The interface, or communications network, between a programmable logic controller (PLC) and motors and drives is critical to precise manufacturing control.
There are several ways to interface PLCs with these devices, including the original methods, which are still in widespread use today. A PLC may send a drive speed and torque references with analog signals, either 0-to-10 Vdc or 4-to- 20 mA. Commands such as Run, Stop, Jog, Reverse, and Fault Reset transmit as discrete 24/48 Vdc or 120 Vac signals, or as contact closures, depending on the distances and noise levels involved.
Similarly, speed and load feedback go from the drive to the PLC through similar analog signals. If the drive has Run and Fault relay contacts, status goes to the PLC as discrete “on” or “off” inputs. Additional contacts or solid-state “open-collector” type discrete outputs contain coded information the PLC uses to decipher faults, then to send diagnostic information to an operator interface.
With this traditional approach, however, interconnecting drives and PLCs with full control and diagnostic capability requires:
• A great deal of discrete I/O wiring.
• Time to verify the wiring was done properly.
• Time for changes and maintenance.
• Additional steps to protect analog signals from noise.
Distributed digital I/O networks can eliminate these problems of discrete point-to-point wiring. There are many types of these systems. You can choose from low-level networks like RS-232 or RS-485, high-level Ethernet and Arcnetbased systems, or distributed I/O midrange networks also known as fieldbuses.
Low-level networks. Some low-level digital communication systems control and monitor HVAC, lighting, fire, and alarm systems for commercial buildings. Not all of these networks, though, can assure a minimum data throughput rate (deterministic). Devices will transmit data at certain intervals, but they may have to repeat that transmission if more than one device sends data at the same time (a network “collision”). These nondeterministic systems can have network response times in the hundreds of milliseconds, too long for some drive-PLC applications.
High-level networks. Other digital communication systems handle highperformance applications like gage control or tension control in steel making. These high-precision closed-loop applications require response times of 20 msec or less. Such fast response is needed because time delays can look like a phase-shift to the closed-loop system, resulting in instability. In a gage application, for example, this could lead to excessive ripple in the sheet metal. In an tension application, it could lead to excessive stretching or even breakage of the material. (For more on highspeed networks, see “More Choices Link Motors and Drives to Controls,” in this issue.)
Mid-level networks. Fieldbus networks (not a reference to the Fieldbus specification sponsored by the Fieldbus Foundation) handle a broad range of applications requiring response times in the 50-to-100-msec range. Many PLC manufacturers offer these networks. Industries that use them include food and beverage, pharmaceuticals, rubber, and the auxiliary equipment in metal and paper mills, like mixers, conveyors and extruders. Although some of these applications may require speed and torque performance available only with dc or with ac vector drives, general-purpose open-loop ac drives fill most requirements.
Each of these categories of networks has the computer hardware and software components necessary to put data into an appropriate digital format, send it out on one or several wires to a particular digital address on the wire, receive return data, and decode them.
Benefits of fieldbus networks
Networks offer more than a reduction in the amount of drive-to-PLC wiring. Because they send and receive digital data, they are more immune to electrical noise. They also improve system reliability because there are fewer wires and interface circuits. Thus, there is less chance that something will go wrong.
Most of these networks also use errordetection schemes to ensure data transmission integrity. These schemes range from parity-checking and checksum methods, to more complex schemes using redundancy or 2-out-of-3 voting.
In parity, the PLC expects to receive data with one bit set as either even or odd (parity). Failure of a message to match the parity is indication of a transmission error. Checksum is a more accurate method of error-detection. The binary data in a message are added to determine a finite number. This number is checked at send and receive points. If any bit in the string changes, the total changes. For redundancy, a message is sent more than once, and must match previous transmissions. In 2-out-of-3 voting, a message is sent three times, two of which must match. This last method eliminates many of the nuisance shutdowns you can get with some transmission checking schemes.
Fieldbus communications for drives also provide long-term repeatability and improved diagnostic capabilities. All operational amplifiers (op amps), digitalto- analog converters (DACs) and analogto- digital converters (ADCs) have time and temperature coefficients. In general, stable characteristics are found in the more expensive versions of these devices. A sure way to avoid concern over device “instablity,” is to send “analog” references, like speed and torque commands, across a serial link. This can provide drift-free service regardless of age or ambient conditions.
A fieldbus-type network lets you use a hand-held monitoring device to verify the network wiring between motor and drive. The user can force the drive to run at a desired reference, isolating the problem in minutes at the drive-motor level or in the PLC application program. Hand-held monitors read inputs from any device on the network, permitting quick isolation of problems.
These networks also give users access to various drive diagnostic codes stored in the inverter (drive). Without this type of network, these codes can only be accessed by a technician who physically has to walk to the drive and interrogate it with a keypad display.
There are also ways to decode this information from the on/off status of the drives discrete outputs. A fieldbus network, though, lets you feed this diagnostic data automatically into the PLC programmed to take the necessary action. And these diagnostics usually come with no additional I/O wiring. Table 1 lists common adjustable-speed drive parameters available for exchange through such a fieldbus interface. Drive bandwidth (response) is not affected, because diagnostics operate when a drive trips or faults.
