Installing machine controls on the actual machine is not a new idea. Mechanical control systems, of course, were perforce installed right on the machines they controlled. They had to be, because controls were operated through mechanical linkages. Manual controls for electrically powered machinery continued to be installed on industrial machines to give human operators easy access. There was seldom a reason to do otherwise.
As automatic industrial machinery, especially CNC metal-forming equipment, began to appear in the 1960s, it became advisable to move these controls away from the dangerous environs immediately adjacent to cutting heads and other rapidly moving machine components. Later, as electronic controls became more sophisticated, machine designers continued the trend toward moving controls off machine into separate cabinets to protect them from high/low temperature extremes, dust, fluid spills, etc.
“On-machine controls are a truly unique segment of the controls market,” says Graham Harris, president of Beckhoff Automation. “The idea of moving control devices directly onto machines to reduce or eliminate cabinets and their associated wiring has been around for years. However, technical limitations in designing all components for IP65/67 enclosures have minimized the number of implementations. Devices such as power supplies, modems and safety relays are not commonly available in such device housings.”
The IP code as defined in the IEC 60529 standard classifies the level of protection against the intrusion of solid objects, dust, accidental contact, and water. IP65 is defined as “dust tight,” meaning there is no way for dust to get in, there is complete protection against contact, and water jets. IP67 is similar, but gives protection against immersion in water to a depth of 1 m.
Today, Beckhoff and other automation vendors have begun to reverse the trend away from on-machine controls by introducing ruggedized industrial electronics packages that make it possible to move controls back onto machines. System integrators are taking advantage of this fact to shorten cable runs and clean up spaghetti-like wiring interconnects.
For example, the ET 200pro CPU, available from Siemens Energy & Automation, is a machine mountable controller with IP65/67 rating, allowing it to be mounted in tough washdown areas without need of a cabinet. It also operates in an extended temperature range of -25
On-motor drive mounting is something Bosch Rexroth has promoted for some time with the company’s IndraDrive Mi integrated servomotor/drive product. Careful thermal management has made it possible to mount a fully-enclosed drive-electronics package directly on the motor case. Heat generated in the drive flows by conduction to the motor case as well as dissipating to the atmosphere through a heat sink on the drive’s top.
Mounting the drive to the motor, which of course is part of the machine, greatly reduces cabling. In addition, IndraDrive Mi allows daisy chaining the power and control conductors from motor to motor through water-resistant connectors, saving copper, complexity, and making the package resistant to dust and water.
|Mounting control electronics directly onto machine structures can expose them to shocks and vibrations.|
Pros and cons
“The main difference between the machine-mountable controllers and cabinet-mounted controllers is their environmental ratings,” says Jim Wilmot, PLC marketing manager at Siemens Energy & Automation. “[Machine-mountable units] need additional protection from water sprays and increased temperature ranges. In addition, if these machine-mounted controllers are part of a large distributed control scheme, they also need advanced networking capabilities to allow simple and fast connectivity.”
Mike Massie, director of drive-product marketing at Yaskawa Electric America, lists the main advantages of on-machine controls as: less wiring, lower cost, fewer issues with EMI/RFI because of shorter wiring distances, and easier equipment maintenance.
“One of the biggest ‘pros’ is the money savings,” says Wilmot. “[Mounting controls on the machine] removes the necessity for cabinets.”
“The most visible on-machine application has been in the area of distributed I/O,” Beckhoff’s Harris points out. Larger machines achieve wiring savings by placing the I/O close to the sensors and actuators, and running a network cables back to the central controller.
“In the traditional PLC market,” he continues, “the initial movement into the on-machine control arena has been with small controllers with a fixed amount of I/O, or a well-thought-out expansion capability and a low power demand. However, the types of machines that normally need a small control system are also the ones that have less of a need to reduce wiring.”
Wilmot says: “IP65/67 rated controls can be operated in tough, washdown environments without the need for a cabinet. Many also operate over an extended temperature range. These controllers can often be easily linked via Ethernet, so the constraint of having all controllers located in one place is removed.”
Removing the need to mount control electronics in a separate cabinet also makes engineering easier, since it allows more flexibility in system layout. For larger control systems where you have lots of separate physical areas in the plant being controlled, being able to use distributed machine-mounted controllers provides the ability to configure, commission, diagnose, and operate different areas of the plant independently. This means faster engineering, commissioning, and startup.
Yaskawa’s Massie points out that higher vibration, shock, and heat in the on-machine environment can lead to lower reliability. Just IP65/67 rating may not be enough. The enclosure may need to be mounted with shock and vibration damping attachments.
“One of the ‘cons’ to mounting control electronics [on machines] is that, even though you can mount the equipment without a cabinet, you still need to be careful that it is not mounted in a location where it will get damaged,” Wilmot points out. Carelessly driven fork trucks and employees using controllers as ladder rungs can easily damage field-mounted controllers.
“You can always build protection around field-mounted devices,” he continues, “but this can eat up the savings gained.”
Another “con” would be that if a controller is a small part of a large control system involving cabinet-mounted controllers, its different form factor would make it an extra spare part. This mitigates the capital savings as well as reducing the advantage to maintenance and repair operations.
“In the case of the ET 200pro CPU,” Siemens’ Wilmot says, “servicing in the field is made easy by having an RJ45 Ethernet connection available to locally connect up a programming device in the field.”
Servicing the software may become as simple as removing the jack’s waterproof cover, and plugging in a programming unit. If the field mounted controller connects via local area network (LAN) to a programming unit in a clean computer area, then you never have to go into the field.
“Because of the increased vibration and heat on-machine control electronics have to deal with, they may prove less reliable,” according to Massie, “but typically they are easier to service.”
Another important trend making on-machine controls more desirable from a maintenance standpoint is better and more rugged user interface components. Improved switch, display, and packaging technology has made it possible to put fully functional displays on light weight, compact panels or pendants right at the machine for service or operator use. Bosch Rexroth’s IndraControl VCH, for example, includes a three-step enabling button and 3-circuit stop button as well as 3.8 in. graphics display.
These technological changes have fueled the movement back to on-machine controls. Does this mean that cabinets full of PLCs and motor drives will go the way of the dinosaurs? Perhaps, but it is always dangerous to predict the demise of any form factor. Let’s just say that from now on, we will have options we never had before.
|C.G. Masi is a senior editor with Control Engineering. Contact him at email@example.com .|