数字电机控制中rs -485的应用

1、Application ReportSLLA143 - May 2003RS-485 for Digital Motor Control Applications1Clark KinnairdHPLInterfaceABSTRACTThis application report focuses on the benefits of using RS-485 signaling for motor control and motion control applications. This technology has several benefits for these applications

2、 in terms of noise immunity, wide common-mode voltage range, adequate data rate, and multipoint capability. Other applications also use RS-485 signaling to take advantage of these same benefits. Applications such as process control networks, industrial automation, remote terminals, building automati

3、on and security systems apply RS-485 extensively to solve their requirements for robust data transmission over relatively long distances.ContentsIntroduction31.41.5Motor Devices3Feedback4Controllers4Data Transmission5Basic Topology52Data Transmission Concerns and How RS-485 Addresses Each6

4、2.1Environment..4EMI/Noise Immunity7Ground Potentials/Common Mode8Electrostatic Discharge9General Ruggedness102.2Speed..4Feedback Loop Delay10Propagation Delay (Cable Transmission Delay, Transceiver Delay.)10Signaling Rate11Larger Payload for Serial Communication

5、112.3Multipoint Topologies113Application Example123.1 Encoder Feedback Signals From High-Resolution Incremental Encoder12Conclusion13References13451 Originally featured on ChipCenter Analog Avenue at /knowledge_centers/analog/ Trademarks are the property of their respective owners.

6、1SLLA143List of FiguresDigital Motor Control Block Diagram Showning Components Offered by Texas Instruments3Rotary and Linear Electric Motors4Interfaces in a Motor Control System (Single-Axis Shown)6Receiver Function With and Without Hysteresis7Hysteresis Eliminates Spurious Transitions8System With

7、Ground Potential Shift9Typical Application, Encoder Feedback Signals121234567List of TablesSignals in a Typical Motion Control System51RS-485 for Digital Motor Control Applications12SLLA1431IntroductionDigital motor control refers to using digital processors to control the motion of electric motors.

8、 Typically one or more methods of feedback are available to the digital processor, making this a closedloop system (See Figure 1). This contrasts to analog control systems and open-loop motion systems.Digital motor control is found in many applications. These include storage devices (such as disk dr

9、ives), industrial robotics, high-precision semiconductor manufacturing, and copiers.Figure 1. Digital Motor Control Block Diagram Showning Components Offered by Texas Instruments1.1Motor DevicesThe motor involved in digital motor control may be one of several types. The most common is the subfractio

10、nal horsepower rotary motor (see Figure 2a). These may be further classified as ac, dc brush, or dc brushless type, depending on the method of commutation. Small motors are typically sized according to their frame size and power in watts. Larger motors, typically ac type, are classified by their pow

11、er in horsepower. Although rotary motors are the most common, other configurations are available, such as linear motors (see Figure 2b), and gearhead motors with various implementations of actuators built on.RS-485 for Digital Motor Control Applications13SLLA143(a) Rotary Electric Motor(b) Linear El

12、ectric MotorFigure 2. Rotary and Linear Electric Motors1.2FeedbackIn order to provide feedback on the position, speed, torque or other dynamic properties of the motion system, feedback sensors are necessary. Perhaps the most common feedback sensor is a rotary encoder, consisting of a wheel with alte

13、rnating stripes, mounted on the motor shaft. As the motor rotates, an optical sensor detects the passing of the stripes, and produces electrical signals, which a controller can use to determine the motors motion. Other types of sensors are tachometers, synchros and resolvers, which are induction-bas

14、ed sensors, Hall-effect sensors, which are magnetic-based, and potentiometers, which are resistance-based.No matter which sensor method is used, the digital controller must repetitively sample the sensor signal, in order to constantly maintain current knowledge of the systems dynamic motion. Dependi

15、ng on the system requirements for speed, dynamic response and accuracy, the rate of feedback sampling may be over several thousand samples per second.1.3ControllersThe controller, whether digital or analog, compares the commanded motion and actual dynamics of the system, and processes these inputs t

16、o create a control signal to the actuator. In the case of 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 as single-chip

17、 programmed gate arrays. Texas Instruments offers digital signal processors with features optimized for motion control, and microcontrollers with varying features for best-fit solutions to a wide array of applications. For more information, search on Digital Control on the Texas Instruments web site

18、 at .RS-485 for Digital Motor Control Applications14SLLA1431.4Data TransmissionThis discussion focuses on the benefits of using RS-485 signaling for motor control and motion control applications. As discussed below, this technology has several benefits for these applications in terms of no

19、ise immunity, wide common-mode voltage range, adequate data rate, and multipoint capability. Other applications also use RS-485 signaling to take advantage of these same benefits. Thus applications such as process control networks, industrial automation, remote terminals, building automation and sec

