Clark Kinnaird ABSTRACT This application report focuses benefits using

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RS-485 Digital Motor Control Applications
Clark Kinnaird ABSTRACT This application report focuses benefits using RS-485 signaling motor control motion control applications. This technology several benefits these applications terms noise immunity, wide common-mode voltage range, adequate data rate, multipoint capability. Other applications also RS-485 signaling take advantage these same benefits. Applications such process control networks, industrial automation, remote terminals, building automation security systems apply RS-485 extensively solve their requirements robust data transmission over relatively long distances. HPL-Interface
Contents Introduction Motor Devices Feedback Controllers Data Transmission Basic Topology
Data Transmission Concerns RS-485 Addresses Each Environment 2.1.1 EMI/Noise Immunity 2.1.2 Ground Potentials/Common Mode 2.1.3 Electrostatic Discharge 2.1.4 General Ruggedness Speed 2.2.1 Feedback Loop Delay 2.2.2 Propagation Delay (Cable Transmission Delay, Transceiver Delay.) 2.2.3 Signaling Rate 2.2.4 Larger Payload Serial Communication Multipoint Topologies Application Example Encoder Feedback Signals From High-Resolution Incremental Encoder Conclusion References
Originally featured ChipCenter Analog Avenue Trademarks property their respective owners.
SLLA143
List Figures Digital Motor Control Block Diagram Showning Components Offered Texas Instruments Rotary Linear Electric Motors Interfaces Motor Control System (Single-Axis Shown) Receiver Function With Without Hysteresis Hysteresis Eliminates Spurious Transitions System With Ground Potential Shift Typical Application, Encoder Feedback Signals List Tables Signals Typical Motion Control System
RS-485 Digital Motor Control Applications
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Introduction
Digital motor control refers using digital processors control motion electric motors. Typically more methods feedback available digital processor, making this closed-loop system (See Figure This contrasts analog control systems open-loop motion systems. Digital motor control found many applications. These include storage devices (such disk drives), industrial robotics, high-precision semiconductor manufacturing, copiers.
Figure Digital Motor Control Block Diagram Showning Components Offered Texas Instruments
Motor Devices
motor involved digital motor control several types. most common subfractional horsepower rotary motor (see Figure 2a). These further classified brush, brushless type, depending method commutation. Small motors typically sized according their frame size power watts. Larger motors, typically type, classified their power horsepower. Although rotary motors most common, other configurations available, such linear motors (see Figure 2b), gearhead motors with various implementations actuators built
RS-485 Digital Motor Control Applications
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Rotary Electric Motor
Linear Electric Motor
Figure Rotary Linear Electric Motors
Feedback
order provide feedback position, speed, torque other dynamic properties motion system, feedback sensors necessary. Perhaps most common feedback sensor rotary encoder, consisting wheel with alternating stripes, mounted motor shaft. motor rotates, optical sensor detects passing stripes, produces electrical signals, which controller determine motor's motion. Other types sensors tachometers, synchros resolvers, which induction-based sensors, Hall-effect sensors, which magnetic-based, potentiometers, which resistance-based. matter which sensor method used, digital controller must repetitively sample sensor signal, order constantly maintain current knowledge system's dynamic motion. Depending system requirements speed, dynamic response accuracy, rate feedback sampling over several thousand samples second.
Controllers
controller, whether digital analog, compares commanded motion actual dynamics system, processes these inputs create control signal actuator. case digital controllers, additional tasks include system start-up routines, diagnostics, communications control, sampling multiple sensors. Digital controllers complex dedicated computer processors, simple single-chip programmed gate arrays. Texas Instruments offers digital signal processors with features optimized motion control, microcontrollers with varying features best-fit solutions wide array applications. more information, search Digital Control Texas Instruments site www.ti.com.
RS-485 Digital Motor Control Applications
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Data Transmission
This discussion focuses benefits using RS-485 signaling motor control motion control applications. discussed below, this technology several benefits these applications terms noise immunity, wide common-mode voltage range, adequate data rate, multipoint capability. Other applications also RS-485 signaling take advantage these same benefits. Thus applications such process control networks, industrial automation, remote terminals, building automation security systems apply RS-485 extensively solve their requirements robust data transmission over relatively long distances. Often RS-485 signaling bundled with protocol such Profibus, Interbus, Modbus BACnet, each tailored specific requirements user. Other signaling technologies used when features RS-485 best fit. example, RS-232 RS-422 signaling adequate some applications-sometimes controller area network (CAN) EtherNet/IP (Industrial Protocol) preferred compatibility with existing network. higher speed applications, where long distance common-mode voltages rigorous, M-LVDS provide lower power dissipation. Several these alternatives discussed application note Comparing Solutions, available site.
