Digital drive technology offers precision
for many demanding industrial applications
Printing presses are understandably very demanding machines with regards to motion control. The presses run at high speeds, many passes are required for full color printing, and image and print alignment have to be precise or the pages will appear blurry. According to a story by Terry Costlow in Design News, “Print Press Goes Digital,” RG Engineering, a Virgina Beach, Va.-based printing press manufacturer, recently installed a digital drive into one of the towers in its new eight-color printing machine, recognizing the performance improvements that digital control technology can offer.
For many years, RG used analog drives to meet strict specifications for speed and precision, knowing that someday a switch to digital drives would come. This changeover is a demanding test, but RG believes that customers will find it much easier to setup and use the new digital equipment, and that improvements in the digital products will be continuous. According to David Ellingsworth, engineering vice president, “With a digital drive you can use a variety of motors, and this helps in terms of inventory.”
One drive manufacturer's new digital drives rely on a software technique that lets them match the speed of an analog control system. While a digital drive can't run the loop as fast as an analog drive, which does continuous sampling, it does something no analog drive can do: speed up the feedback conversion process using an observer.
An observer is a software algorithm that runs inside the drive, alongside the drive system. It compares the output of the real system with the model system and forces the model to follow the real system. In this way, the observer can calculate what's happening in the motor as soon as current is applied, rather than waiting for the position feedback device. The algorithm comparison technique speeds up the servo loops, allowing higher servo gains and thus better performance. An observer helps the digital drive more than compensate for the sample delay.
The digital line has a number of network connections, including CANOpen, DeviceNet, and fieldbus. It's also rated from 3A to 70A, which is an important feature, given the many motor sizes used in printing equipment.
RG's experience shows that digital signaling is encroaching into measurement motion control systems that in the past were handled by analog circuit (4-20mA) signaling devices. In addition to handling more complex motion, digital control systems are generally much smaller than their analog counterparts; drives and controls are now the same size as the motors they serve. For example, a small amplifier and microprocessor controller can be attached to the back of a servomotor.
These smaller control systems can easily handle simple tasks, like adjusting the width of a conveyor belt to different sized boxes in a line picking operation. But they can also handle demanding applications, such as semiconductor manufacturing where equipment positions within plus or minus one micron, with 3 sigma repeatability, are needed.
Three data communications technologies, or protocols, are used in motion control: fieldbus (DeviceNet, Profibus-DP, and Profibus-FMA), PC bus, such as peripheral component interconnect (PCI), and Ethernet. Each has its place in industrial control (Table 1), but only Ethernet has the advantage of scalability and worldwide acceptance. It's also able to transfer both device and high-level information at relatively high speeds. Let's look at the differences among the three.
Fieldbus. Originally designed as a replacement for the 4-20mA analog control method, fieldbus is a generic term that covers many different industrial network protocols. Two of the most popular protocols are DeviceNet and Profibus, as mentioned above. Generally, fieldbus protocols originate with specific programmable logic controller (PLC) manufacturers, and their performance and hardware interfaces differ.
Typically, a PLC serves as the fieldbus master, communicating to distributed slave devices, such as industrial I/O or motion controllers. A motion controller is typically a stand-alone or bus-based slave on the fieldbus that can achieve data transfer rates from 500 kbps to 12 Mbps.
Software is a key component in the fieldbus standard, whereas hardware is the key component in the 4-20mA standard. Table 2 compares the cost and complexity of the two standards.
PC bus. PCI bus architecture offers the highest data transfer rate between peripheral devices and a PC — about 20 times greater than either Ethernet or fieldbus. In the future it will be capable of supporting 1 Gbps. However, speed isn't the only requirement for creating a good motion control system. Other important factors are flexibility, package size, PLC compatibility, long-term usefulness, and maintenance and installation issues.
A variety of controllers handle motion control today. Depending on the application, a stand-alone, bus-based, or network motion controller may be needed. However, the PCI architecture offers only a bus-based approach. In applications where the motion controller has to handle functions independent of a PC or in cases where the machine doesn't have PC control, PCI architecture doesn't work. However, most fieldbus or Ethernet controllers can operate as stand-alone devices. Consider also, that a motion controller must connect to a factory network through a network interface. Most factory networks are Ethernet-based, which further simplifies the decision.
Many users are reluctant to change from a traditional analog system to a PC-based motion control system because transition costs can be very high. Many users have already invested heavily in the older technology. Also, while technicians and control engineers usually maintain traditional systems, software-based systems need IT-trained personnel or skilled programmers. Additionally, in the event of a hardware failure, the entire host PC must be shut done to replace the device. Nevertheless, the trend toward integrating vision and motion system software can increase the use of the PCI bus among applications that need high-level precision.
Ethernet. Ethernet offers a variety of advantages for today's motion control needs. It's usually incorporated into a motion control system through a stand-alone controller connected to the PC or network using a standard Ethernet cable.
Using Ethernet TCP/IP can help eliminate the problems inherent with PCI architecture. Ethernet devices are stand-alone and reside outside the PC. This means that the device is accessible in the event of a hardware failure. Additionally, the host system doesn't have to shut done to maintain or support an Ethernet device. It's simply unplugged from the network. The last, but perhaps most important advantage of the Ethernet protocol is its inherent scalability.
Keep in mind this additional advantage of Ethernet protocol: Most supervisory control and data acquisition (SCADA) networks use TCP/IP over Ethernet as the network protocol and physical layer. Thus, a PLC must act as a gateway between the device level using a fieldbus and the SCADA level using Ethernet TCP/IP. A PLC no longer has to be used as a gateway between dissimilar networks, since both the device level and the SCADA level can be set up with Ethernet TCP/IP.
The most common Ethernet data transfer rate is 10 Mbps, although new installations are converting to Fast Ethernet, or 100 Mbps. The next generation is Gigabit.
The Open DeviceNet Vendor Association and ControlNet joint special interest group helped develop a standard specifically for industrial Ethernet. A special RJ-45 plug and outlet design with a protective bayonet style outer housing is designed for harsh industrial environments. The TIA's TR-42.9 Industrial Telecommunications Infrastructure Group is also working on the environmental requirements for this type of hardware.
With Ethernet networking already a staple in most industrial environments, the choice between protocols will most likely be easy — even in the decision to convert to digital, PC-based motor control systems.