiver Delay.)10.2.2.3Signaling Rate11.2.2.4Larger Payload for Serial Communic
5、ation11.2.3 Multipoint Topologies11.3Application Example12.3.1 Encoder Feedback Signals From High-Resolution Incremental Encoder12.4Conclusion13.5References13.1 Originally featured on ChipCenter Analog Avenue at are the property of their respective owners.SLLA1432RS-485 for Digital Motor Control App
6、lications1List of Figures1 Digital Motor Control Block Diagram Showning Components Offered by Texas Instruments3.2 Rotary and Linear Electric Motors4.3 Interfaces in a Motor Control System(Single-Axis Shown)6.4 Receiver Function With and Without Hysteresis7.5 Hysteresis Eliminates Spurious Transitio
7、ns8.6 System With Ground Potential Shift9.7 Typical Application,Encoder Feedback Signals12.List of Tables1 Signals in a Typical Motion Control System5.SLLA1433 RS-485 for Digital Motor Control Applications11IntroductionDigital motor control refers to using digital processors to control the motion of
8、 electric motors.Typically one or more methods of feedback are available to the digital processor,making this aclosedloop system(See Figure 1).This contrasts to analog control systems and open-loopmotion systems.Digital motor control is found in many applications.These include storage devices(such a
9、s diskdrives),industrial robotics,high-precision semiconductor manufacturing,and copiers.Figure 1.Digital Motor Control Block Diagram Showning Components Offered by TexasInstruments1.1Motor DevicesThe motor involved in digital motor control may be one of several types.The most common isthe subfracti
10、onal horsepower rotary motor(see Figure 2a).These may be further classified asac,dc brush,or dc brushless type,depending on the method of commutation.Small motors aretypically sized according to their frame size and power in watts.Larger motors,typically ac type,are classified by their power in hors
11、epower.Although rotary motors are the most common,otherconfigurations are available,such as linear motors(see Figure 2b),and gearhead motors withvarious implementations of actuators built on.SLLA1434RS-485 for Digital Motor Control Applications1(a)Rotary Electric Motor(b)Linear Electric MotorFigure
12、2.Rotary and Linear Electric Motors1.2FeedbackIn order to provide feedback on the position,speed,torque or other dynamic properties of themotion system,feedback sensors are necessary.Perhaps the most common feedback sensor isa rotary encoder,consisting of a wheel with alternating stripes,mounted on
13、the motor shaft.Asthe motor rotates,an optical sensor detects the passing of the stripes,and produces electricalsignals,which a controller can use to determine the motors motion.Other types of sensors aretachometers,synchros and resolvers,which are induction-based sensors,Hall-effect sensors,which a
14、re magnetic-based,and potentiometers,which are resistance-based.No matter which sensor method is used,the digital controller must repetitively sample thesensor signal,in order to constantly maintain current knowledge of the systems dynamicmotion.Depending on the system requirements for speed,dynamic
15、 response and accuracy,therate of feedback sampling may be over several thousand samples per second.1.3ControllersThe controller,whether digital or analog,compares the commanded motion and actual dynamicsof the system,and processes these inputs to create a control signal to the actuator.In the caseo
16、f digital controllers,additional tasks may include system start-up routines,diagnostics,communications control,and sampling multiple sensors.Digital controllers may be as complex as dedicated computer processors,or as simple assingle-chip programmed gate arrays.Texas Instruments offers digital signa
17、l processors withfeatures optimized for motion control,and microcontrollers with varying features for best-fitsolutions to a wide array of applications.For more information,search on Digital Control on theTexas Instruments web site at .SLLA1435 RS-485 for Digital Motor Control Applications11.4Data T
18、ransmissionThis discussion focuses on the benefits of using RS-485 signaling for motor control and motioncontrol applications.As discussed below,this technology has several benefits for theseapplications in terms of noise immunity,wide common-mode voltage range,adequate data rate,and multipoint capa
19、bility.Other applications also use RS-485 signaling to take advantage ofthese same benefits.Thus applications such as process control networks,industrial automation,remote terminals,building automation and security systems apply RS-485 extensively to solvetheir requirements for robust data transmiss