20、urity systems apply RS-485 extensively to solve their requirements for robust data transmission over relatively long distances. Often the RS-485 signaling is bundled with a protocol such as Profibus, Interbus, Modbus or BACnet, each tailored for the specific requirements of the end user.Other signal

21、ing technologies may be used when the features of RS-485 are not the best fit. For example, RS-232 or RS-422 signaling may be adequate in some applicationssometimes controller area network (CAN) or EtherNet/IP (Industrial Protocol) are preferred due to compatibility with an existing network. For hig

22、her speed applications, and where long distance and common-mode voltages are not as rigorous, M-LVDS can provide 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 a

23、pplication example shown in Figure 3, there are several different interfaces that may require special attention with regard to data transmission. Table 1 identifies several categories of signals, and summarizes the critical characteristics of signaling speed and signal level.Table 1. Signals in a Ty

24、pical Motion Control SystemFrom this table it can be observed that any data transmission scheme must have a wide range of operation to fit the spectrum of digital motion control needs. RS-485, with signaling from dc to rates of over 10 Mbps and robust signal levels, suits most of these requirements

25、well. These signals are illustrated in Figure 3. Note this figure shows a single-axis system; multiaxis systems may share the same controller and have coupled mechanics to the same tool or load.RS-485 for Digital Motor Control Applications15SIGNALDESCRIPTIONTYPICAL SPEEDSIGNAL LEVELSMotion CommandsD

26、igital (pulse or encoded binary)Up to 10 MbpsTTL or CMOS logicAnalogUp to the servo bandwidth of the 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 an

27、d Index pulses)Up to 10 Mbps (after interpolation)TTL or CMOS logicDrive VoltageMotor coil voltage, one to three phasesUp to 100 kHz depending on commutation schemeUp to 200 V depending on motor power and windingCommutation SignalsBinary signals, usually three phases, for determining motor commutati

28、on based on winding positionUp to 3 kbpsTTL or CMOS logicTool/Load CommandsApplication-specific command signals, usually coordinated with motion trajectoryApplication specificApplication specificActuator Limits/StatusLimit switches, interlocks, homing sensors, etc.Up to 1 kbpsTTL, CMOS or up to 24VS

29、LLA143Tool/Load CommandsActuatLoadMotion FeedbackFigure 3.Interfaces in a Motor Control System (Single-Axis Shown)Depending on the physical arrangement of the specific application, there may be significant distance between the controller, servo amplifiers, motors, and load. In addition to distance,

30、other factors such as electrical noise, temperatures, and cable faults should be considered when designing these systems. The goal of effective data transmission should be to provide reliable communication between these components, regardless of the distance or environmental conditions.2Data Transmi

31、ssion Concerns and How RS-485 Addresses EachDigital motion control applications pose several challenges to reaching the goal of efficient, robust communication between system components. Inherently an electromechanical actuator is involved, with its associated electrical noise and relatively high cu

32、rrent levels. Safety and dependability further require that the communication path is very reliable for controlling the moving mechanism. Also associated with the moving application are constraints on cable routing, which may require extra lengths of cabling. Stability of the servo system also puts

33、additional demands on the signaling rate. In the following paragraphs, the suitability of RS-485 to meet these needs is discussed.RS-485 for Digital Motor Control Applications16Controllerr/AmplifierActuatorEncoderLimits/StatusCommutationSignalsMotorPositionDriveFeedbackVoltageMotion CommandsServoSLL

34、A14EnvironmentEMI/Noise ImmunityElectromagnetic interference (EMI) can corrupt the signals in a motor control system. Typical sources of EMI are motor drive voltages, motor brush noise, tool sources, and electrical noise from clocks, displays, and other computer-based components. In an anal

35、og system, noise signals might cause unwanted motion or instability. Due to the inherent signal-to-noise ratio of binary coding, the main concern with digital systems is spurious pulses, which may be interpreted as commands or feedback signals.The RS-485 signaling standard includes features that are

36、 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 both lines. This noise is therefore rejected, while the difference in the voltages continues

37、to carry the signal information.The RS-485 signal levels are defined such that for any active driver, one line is driven high and one is driven low. The magnitude of the difference between the voltages on the two lines must be greater than 1.5 V at the driver to transmit a valid state. This is true

38、for all valid loading conditions.The receiver specifications are very important for EMI noise rejection. The RS-485 standard requires detection of a valid state when the received differential signal has amplitude of 200 mV or more. This sensitivity accounts for losses in the cable that reduce the si

39、gnal at the receiver below the 1.5 V amplitude generated by the driver.Also important, and not specified by the RS-485 standard, is the receiver hysteresis (shown in Figure 4), which is the difference between the thresholds for low-to-high and high-to-low transitions.Receiver Output VoltageReceiver

