Design Robust, Fault-Tolerant Motion-Control Feedback Systems

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Successful designers will understand the key role played by receiver ICs, encoder signal cabling, terminations, and pc-board layout. Today's demanding industrial applications require rugged, reliable robots and automated machines that operate under harsh conditions and run 24 hours a day, seven days a week.

Fortunately, modern high-speed servo systems can be designed for a robust and fault-tolerant motion-control feedback system. Receiver circuits for the motion controller must respond predictably to potential faults, and proper pc layout for the receiver circuit prevents noise problems in the encoder data. A designer should also consider the quadrature encoder's signal cabling, including terminations at the receiver circuit. These precautions will produce a motion-control system that's stable and predictable during fault conditions.

Much of the improved performance seen today results from newer technologies and microelectronics. Those innovations provide more robust automated systems by eliminating robot collisions in shared workspaces, improving task assignments, and honing servo accuracies. The key to robust system operation comes down to how it handles mechanical and electrical faults. To that end, this article discusses the design of a robust and fault-tolerant motion-control system whose feedback paths incorporate quadrature encoders.

HIGHLIGHTS

Servo Systems
Automated systems typically include feedback elements to ensure accurate and stable control over speed and position.

Controller Receiver Circuits
The controller's receiver converts the quadrature encoder's signals to logic levels for processing. The receiver provides fault detection and ESD protection.

Receiver Circuit Layout
Pay attention to board layout to minimize signal imbalance, crosstalk, common-mode noise, and noise coupling.

Encoder Sigal Cable
Shielded twisted-pair cable should be used for best performance. The cable should carry only the encoder's signals.

Sidebar: Encoder Types
A look at some of the types of components that supply feedback in servo systems.

Sidebar: Fault Types
A discussion of the most common faults in control systems and what problems could cause them.

Full article begins on Page 2

Fortunately, modern high-speed servo systems can be designed for a robust and fault-tolerant motion-control feedback system. Receiver circuits for the motion controller must respond predictably to potential faults, and proper pc layout for the receiver circuit prevents noise problems in the encoder data. A designer should also consider the quadrature encoder's signal-cabling system, including terminations at the receiver circuit. These precautions will produce a robust motion-control feedback system that's stable and predictable during fault conditions.

Much of today's improved performance results from newer technologies and microelectronics. Those innovations provide more-robust automated systems by eliminating robot collisions in shared workspaces, improving task assignments, and honing servo accuracies. The key to robust system operation comes down to how it handles mechanical and electrical faults. This article discusses the design of a robust and fault-tolerant motion-control system whose feedback paths incorporate quadrature encoders.

SERVO SYSTEMS Modern automated systems incorporate closed-loop feedback for motion control. They typically include a servo system that combines a motor driver and feedback elements to provide accurate and stable control over speed and position. Figure 1 illustrates the various system-level components of a typical servo system.

Dc brushless motors are preferred for high-performance and high-speed applications. Dc brush and stepper motors suit low-speed and less demanding applications. Brushless motors are assumed throughout this article. Such motors normally include a quadrature encoder on their end shaft that determines the shaft velocity and commutation point for controlling the motor's coil-switching sequences (see "Feedback Encoder Types," p. 3). A second quadrature encoder on the machine's rotating shaft supplies position data for that shaft, which generally differs from the motor-shaft position due to inaccuracies caused by backlash in the gearhead and lead-screw assemblies.

Typical motion-controller cards and modules include a motion-control IC, a microprocessor, and a DSP or custom ASIC for processing the high-speed encoder signals. The controller provides velocity and direction-of-rotation signals to the driver or amplifier, which supplies the proper levels of voltage and current (power) to operate the motor. To design a robust and fault-tolerant motion-control system with feedback, address the following at the system-level design phase:

Controller-encoder input circuits (receiver circuits)
Receiver circuit pc-board layout
Encoder-signal cabling system

Motion-controller inputs, such as hard-wired emergency stop, and limit inputs should also be considered when designing a fault-tolerant feedback system.

CONTROLLER RECEIVER CIRCUITS The motor's quadrature encoder sends six RS-422/RS-485 signals (A, A—, B, B—, Index, and —Index) down the cable to the motion controller's receiver circuit (the encoder input). The receiver converts the RS-422 signals to logic-level signals (we assume RS-422 signals because the system has only one transmitter) and feeds them to the motion-controller circuit for processing. (For RS-422 and RS-485 differences, refer to the short online tutorial, "RS-485 Basics," at www.maxim-ic.com.) The receiver circuit must respond to various faults in the servo-system environment, including opens, shorts, and noise (see "Fault Types," p. 4).

Figure 2 illustrates the encoder-input receiver circuit in a typical motion controller. The MAX3095 is a 10-Mbit/s, 5-V, quad RS-422/RS-485 receiver with ±15-kV ESD protection. For fault-tolerant systems with encoder inputs that connect to external components, ESD protection is a must. Here, ESD protection on all encoder input lines is internal to the device. The absence of external ESD-protection components substantially reduces the pc-board area required for this circuit.