20、ion over relatively long distances.Often the RS-485signaling is bundled with a protocol such as Profibus,Interbus,Modbus or BACnet,each tailoredfor the specific requirements of the end user.Other signaling technologies may be used when the features of RS-485 are not the best fit.Forexample,RS-232 or
21、 RS-422 signaling may be adequate in some applicationssometimescontroller area network(CAN)or EtherNet/IP(Industrial Protocol)are preferred due tocompatibility with an existing network.For higher speed applications,and where long distanceand common-mode voltages are not as rigorous,M-LVDS can provid
22、e lower power dissipation.Several of these alternatives are discussed in the application note Comparing Bus Solutions,available on the TI web site.1.5Basic TopologyIn the motion control application example shown in Figure 3,there are several differentinterfaces that may require special attention wit
23、h regard to data transmission.Table 1 identifiesseveral categories of signals,and summarizes the critical characteristics of signaling speed andsignal level.Table 1.Signals in a Typical Motion Control SystemSIGNALDESCRIPTIONTYPICAL SPEEDSIGNAL LEVELSMotion CommandsDigital(pulse or encoded binary)Up
24、to 10 MbpsTTL or CMOS logicAnalogUp to the servo bandwidth ofthe system10V typical rangeMotion FeedbackDigital(pulse or encoded binary)Up to 10 MbpsTTL or CMOS logicPosition FeedbackSynchro,Resolver(sinusoidal)Up to 10 kHz20 VacEncoder,digital outputs(A,B and Index pulses)Up to 10 Mbps(afterinterpol
25、ation)TTL or CMOS logicDrive VoltageMotor coil voltage,one to three phasesUp to 100 kHz depending oncommutation schemeUp to 200 V depending on motorpower and windingCommutationSignalsBinary signals,usually three phases,for determiningmotor commutation based on winding positionUp to 3 kbpsTTL or CMOS
26、 logicTool/LoadCommandsApplication-specific command signals,usuallycoordinated with motion trajectoryApplication specificApplication specificActuatorLimits/StatusLimit switches,interlocks,homing sensors,etc.Up to 1 kbpsTTL,CMOS or up to 24VFrom this table it can be observed that any data transmissio
27、n scheme must have a wide rangeof operation to fit the spectrum of digital motion control needs.RS-485,with signaling from dc torates of over 10 Mbps and robust signal levels,suits most of these requirements well.Thesesignals are illustrated in Figure 3.Note this figure shows a single-axis system;mu
28、ltiaxis systemsmay share the same controller and have coupled mechanics to the same tool or load.SLLA1436RS-485 for Digital Motor Control Applications1MotorServoAmplifierActuator/LoadEncoderCommutationSignalsPositionFeedbackDriveVoltageActuatorLimits/StatusMotionCommandsMotionFeedbackTool/Load Comma
29、ndsControllerFigure 3.Interfaces in a Motor Control System(Single-Axis Shown)Depending on the physical arrangement of the specific application,there may be significantdistance between the controller,servo amplifiers,motors,and load.In addition to distance,otherfactors such as electrical noise,temper
30、atures,and cable faults should be considered whendesigning these systems.The goal of effective data transmission should be to provide reliablecommunication between these components,regardless of the distance or environmentalconditions.2Data Transmission Concerns and How RS-485 Addresses EachDigital
31、motion control applications pose several challenges to reaching the goal of efficient,robust communication between system components.Inherently an electromechanical actuator isinvolved,with its associated electrical noise and relatively high current levels.Safety anddependability further require tha
32、t the communication path is very reliable for controlling themoving mechanism.Also associated with the moving application are constraints on cablerouting,which may require extra lengths of cabling.Stability of the servo system also putsadditional demands on the signaling rate.In the following paragr
33、aphs,the suitability of RS-485to meet these needs is discussed.SLLA1437 RS-485 for Digital Motor Control Applications12.1Environment2.1.1EMI/Noise ImmunityElectromagnetic interference(EMI)can corrupt the signals in a motor control system.Typicalsources of EMI are motor drive voltages,motor brush noi
34、se,tool sources,and electrical noisefrom clocks,displays,and other computer-based components.In an analog system,noisesignals might cause unwanted motion or instability.Due to the inherent signal-to-noise ratio ofbinary coding,the main concern with digital systems is spurious pulses,which may beinte
35、rpreted as commands or feedback signals.The RS-485 signaling standard includes features that are well suited to these EMI concerns.RS-485 signaling is balanced and differential,and is typically transmitted over twisted wire pairs.This results in any electrical noise being coupled nearly equally onto
