Communications interfaces provide the all-important link between sensors (the “nerves” of a control system) and the controllers (the “brain”). An impressive variety of communications technologies has been introduced to provide this link, usually with features and capabilities tailored to a certain type of control system. Let’s look at some widely used communications technologies for motion control.
Special Solutions for Special Cases
For some sensors that provide feedback for motion control, the measurement technology will dictate the communications interface. Incremental encoders deliver a continuous stream of signal pulses — one for each time the encoder’s shaft rotates a certain amount. They excel at speed control, as the controller can accurately determine the rate of rotation from the interval between pulses.
Many incremental encoders transmit two output signals, termed A and B, with a 90° phase difference, enabling the controller to determine the direction of rotation. Some also output a Z signal once every rotation at a defined rotation angle. This provides a precise reference point.
The connection between an incremental encoder and its controller must be point-to-point, with a dedicated cable connecting each encoder to its controller. Communications is usually based on a differential signal transmitted over twisted pair wiring, with the number of conductor pairs in the cable depending on the number of signals (A, B, and Z).
The output drivers in the encoders must be compatible with the interface on the controller — Push-Pull (HTL) or RS-422 (TTL) output drivers are commonly used. These standards specify signal voltage.
Absolute Position Sensors
The remainder of this article will focus on absolute position sensors such as encoders and inclinometers. These devices report a measured position at a specific instant in time, either as a voltage/current level (analog encoders) or by transmitting a digital word or “telegram” (digital encoders). These devices are ideal for position control applications.
Analog sensors have a long history, with earlier control systems using potentiometers (variable resistors). More recently, digital sensors with built-in D/A converters have been introduced. These are available with either voltage (e.g., 0-5 V) or current (e.g., 0-20 mA) outputs. They feature programmable D/A converters so that a predetermined range of mechanical motion (anything from a fraction of a turn to multiple revolutions) can be set to span the full system’s electrical output range (e.g., 0-5 V, 0-20 mA). This improves accuracy and resolution over the most significant range of motion. Analog sensors require point-to-point connections, often with relatively large-gauge wire to limit electrical resistance.
Absolute encoders with bit parallel interfaces report measurements as a digital word, with a separate conductor for each bit. Response is virtually instantaneous. The connection is point-to-point, typically through a ribbon-type cable. As this type of cable is relatively bulky and inflexible, bit parallel systems work best over short distances.
Digital Point-to-Point Wiring
With point-to-point wiring, a dedicated cable runs from the controller to each individual sensor. SSI (Serial Synchronous Interface) and BiSS (Bidirectional Serial Synchronous) interfaces for absolute encoders use point-to-point wiring systems. These are digital interfaces that can connect directly to PLCs or other controllers. SSI connections offer good speed (clock rates up to 2 Mhz), high resolution, flexible cabling, and reliable communication up to a few hundred meters (although baud rates are reduced for longer distances). SSI protocols provide basic error detection (broken wire, short circuit, data consistency).
BiSS is an advanced version of SSI that supports real-time communications between control devices and sensors/actuators in servomotors, robots, and other automation systems. The interface also enables the controller to set operational parameters in slave devices. There are several BiSS variants including BiSS C (continuous communications) and BiSS Line (designed for configurations that combine power delivery and data transmission in a single cable). Open-source SSI and BiSS interface standards are non-proprietary, with no-cost licenses.
SSI and BiSS communications use point-to-point connections — typically RS-422. Several devices can be daisy-chained together for more efficient cable layouts.
Fieldbus: Shared Access Wiring Systems
Point-to-point wiring works well for systems with short distances and a limited number of devices but as the number of sensors increases, wiring layouts can become unwieldy. As automated systems became more sophisticated and the number of connected devices grew, several manufacturers introduced fieldbus systems. These feature networks are based on a bus topology, with multiple devices sharing a common wiring backbone. Fieldbus systems are reliable, fast, and relatively cost effective. Applications range from conveyors and manufacturing facilities to mobile equipment, medical equipment, wind turbines, and solar panels.