The 150-Ω resistors provide proper terminations for each group of complementary signal pairs transmitted down the twisted-pair cable from the quadrature encoder. A break or disconnect in the cable produces an open-circuit fault that must be detected before the motion controller can take appropriate action. As a failsafe measure, the MAX3095 receiver outputs assert a logic high in response to an open-circuited input pair. The 1-kΩ resistors bias the receiver's "A" inputs at least 200 mV above its "B" inputs. They are also necessary for maintaining fail-safe outputs in the presence of the input-termination resistors. This circuit provides ESD protection, open-circuit detection, and output short-circuit protection, but it doesn't detect short-circuited inputs.

An improved circuit uses two MAX-3098E ICs. Each includes three RS-422/RS-485 receivers (Fig. 3). Every receiver provides built-in fault detection, ±15-kV ESD protection, and a 32-Mbit/s data rate. The MAX3098E detects open- and short-circuited encoder inputs. It also detects low-voltage differential signals, common-mode range violations, and other faults. Its logic-level outputs indicate which receiver input has the fault condition. By reporting the fault directly, that feature reduces software overhead and minimizes the need for external logic components.

A fault on any encoder input produces an immediate logic high on the corresponding output: ALARMA, ALARMB, or ALARMZ. Slow movement of the servo system can produce transient faults at the quadrature-encoder signal's zero-crossing region, triggering a "false fault." Delay the ALARMD output (logic OR of ALARMA, ALARMB, and ALARMZ) for a desired interval by selecting the C_Delay value. The 120-Ω resistors provide proper RS-422 terminations for the cable. Because the IC is available in a 16-pin QSOP package, this circuit requires fewer components and occupies a very compact space on the pc board.

RECEIVER CIRCUIT PC-BOARD LAYOUT A proper receiver-circuit layout starts with the RS-422 encoder's input connector. The differential signal pairs A A—, B —B, and Index —Index must occupy adjacent pins on the connector. That configuration minimizes signal imbalance by ensuring that the differential pair's returning signal-current paths overlap and cancel. Figure 4 shows typical component placements. To ensure that each pc-board trace has the same parasitic capacitance, route each differential pair of traces close together, with equal lengths, and with symmetrical bends.

To minimize inductive and capacitive crosstalk on the digital outputs and provide lower inductance, differential RS-422 signals from the connector and receiver circuit should be laid over a solid ground-plane layer within the pc board. No high-current signals should flow in this ground plane.

The high-speed current switching in motion-controller circuits can produce common-mode noise. Using filters and bypass capacitors helps to reduce the effect of common-mode voltages coupled onto the power-supply lines. You should place a 0.1-µF bypass capacitor close to the receiver's V_CC input. To minimize inductance in the bypass loop, the capacitor's ground lead should connect directly to the solid ground plane, as should the IC ground pin through a via placed adjacent to it. Finally, to minimize noise coupling to the receiver circuits, avoid routing receiver traces near or adjacent to any power circuits.

ENCODER SIGNAL CABLE Because the differential signals from a quadrature encoder are balanced, they can be transmitted on regular paired cable, but twisted-pair cable is preferred. Twisted-pair cables have very low-inductance coupling and a constant impedance up to several megahertz for exceptionally high-speed performance in motion-control systems. Twisted-pair cables also help reduce radiated and received electromagnetic interference (EMI).

Both shielded and unshielded twisted-pair cables are available. Unshielded cable is smaller, costs less, weighs less, and can bend in a smaller radius. But shielded twisted-pair cable must be used for the differential quadrature-encoder signals. Shielded twisted-pair cable provides better common-mode rejection, because the shield offers additional protection from EMI. The nonideal twists in an actual unshielded twisted-pair cable allows for a dramatic increase in EMI noise. The shield wire should be connected to the receiver's ground plane at the encoder-input connector.

The encoder's signal cable should not carry power-level signals—or any other signals for that matter. Nor should it be routed close to or parallel to other cables or conduits that carry power-level signals or other noisy signals, including 60-Hz power.

Modern, high-speed servo-control systems operate with encoders that use data rates up to several megahertz. At such high rates, the encoder-signal cable must be properly terminated with a terminating resistor or network at the receiver end. Ideally, the terminating resistor has the same value as the cable's characteristic impedance.

Because only one transmitter (encoder output) is on the RS-422 network (one transmitter and one receiver), the transmitter doesn't require a terminating resistor. However, ringing and reflections on a nonterminated receiver input can restrict the data throughput to several hundred kilobits per second. Matching the characteristic impedance of a cable to within ±20% is usually more than adequate. Figures 2 and 3 demonstrate the proper termination of encoder cables.

Recommended Reading:
Barnes, John R., Electronic System Design, Interference And Noise Control Techniques, Prentice-Hall, Englewood Cliffs, N.J., 1987.

"New RS-485 IC Increases System Reliability and Fault Detection in Motor-Control Circuits," www.maxim-ic.com.

Thomas, Sokira J., and Jaffe, Wolfgang, Brushless DC Motors, Electronic Commutation and Controls, TAB Books Inc., Blue Ridge Summit, Pa., 1990.

GLOSSARY OF TERMS

Backlash: The mechanical play between two or more adjacent gears.
Index: On a quadrature encoder, it's the output signal that provides one pulse per revolution.
Latchup: Complete failure in an IC, or momentary loss of operation.
Resolution: The number of bits in an output signal, or for quadrature encoders, the number of cycles per revolution.