Drives and Controls: Future Trends in Sensor Technology

1 Smart Sensors

Smart sensors may be used in temperature logging, diagnostics, failure detection, and fault isolation. Also, they may be used in device parameter storage, allowing a drive to interrogate the motor, determine what type it's, and set itself up automatically in plug-and-play drives, databus interfaces, and local interloop control systems, where a localized velocity loop could be implemented using a very high sample rate, allowing the position loop to be closed over a slower network bus. Full local control allows some companies to put the entire servo inside the motor. Databus interfaces need only provide the set point.

2 Redundant Sensors

Some technologies may benefit from massively redundant sensors and activators, such as robotics, teleoperated devices, and so forth. There are Advanced Research Project Administration (ARPA) programs, such as the microelectromechanical systems (MEMS) project, where the development of distributed arrays of miniature low-cost sensor elements is being promoted. One example of this program is a 13-bit single-turn absolute encoder the size of a dime.

3 Distributed Sensor Networks-Sensor Databus Systems

One less fanciful area with what appear to be near-term opportunities is in higher-end sensors capable of interfacing with factory automation networks. The only problem is that there are so many candidates that a manufacturer will never build the right one. The problem is that every one has a better idea. The military has developed standards: MIL-STD-1553, MIL-STD-1760, and ARINC 429. In the commercial sector, there are many standards that are in wide use at the hardware level, such as RS-485, but no real application interface standards. As an example of current and evolving approaches, the following are examples of three bus systems that are in use today, and one that's in development.

Synchronous Serial Interface (SSI). This is a simple, dedicated bus structure which allows a single device to be connected to a remote processor using only four wires. SSI is used as a device-level bus to connect multiturn encoders to controls. It is a single-drop clocked serial bus using RS-422 or RS-485 media for data transmission.

The controller sources the clock, and the encoder returns data at the appropriate bit times. The data format consists of 25-bit words. The first 12 are dedicated to turn counting, the next 12 to the position within the turn or measuring steps. The last bit's always 0. This allows a simple serial-in/parallel-out shift-register implementation to be used.

Some companies are putting the entire servo inside the motor for full local control, and the interface need only provide the set point. This is an efficient, low-overhead system allowing economical interconnection between two devices to be made. It does not allow for expansion or multidrop applications.

SERCOS. This is a network that has grown out of the machine-tool industry but is being used more and more for general-purpose applications. Started in Germany around 1986, it became a European standard in 1995 (IEC 1491). SERCOS is now being championed in the United States by the SERCOS North America group.

[Membership information is available by calling (800) 5-SERCOS.] Worldwide, there are more than 25 manufacturers supplying SERCOS-compatible controls and drives, with Indramat being one of the large companies involved. In the United States , Pacific Scientific has committed to SERCOS and has developed its product line and software products around SERCOS device interfaces.

The SERCOS architecture is flexible and allows the combination of many types of devices on the same backbone. The topology and technology are sound and make good use of current technology. A master/slave architecture is used with a ring topology and time-division multiplexing for data transmission. The bus media is fiberoptic, and can transmit at up to 4Mbit/s. The similarity to MIL-STD-1553 is strong, and the reliability of that bus has been proven in thousands of applications. The SERCOS technology exceeds MIL-STD-1553 in a few important areas, such as datarate and the number of slaves allowed. The fiberoptic media is also a bonus for noise immunity, and it reduces cost over the MIL-STD-1553 hardware.

SERCOS can address up to 254 slaves per ring. A novel feature is the ability to exactly synchronize all devices on the network through what is called the master synchronization telegram. This is very advantageous in control applications where harmonics could result from slightly out-of-phase actions by distributed actuators.