36、 both lines.This noise istherefore rejected,while the difference in the voltages continues to carry the signal information.The RS-485 signal levels are defined such that for any active driver,one line is driven high andone is driven low.The magnitude of the difference between the voltages on the two
37、 lines mustbe greater than 1.5 V at the driver to transmit a valid state.This is true for all valid loadingconditions.The receiver specifications are very important for EMI noise rejection.The RS-485 standardrequires detection of a valid state when the received differential signal has amplitude of 2
38、00 mVor more.This sensitivity accounts for losses in the cable that reduce the signal at the receiverbelow the 1.5 V amplitude generated by the driver.Also important,and not specified by the RS-485 standard,is the receiver hysteresis(shown inFigure 4),which is the difference between the thresholds f
39、or low-to-high and high-to-lowtransitions.Receiver DifferentialInput VoltageReceiver OutputVoltageReceiver OutputVoltageReceiver DifferentialInput VoltageWithoutHysteresisWithHysteresisFigure 4.Receiver Function With and Without HysteresisBecause no wire pair is perfectly balanced,there will be some
40、 differential-mode noise inducedby EMI sources.Without receiver hysteresis,the receiver would change state each time theinputs intersect(a differential voltage of zero),whether due to valid signal changes or inresponse to noise(see Figure 5).Therefore hysteresis is needed to avoid spurious pulses,es
41、pecially during idle-bus or transition periods.These spurious pulses could be interpreted asencoder counts,step commands,or actuator signals,depending on the location in the systemwhere they occur.Receivers with higher values of hysteresis are more immune to EMI noise.Typical RS-485 receivers have 4
42、0 mV to 60 mV of hysteresis;Texas Instruments offers receiverswith up to 100 mV of hysteresis for especially harsh electrical noise environments,such asdigital motor control.SLLA1438RS-485 for Digital Motor Control Applications1Receiver InputsReceiver OutputReceiver Outputwithout hysteresiswith hyst
43、eresisFigure 5.Hysteresis Eliminates Spurious Transitions2.1.2Ground Potentials/Common ModeAnother type of electrical challenge that can affect communication in a motion control applicationis offsets in the ground reference between the driver and the receiver nodes.Current loading,such as may occur
44、with a high-power tool,can cause this type of problem.Localized voltagesurges may also occur due to motor back-emf,equipment failures,and secondary surges fromnearby lightning strikes.Figure 6 illustrates how ground offset can occur in a motion control application.Consider atypical motor and amplifi
45、er/controller,with some length of cable connecting them forcommunication and providing electrical power.If the 24-V power supply between node 1 and node 2 is connected by 50 meters of 14 AWGcable,we expect RCOPPER to be approximately 0.5?.Under normal operation,we assume themotor current is less tha
46、n 2 A.But under a stall fault condition,the current may quickly spike to10 A.This causes a difference between GND1 and GND2 of 5 V,due to the drop across theground line.Therefore any signal referenced to GND1 appears to be shifted by 5 V whenreceived at node 2.Since all signals are affected by a com
47、mon offset,this is known as acommon-mode voltage shift.While this scenario prevents reliable communications with single-ended data transmission,a 5-Vground shift is within the standard RS-485 common-mode voltage(VCM)range.Since thesignals from node 1 are both shifted equally,the differential-mode si
48、gnal is still valid,and theRS-485 receiver reliably receives the correct signal.SLLA1439 RS-485 for Digital Motor Control Applications1Power SupplyGNDNODE 1NODE 2VoltageRegulatorGND2VoltageRegulatorGND124 V dcVCC1VCC2ILOADR(Copper)R(Copper)Figure 6.System With Ground Potential ShiftAll of Texas Inst
49、ruments RS-485 transceivers meet or exceed the requirements of theTIA/EIA-485 standard for operation with a common-mode voltage range of 7 V to 12 V.Foroperation over an even wider range of VCM,new products such as the SN65HVD22 operate witha common-mode range of 20 V to 25 V.2.1.3Electrostatic Disc
50、hargeElectrostatic discharge(ESD)is a hazard for any circuit that is connected via a cable,and whichmay be exposed to handling or external high voltages.Various test methods,such as theJEDEC human body model(HBM)and the IEC ESD immunity test(IEC 61000-4-2)are used tosimulate different ESD hazards.Te