40、Output VoltageWithout HysteresisWith HysteresisReceiver DifferentialReceiver DifferentialInput VoltageInput VoltageFigure 4. Receiver Function With and Without HysteresisBecause no wire pair is perfectly balanced, there will be some differential-mode noise induced by EMI sources. Without receiver hy

41、steresis, the receiver would change state each time the inputs intersect (a differential voltage of zero), whether due to valid signal changes or in response to noise (see Figure 5). Therefore hysteresis is needed to avoid spurious pulses, especially during idle-bus or transition periods. These spur

42、ious pulses could be interpreted as encoder counts, step commands, or actuator signals, depending on the location in the system where they occur. Receivers with higher values of hysteresis are more immune to EMI noise. Typical RS-485 receivers have 40 mV to 60 mV of hysteresis; Texas Instruments off

43、ers receivers with up to 100 mV of hysteresis for especially harsh electrical noise environments, such as digital motor control.RS-485 for Digital Motor Control Applications17SLLA143Receiver InputsReceiver Outputwithout hysteresisReceiver Outputwith hysteresisFigure 5. Hysteresis Eliminates Spurious

44、 Transitions2.1.2Ground Potentials/Common ModeAnother type of electrical challenge that can affect communication in a motion control application is offsets in the ground reference between the driver and the receiver nodes. Current loading, such as may occur with a high-power tool, can cause this typ

45、e of problem. Localized voltage surges may also occur due to motor back-emf, equipment failures, and secondary surges from nearby lightning strikes.Figure 6 illustrates how ground offset can occur in a motion control application. Consider a typical motor and amplifier/controller, with some length of

46、 cable connecting them for communication and providing electrical power.If the 24-V power supply between node 1 and node 2 is connected by 50 meters of 14 AWG cable, we expect RCOPPER to be approximately 0.5 . Under normal operation, we assume the motor current is less than 2 A. But under a stall fa

47、ult condition, the current may quickly spike to 10 A. This causes a difference between GND1 and GND2 of 5 V, due to the drop across the ground line. Therefore any signal referenced to GND1 appears to be shifted by 5 V when received at node 2. Since all signals are affected by a common offset, this i

48、s known as a common-mode voltage shift.While this scenario prevents reliable communications with single-ended data transmission, a 5-V ground shift is within the standard RS-485 common-mode voltage (VCM) range. Since the signals from node 1 are both shifted equally, the differential-mode signal is s

49、till valid, and theRS-485 receiver reliably receives the correct signal.RS-485 for Digital Motor Control Applications18SLLA143NODE 1Power SupplyR(Copper)NODE 224 V dcGNDR(Copper)VCC2Voltage RegulatorVCC1Voltage RegulatorILOADGND2GND1 Figure 6. System With Ground Potential ShiftAll of Texas Instrumen

50、ts RS-485 transceivers meet or exceed the requirements of the TIA/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 with a common-mode range of 20 V to 25 V.2.1.3Electrostatic Disch

51、argeElectrostatic discharge (ESD) is a hazard for any circuit that is connected via a cable, and which may be exposed to handling or external high voltages. Various test methods, such as the JEDEC human body model (HBM) and the IEC ESD immunity test (IEC 61000-4-2) are used to simulate different ESD

52、 hazards. Texas Instruments offers a selection of transceivers with ESD protection integrated into the bus circuits.Typical levels of protection range from 2 kV to 15 kV. Some transceivers, such as the SN65LBC184, providing protection to events of over 30 kV. The level of protection needed by any pa

53、rticular application is difficult to predict, but designers should consider such factors as:The electrical environment where the transceiver is located Handling conditions and frequency of cable access Diagnostic procedures to determine failure pointsReplacement downtime and associated labor costsAn

54、other type of electrical hazard is damage due to transient (surge) overvoltages. This type of event can be caused by lightning strikes coupled through a power transformer secondary, or by localized power faults due to machine failures. Test methods for this type of hazard are documented in IEC61000-

55、4-5. External protection diodes are typically added to provide a safe path for this energy to dissipate. Texas Instruments offers the SN65LBC184 with integrated transient voltage suppression circuits, capable of protecting the bus inputs to over 400 W of surge power.RS-485 for Digital Motor Control

56、Applications19SLLA1432.1.4General RuggednessBecause digital motor control products must frequently operate in harsh environments, there are additional considerations when specifying transceivers. For high-power and industrial applications, performance over an extended temperature range may be requir

57、ed. TI offersRS-485 transceivers specified for commercial, industrial, automotive, and military temperature ranges.Another concern may be the power supply for the transceiver and the power supply tolerance. Recognizing that high-current motor applications may induce voltage sag in the power supplies, TI offers a selec