Basic Topology
motion control application example shown Figure there several different interfaces that require special attention with regard data transmission. Table identifies several categories signals, summarizes critical characteristics signaling speed signal level. Table Signals Typical Motion Control System
SIGNAL
DESCRIPTION
Digital (pulse encoded binary) Analog
TYPICAL SPEED
Mbps servo bandwidth system Mbps Mbps (after interpolation) depending commutation scheme kbps Application specific kbps
SIGNAL LEVELS
CMOS logic ±10V typical range CMOS logic CMOS logic depending motor power winding CMOS logic Application specific TTL, CMOS
Motion Commands
Motion Feedback Position Feedback
Digital (pulse encoded binary) Synchro, Resolver (sinusoidal) Encoder, digital outputs Index pulses)
Drive Voltage Commutation Signals Tool/Load Commands Actuator Limits/Status
Motor coil voltage, three phases Binary signals, usually three phases, determining motor commutation based winding position Application-specific command signals, coordinated with motion trajectory usually
Limit switches, interlocks, homing sensors, etc.
From this table observed that data transmission scheme must have wide range operation spectrum digital motion control needs. RS-485, with signaling from rates over Mbps robust signal levels, suits most these requirements well. These signals illustrated Figure Note this figure shows single-axis system; multiaxis systems share same controller have coupled mechanics same tool load.
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Tool/Load Commands Actuator Limits/Status
Encoder
Actuator/ Load Commutation Signals Motor Drive Voltage Position Feedback
Motion Commands Servo Amplifier Motion Feedback Controller
Figure Interfaces Motor Control System (Single-Axis Shown) Depending physical arrangement specific application, there significant distance between controller, servo amplifiers, motors, load. addition distance, other factors such electrical noise, temperatures, cable faults should considered when designing these systems. goal effective data transmission should provide reliable communication between these components, regardless distance environmental conditions.
Data Transmission Concerns RS-485 Addresses Each
Digital motion control applications pose several challenges reaching goal efficient, robust communication between system components. Inherently electromechanical actuator involved, with associated electrical noise relatively high current levels. Safety dependability further require that communication path very reliable controlling moving mechanism. Also associated with moving application constraints cable routing, which require extra lengths cabling. Stability servo system also puts additional demands signaling rate. following paragraphs, suitability RS-485 meet these needs discussed.
RS-485 Digital Motor Control Applications
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2.1.1
Environment EMI/Noise Immunity
Electromagnetic interference (EMI) corrupt signals motor control system. Typical sources motor drive voltages, motor brush noise, tool sources, electrical noise from clocks, displays, other computer-based components. analog system, noise signals might cause unwanted motion instability. inherent signal-to-noise ratio binary coding, main concern with digital systems spurious pulses, which interpreted commands feedback signals. RS-485 signaling standard includes features that well suited these concerns. RS-485 signaling balanced differential, typically transmitted over twisted wire pairs. This results electrical noise being coupled nearly equally onto both lines. This noise therefore rejected, while difference voltages continues carry signal information. RS-485 signal levels defined such that active driver, line driven high driven low. magnitude difference between voltages lines must greater than driver transmit valid state. This true valid loading conditions. receiver specifications very important noise rejection. RS-485 standard requires detection valid state when received differential signal amplitude more. This sensitivity accounts losses cable that reduce signal receiver below amplitude generated driver. Also important, specified RS-485 standard, receiver hysteresis (shown Figure which difference between thresholds low-to-high high-to-low transitions.
Receiver Output Voltage Without Hysteresis Receiver Differential Input Voltage With Hysteresis Receiver Differential Input Voltage Receiver Output Voltage
Figure Receiver Function With Without Hysteresis Because wire pair perfectly balanced, there will some differential-mode noise induced sources. Without receiver hysteresis, receiver would change state each time inputs intersect differential voltage zero), whether valid signal changes response noise (see Figure Therefore hysteresis needed avoid spurious pulses, especially during idle-bus transition periods. These spurious pulses could interpreted encoder counts, step commands, actuator signals, depending location system where they occur. Receivers with higher values hysteresis more immune noise. Typical RS-485 receivers have hysteresis; Texas Instruments offers receivers with hysteresis especially harsh electrical noise environments, such digital motor control.