FieldBus. FieldBus is almost a generic name for a factory-floor databus. The Field-Bus Foundation has been developing the specifications for what was to have been the definitive bus implementation. Called the Foundation FieldBus, its only problem is that the specification has been in development for years, and it's not completed yet. As a result, several manufacturers have taken the lead and developed their own implementations. This has caused many versions of "FieldBus" systems to come into existence. The parameters that vary include the number of connected devices, data packet size, connectors, certification requirements, datarate, bus length, and implementation cost. The specification of a FieldBus usually follows the definitions set forth by the Open Systems Interconnection (OSI) terminology, and will generally apply to the following layers:

Layer 7 Application layer Management functions Layer 2 Data-link layer Definition of dataframe format Layer 1 Physical layer Circuit for bus electrical protocol Layer 0 Media Cable (not a real OSI layer) Layer 0 varies significantly between bus systems. It can be as simple as two wires in flat cable, as used in AS-1, or it can be a fiberoptic cable, as in SERCOS. As an example of how FieldBus networks are implemented, the following gives the details of three fairly popular examples.

Process FieldBus (PROFIBUS) DP. There are actually three variations of this bus:

PROFIBUS FMS, for general-purpose automation PROFIBUS DP, for high-speed data transfer to decentralized peripherals PROFIBUS PA, for process control with support for intrinsically safe operation PROFIBUS is used by approximately 40 percent of the German FieldBus market, and certification to market it's required. It is used by Siemens, AEG, and Mitsubishi Electric, among others. This bus differs from SERCOS in both topology and architecture.

The PROFIBUS physical architecture allows for multimaster/multislave systems, and is a linear bus topology. A multimaster system can be of benefit in that it will allow the system to withstand single-point failures more readily. The physical bus can be twisted-pair based upon the RS-485 standard or fiberoptic cable, allowing for up to 12Mbit/s datarates with 32 terminals. The physical-layer implementation has been developed by Siemens in a single-chip ASIC.

INTERBUS-S. This bus was developed by Phoenix Contact as a sensor/actuator bus, and it's used in the automotive industry. This is a single-master/multislave ring bus, similar to that of SERCOS. The bus is implemented as a twisted-pair wire using the RS-485 protocol, and it transmits at up to 2Mbit/s with a bus length of up to 400 m. INTERBUS-S can accommodate more slaves than PROFIBUS, and it allows up to 64 stations. The physical-layer implementation has been developed by and is available from Phoenix .

Controller Area Network (CAN). This bus was developed by Bosch and Intel for automotive real-time applications. It is a very low cost and reliable system, with many features built in to support redundancy and failure detection or isolation. It is currently used and supported by Honeywell (ADA Smart Distributed System), Allen-Bradley (DeviceNet), and Selectron (SELECAN).

This is an example of a multimaster bus in which the masters all arbitrate for control based on priorities assigned in their object lists. The physical media uses a twisted-pair cable which will transmit data at up to 1Mbit/s over a 40-m cable. Up to 110 stations can be on the bus at one time. It is a linear bus with terminations at both ends. Interface hardware is marketed by Intel, Motorola, Phillips, National, Siemens, and NEC. This bus may become the dominant embodiment of FieldBus as time goes on. Many manufacturers are producing products with interfaces to the DeviceNet protocol in particular. Sensor manufacturers are getting onto the bus, too. Leine and Linde, a Swedish encoder manufacturer, is providing multiturn absolute encoders which interface to the CAN bus.

Smart Transducer Interface for Sensors and Actuators. The National Institute of Standards and Technology (NIST) is actively developing a standard for smart transducer interfaces via its TC-9 committee. Using the fact that more and more sensors are utilizing built-in intelligence via micro-processors, NIST wants to leverage this capability into provision of a unified bus interface system.

The standard for this new interface system is being encapsulated in IEEE Standards 1451.1 and 1451.2, which are intended to define an interface that transducer manufacturers can build. The interface system is built upon three components: the smart transducer interface module (STIM), the network-capable application processor (NCAP), and the transducer independent interface (TII). The STIM may handle up to 255 transducers of seven defined types. The different transducer types are managed using transducer electronic data sheets (TEDS), which appear to be similar to the INTERBUS-S device profiles. The basic idea is that the user network communicates to the NCAP, which routes data to the STIM via the TII.

The network communications and NCAP are defined in IEEE 1451.1.The STIM, TEDS, and TII are defined in IEEE 1451.2.The network aspects of these systems are being heavily emphasized, and go beyond the single factory implementation. In May 1997, NIST demonstrated real-time control of remote sensors over the Internet at Sensors Expo. It may soon be possible for engineers to monitor and control plant processes anywhere in the world from any location having Internet access.