Having multiple devices sharing a common communications channel can cause problems with response times — when traffic on the bus is heavy, individual sensors may have their messages delayed by an unpredictable amount of time. To get around this, most fieldbus designs allow the operator to rank devices in order of importance. This helps to ensure that critical messages get priority treatment. The physical layer of fieldbus systems is usually based on twisted pair cables (e.g., EIA-485).
Popular fieldbus standards include Controller Area Network (CAN) from Bosch, CANopen, Profibus (Process Field Bus) from Siemens, and DeviceNet from Allen-Bradley/Rockwell. DeviceNET, which is widely used in North America, combines the CAN physical layer with CIP (Common Industrial Protocol) higher-level protocols. SAE J1939, which makes use of the CAN data transport standards, is optimized for heavy vehicles.
Networks consist of physical elements (wires, connectors, and the electronic components that control signal flow) and logical elements (that address schemes, communications protocols, device profiles, etc. that are implemented in software). In the world of fieldbus, many systems use CAN standards to define the physical aspects of the networks, while higher-level protocols — such as CANopen, DeviceNet, J1939, etc. — provide end-to-end functionality.
Industrial Ethernet uses the same technical underpinnings as office Ethernet but with enhancements that make it better suited to harsh factory conditions. Industrial-grade switch units may feature rugged water- and dust-proof enclosures, while many devices use robust M12 connectors in place of more vulnerable RJ45 connectors.
There are also important upgrades to the communications protocols. Industrial applications — especially motion control — often require controls to operate in real time, with none of the unpredictable transmission delays (latency or jitter) that can occur in ordinary Ethernet networks. Industrial Ethernet systems such as Profinet IRT, EtherNet/IP, and Ethernet Powerlink address this through modified protocol stacks and special hardware that give critical messages priority access to network bandwidth. The special components needed to achieve this can add to system complexity and cost.
It should be noted that while Ethernet offers a vision of an unlimited number of devices and flexible topology options, issues of system performance — especially for real-time motion control applications — can require design compromises that reduce local traffic and provide direct communications paths for critical components.
IO-Link is a low-cost, easy-to-implement communications system designed to simplify connections between fieldbus or industrial Ethernet networks and simple sensors or actuators located on the factory floor. On one side of an IO-Link master gateway device is an interface to the plant-wide network, while the other side has multiple point-to-point connections to individual sensor devices.
The IO-Link interface for end devices is relatively simple, eliminating the need to support complex communications protocols at the sensor/actuator level. IO-Link supports a variety of data types including measurement data, device configuration instructions, and information about operating condition parameters such as temperature.
Wireless technologies enable communications with mobile machinery (e.g., autonomous guided vehicles) or equipment that must be relocated frequently. Wi-Fi (IEEE 802.11) and Bluetooth are widely used standards for short-range wireless communications. Other standards are available for longer-range communications, although these may have lower bit rates. Emerging 5G networks promise high data rates and low latency.
Wireless communications can be less reliable than wired connections in electrically noisy environments and may not be suitable for highly time-dependant feedback signals. In the case of a warehouse robot, for example, a wireless signal can be used to instruct the machine to retrieve material from a particular location. However, sensors for steering, speed control, and collision avoidance would typically be hard-wired to the control system to ensure reliable, instantaneous response.
Open Standards for Compatibility
No single vendor can supply best-inclass equipment for every part of a complex automation system and suppliers of industrial networking technologies have moved from proprietary networking systems towards supporting open (vendor-neutral) interface and networking standards. With these standards, buyers of motion control equipment can mix and match standards-certified components from different vendors with the expectation that everything will work together.
Important industry standards organizations include the Open Device Vendors Association , sponsors of DeviceNet and Ethernet/IP standards; the CAN in Automation Association , sponsors of the CANopen protocols; and Profibus Nutzerorganisation , sponsors of Profibus and Profinet interfaces.
This article was written by Christian Fell, General Manager at FRABA Inc., Hamilton, NJ. For more information, visit here .