PLCs also benefit
Fieldbus interfaces, known as distributed I/O networks in the PLC world, offer benefits to PLCs too. A major benefit is that the networks let engineers configure and communicate across a network in a straightforward process.
In one such system, a proprietary menu-driven, DOS-based configuration software package lets the user select the parameters to transmit across the network and specify the exact PLC memory locations to read and write those parameters.
Thus, you can consistently configure drives and other I/O devices. Once done, network communication occurs automatically, insulating the user from the nutsand- bolts of network operation.
If several drives must network to a PLC, each is assigned its own digital address. When the PLC sends data to a specific drive, it precedes the data with the correct address, which causes only the specified drive to listen to the remainder of the message.
Another benefit is the ability to send data over long distances with high transmission rates. In one system, the distance ranges from 2,000 ft at the fastest transmission rate (150 Kbaud), to 7,500 ft at a reduced rate. Previous-generation drive networks, based on an RS-232 (singleended) or RS-422/485 (differential) platform, were limited to several hundred feet and baud rates of 50 Kbaud or less.
For even longer distances, modem, fiberoptic transducers and satellite uplink converters are available, making transmission distances almost unlimited. Of course, these networks use circuits and cables with excellent noise-rejection features.
The distributed I/O, or fieldbus, networks can be mixed with other signaling systems and 120 Vac circuits without additional shielding or conduits. (Conservative wiring practices, and local and national codes, require physical separation between control circuits and power distribution or motor wiring. Refer to sections 430 and 725 of the National Electric Code, NFPA 70-1993.)
Tips and potential pitfalls
Fieldbus networks have varying degrees of performance. The first criterion to evaluate is bandwidth. Overall bandwidth is determined by several items, including baud rate, number of drops on the network, and the level of overhead associated with error-detection and correction circuitry. System bandwidth requirements vary widely and depend on the application.
Adjustable-speed drive networks connected with a distributed I/O network may experience single-point failure. Most network design permits device connection and disconnection without disturbing the operation of the other devices on the network. A single failure in the bus controller unit or at particular points in the cabling, though, could bring down the entire system.
Fieldbus networks offer different degrees of protection in case of network failure. Equipment can be pre-programmed to revert to “limp along” states automatically, or shut down completely. For example, say that the cooling fans for a drive run at variable speeds for energy efficiency. If a network fails, you might want the cooling fans to run automatically or to go to full-speed operation. This will consume more energy until the problem is solved, but it will prevent damage from overheating.
In industries like refining and chemical manufacturing, network failure can mean danger because of the possible volatile compounds processed. More sophisticated protection setups include dual bus controllers, dual networks, or fully triplicated TMR functionality, along with the guaranteed fault states.
Table rolls in a steel mill benefit from adjustable-speed drives connected to a PLC with a fieldbus network. Typically, a mill will have a finishing or roughing stand driven by several large, high-performance drives with “tables” that extend for hundreds of feet in either direction. These tables accelerate and decelerate the metal slabs to or from the roughing stand.
The tables have tens or even hundreds of drives and motors, all generally running at the same speed. In some instances, however, the speeds are ratioed to account for differences in roll diameters or gearing.
You can send the same 0-to-10-Vdc reference signal to several drives. But this is not recommended without also installing expensive analog isolators to avoid tying the drives’ circuit commons together. Without isolators, there is a potential for noise pickup, especially if the drives are a significant distance apart.
It is preferable to use 4-to-20-mA signals for references in noisy environments. Yet, while more than one drive can be connected in series, generally not more than three or four are connected because of the limited amount of forcing voltage available in the driver circuit.
A fieldbus network can supply 30 drives, using a daisy-chained shielded twisted-pair. It can send data over a distance of thousands of feet. The network also provides centralized monitoring and fault diagnosis for drives that may be hundreds or thousands of feet apart.
Another application that benefits from drives linked through fieldbuses to PLCs is a gas or oil pipeline. These applications generally have large pump drives and other accessory drives located in pumping stations tens or even hundreds of miles apart. Because these stations are often unattended, they are controlled and monitored from a central facility that can transmit and receive data through a SCADA interface or over phone lines through a modem.
Many fieldbus or distributed I/O networks transmit and receive data via modem, as do personal computers. Meanwhile, many pipeline operators have their own microwave-based or satellite uplinkdownlink communications systems for long-distance communications with remote stations. If the network’s modem connects to a long-distance communications system, users can monitor and control the status of any piece of equipment connected to the network from a central facility.
With drives at remote stations interfaced to a network, an operator would not have to wire them individually.
There are other applications. One of the key elements to evaluate is the amount of data to exchange between the controller and the drives. As the information load increases, the choice swings more toward a distributed I/O solution, rather than discrete I/O.
Ken Hooker is a design engineer with GE Motors & Industrial Systems in Fort Wayne, Ind. Brent Evans is an application Engineer at GE Fanuc Automation in Pittsburgh, Pa.