RS-485 Digital Motor Control Applications
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Receiver Inputs
Receiver Output
without hysteresis
Receiver Output
with hysteresis
Figure Hysteresis Eliminates Spurious Transitions
2.1.2
Ground Potentials/Common Mode
Another type electrical challenge that affect communication motion control application offsets ground reference between driver receiver nodes. Current loading, such occur with high-power tool, cause this type problem. Localized voltage surges also occur motor back-emf, equipment failures, secondary surges from nearby lightning strikes. Figure illustrates ground offset occur motion control application. Consider typical motor amplifier/controller, with some length cable connecting them communication providing electrical power. 24-V power supply between node node connected meters cable, expect RCOPPER approximately Under normal operation, assume motor current less than under stall fault condition, current quickly spike This causes difference between GND1 GND2 drop across ground line. Therefore signal referenced GND1 appears shifted when received node Since signals affected common offset, this known common-mode voltage shift. While this scenario prevents reliable communications with single-ended data transmission, ground shift within standard RS-485 common-mode voltage (VCM) range. Since signals from node both shifted equally, differential-mode signal still valid, RS-485 receiver reliably receives correct signal.
RS-485 Digital Motor Control Applications
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NODE
Power Supply R(Copper) NODE
R(Copper) Voltage Regulator GND1 VCC1
Voltage VCC2 Regulator GND2
ILOAD
Figure System With Ground Potential Shift Texas Instruments' RS-485 transceivers meet exceed requirements TIA/EIA-485 standard operation with common-mode voltage range operation over even wider range VCM, products such SN65HVD22 operate with common-mode range
2.1.3
Electrostatic Discharge
Electrostatic discharge (ESD) hazard circuit that connected cable, which exposed handling external high voltages. Various test methods, such JEDEC human body model (HBM) immunity test (IEC 61000-4-2) used simulate different hazards. Texas Instruments offers selection transceivers with protection integrated into circuits. Typical levels protection range from Some transceivers, such SN65LBC184, providing protection events over level protection needed particular application difficult predict, designers should consider such factors
electrical environment where transceiver located Handling conditions frequency cable access Diagnostic procedures determine failure points Replacement downtime associated labor costs
Another type electrical hazard damage transient (surge) overvoltages. This type event caused lightning strikes coupled through power transformer secondary, localized power faults machine failures. Test methods this type hazard documented IEC61000-4-5. External protection diodes typically added provide safe path this energy dissipate. Texas Instruments offers SN65LBC184 with integrated transient voltage suppression circuits, capable protecting inputs over surge power.
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2.1.4
General Ruggedness
Because digital motor control products must frequently operate harsh environments, there additional considerations when specifying transceivers. high-power industrial applications, performance over extended temperature range required. offers RS-485 transceivers specified commercial, industrial, automotive, military temperature ranges. Another concern power supply transceiver power supply tolerance. Recognizing that high-current motor applications induce voltage power supplies, offers selection transceivers which meet full performance specifications with either variation supply. most cases, RS-485 transceivers operates over even wider power supply variation, meet parametric specifications. TI's selection transceivers includes products supplies, well SN65HVD08 transceiver which operates with power supply voltages range
2.2.1
Speed Feedback Loop Delay
concern engineers designing communications digital motor control whether communications components significant delay servo loop. general, propagation delay associated with RS-485 data transmission negligible typical systems. communications delays categorized into:
2.2.2
propagation delay transceiver media signaling rate (synchronization) delay overhead added coding
Propagation Delay (Cable Transmission Delay, Transceiver Delay.)
propagation delay transceiver media physics transmitting electrical signal through semiconductor devices through copper wire. Typical propagation delay through transceiver order tens hundreds nanoseconds. Propagation delay through cable, such twisted-pair wires RS-485, about meter. comparison, consider very high-performance system, with servo bandwidth kHz. Therefore, even this very quick system, transceiver delay microsecond (1000 corresponds phase shift less than degrees. cable lengths less than meters, phase shift less than cable delay also negligible.
RS-485 Digital Motor Control Applications
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2.2.3
Signaling Rate
data transmission such that data sent received soon available, signaling rate typically limited only data source, data transmission chain. example would encoder, which sends pulses asynchronously, soon motion detected. Rotary encoders able produce 8192 even 32000 counts revolution, rates over million counts second. coupled directly transceiver, these pulses transmitted starting less than microsecond, with typically negligible delay system. However, controller clocks transceiver synchronously, signaling rate much slower, this constraint system performance. Typical synchronous signaling rates 9600 bits second, 19200 bits second, kbps, etc. system designer should consider impact this signaling rate delay latency data, performance system.
2.2.4
Larger Payload Serial Communication
Besides propagation delays synchronous signaling delays, there delays encoding format associated with protocol data. Encoding incorporated data transmission scheme several reasons. reason provide means error checking. typical example parity commonly used verify fidelity each data bits. Another example start stop bits used signal beginning message. Description bits such command/status codes also part message protocol, data source sufficient complexity support these elements. These added bits provide additional features data transmission scheme, also require time transmit decode. Therefore, system designer must sure allow margin these overhead bits when setting system speed requirements signaling rate. example, consider application with encoder providing absolute position data form three 8-bit words. With signaling rate 9600 bits second (bps), feedback speed positions second achieved. However, message protocol requires additional bits message determine most-significant word, start bit, stop bit, parity, etc.) then effective update rate drops position updates second (9600 bits/sec update/ bits updates/sec).
Multipoint Topologies
Another consideration whether more than nodes communicate same bus. node transmits several receivers, this called multidrop configuration. several nodes take control transmit other nodes, this denoted multipoint architecture. course, complexity system increases, signaling protocol must include procedures determining which node will transmit time. This avoids contention, where line drivers fight each other voltages. safeguard, RS-485 standard also requires that each transceiver include protection against damage contention. That drivers should happen active with opposite states, neither shall damaged contending voltage levels shared lines. With available RS-485 signaling technology, nodes with standard (unit-load) transceivers nodes using sub-unit-load transceivers) connected same twisted pair cable multipoint arrangement. This simplify wiring multiaxis, multisensor system.
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signaling rate should selected high enough allow nodes meet their individual update requirements. TIA/EIA-485 standard suggests signaling speeds million bits second Mbps). While this more than adequate most systems, Texas Instruments offers transceivers with signaling rate capability over Mbps, most demanding high-speed systems. There several standard protocols which signaling based RS-485. These protocols implement various methods setting message formats, error checking, multipoint control, negotiating signaling rates. Common protocols that used motor motion control Modbus, Profibus, Interbus-S. Each championed different vendors trade organizations, optimized different network conditions.
Application Example
Encoder Feedback Signals From High-Resolution Incremental Encoder
application example shown Figure where RS-485 signaling being used report encoder information motion controller. necessary locate motion controller some distance from encoder, space constraints need access controller easily. this example, there four signals point-to-point configuration, quad driver quad receiver needed. Termination resistors needed receiving match cable impedance thereby eliminate signal reflections. Selection optimum driver receiver chips will depend several factors:
distance from encoder controller maximum speed motor rotation interpolation factor, which determines encoder resolution Requirements protection, power dissipation, cost
Servo Drive Motion Controller SN65LBC172A Encoder Interpolation Electronics Encoder Phase SN65LBC173A Encoder Phase
Encoder Index Status
Status
Figure Typical Application, Encoder Feedback Signals
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Conclusion
RS-485 signaling provides solutions many challenges digital motor control communications.
overcomes electrical noise with high driver output voltages substantial receiver hysteresis. long distances, strong differential drivers wide common-mode capability ensure reliable signaling. protection surge survivability available integrated features; these enhance reliability harsh environments. speed available with RS-485 signaling sufficient have negligible impact servo performance, even when burdened with error checking protocol overhead. capability operate multipoint architectures makes RS-485 flexible expandable advanced, networked applications.
Overall, moderate signaling rate, robust features, wide selection available transceivers make this technology good most digital motion control applications.
References
ANSI TIA/EIA-485: Electrical Characteristics Generators Receivers Balanced Digital Multipoint Systems, available from Global Engineering Documents, www.global.ihs.com Interface Circuits TIA/EIA-485 (RS-485), Texas Instruments application note (SLLA036), www.ti.com Comparing Solutions, Texas Instruments application note (SLLA067), www.ti.com DSP-Solution High-Resolution Position with Sin/Cos-Encoders, Texas Instruments application report (SPRA496), www.ti.com Electrical Noise Motion Control Circuits, application note #5438, Galil Motion Control Inc., www.galilmc.com Resolver Rotary Encoder, Characteristics Measuring Systems, Heidenhain Technical Article, Peter Polak, www.heidenhain.com Linear Encoders Linear Motors, Heidenhain technical article, www.heidenhain.com Introduction MODBUS, Acromag technical reference, www.acromag.com InterBus Association site, www.interbusclub.com Modbus Organization site, www.modbus.org Profibus Trade Organization site, www.profibus.com
RS-485 Digital Motor Control Applications
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