Technical Review
(Issue 4, 2005)
A Condensed Guide to Automation Control System Specification, Design
and Installation
Part 2: Control Device Specification
by Tom Elavsky,
AutomationDirect
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In Part 1, Winter 2005 Issue 3, we covered some safety aspects involved in the use of automated control systems and discussed some tips used in identifying which
manufacturing equipment and processes can be automated.
In Part 2, we will cover how to specify the various devices required for controlling the equipment in an automated system. Your specifications need to include not only the "controlling" devices for your application, but also items such as the housing or enclosure for the devices, the type of wire required to meet the various codes, agency approvals required for safety and insurance purposes, environmental conditions, etc.
As stated in Part 1, special expertise is generally required to design, wire, install, and operate industrial automation control systems. Persons without such expertise or guidance should not attempt to design control systems, but should consider seeking the services of a qualified system integrator. Control systems can fail and cause serious injury to personnel or damage to equipment. The information in this series of articles is provided "as is" without a guarantee of any kind.
With that said, the first skill we need to develop in this effort will be the gathering of all the equipment parameters and specifications needed to specify the devices required to control the equipment. We need to be the proverbial detective who would ask questions such as;
• What is the operating voltage?
• What is the power rating?
• How much current does it draw?
• What is the operating temperature range?
• What is the relative humidity range?
• What are the mounting dimensions?
• What are the minimum mounting clearances?
• What is the duty cycle?
• How will the system be used?
• Who will be using the control system?
Control System Devices
The devices you need to specify in your control system will generally fall into one of three categories; input devices, output devices and the processing unit.
Input devices
Input devices are used to sense a condition, detect movement or position, indicate a limit or set point has been reached, sense intervention by an operator, detect an alarm, etc. Typical input devices may include limit switches, photoelectric sensors, pushbuttons, proximity sensors, an operator interface, etc. These input signals are generally in an ON or OFF state. We can look at an input from a device, such as a photoelectric sensor used to detect an obstruction, and state that when the sensor sees the obstruction, the sensor is ON; in other words we have a true condition. When the sensor is not obstructed, then the input is OFF; or we can say the condition is false. These types of signals are called discrete signals, meaning they are always one of two states; ON or OFF. They can be wired into a PLC input module and the PLC can be programmed to use the status of the signals to execute the logic to control the automated system. Or these same signals can be used in a "relay logic" system, where control relays are hardwired to create the system logic.
Of importance to our specifications are the ratings of these input devices:
• What is the operating voltage; 24, 120 or 240 Volt AC or DC?
• If they are rated for DC, are they sinking or sourcing?
• What distances can they sense?
• How much force can be applied to the actuator?
• How much current do they require?
Figure 1
As a note, most PLC DC input modules can be configured as sinking or sourcing 1.
Also keep in mind that when selecting a sensor device, such as to detect the presence of product or sense the end of travel for a mechanism, it is very important to consider the environment in which the sensor will operate. This should not only include temperature and humidity ranges, but in some cases, indoor or outdoor use, altitude, ability for the sensor to be washed down, etc. Photoelectric sensors are sensitive to the atmosphere in which they can efficiently work. If there is a lot of dust, dirt and/or mist in the air, then the optics can easily become dirty or coated, reducing their sensitivity and operating distance.
Output devices
Output devices are used to control actions such as motion, start/stop of equipment like conveyors and pumps, on/off control of valves, operator alerts/prompts, status indications, etc. Typical output devices include relays, motor starters, pilot lights, operator interface graphics and numeric display, etc. These output signals, like input signals, are also discrete; either ON or OFF. The signals can be wired from a PLC output module to control the devices, such as starting and stopping motors, energizing a valve to control water flow, illuminating a pilot light to alert an operator to a condition such as "Bin Full", etc. Output signals can also be wired directly to a controlling device using hardwired relay logic.
Of importance to our specifications are the ratings of these output devices:
• What is the operating voltage; 24, 120 or 240 Volt AC or DC?
• If they are rated for DC, are they sinking or sourcing?
• What is the current draw?
• What is the duty cycle?
• What is the operating temperature range?
• What are the mounting dimensions?
For example, if our process uses a solenoid valve to control water flow to a wash station, we would need to know the operating voltage of the valve and how much current it draws. We also need to know not only the on-state current draw of a valve, but also the inrush current, so that we can properly specify a PLC output module or a control relay. Although a valve may be rated to draw 250 mA continuous current, it may have an inrush of 800 mA when first energized. If an output module has eight output points and each point is rated for 1 amp continuous duty, after thermal considerations, the entire output module has a total rating of 6 amps and therefore has a common fuse rated at 6 amps. If we had solenoid valves connected to all eight output points and our program called for them all to energize at the same time, the total inrush current would be 8 times 800mA or 6.4 amps, and most likely would blow the fuse. The solution could be to select an output module with a higher current rating or to use the ladder program to sequence the valves, preventing them all from being energized at the same time. Another option is to split up the valves between several output modules, using the remaining points to power smaller loads such as pilot lights. Certain output types may have derating curves depending on the ambient temperature and the number of outputs energized. Keep in mind that DC output modules can be sinking or sourcing type.
The processing unit
Figure 2
All control systems can typically be defined as having inputs, outputs and some form of decision making going on in between so that the outputs are controlled based on the status of the inputs. This brings us to our third category, the "decision making" element. This element can be performed by a PLC, where we have inputs, outputs and a central processing unit (CPU) that uses ladder logic programming to make decisions based on input status and the logical conditions in the program (see figure 3). A similar device that can be looked at in the same manner is a personal computer (PC). The keyboard, mouse, scanner, etc. would serve as input devices and the monitor, printer, speakers, etc. would be the output devices. The microprocessor used on the motherboard, along with its memory, the operating system, and the application program would serve as the decision making element. As a matter of fact, PCs are used in some automated control systems as the decision making element, together with industrial input and output (I/O) modules. These PC-based systems rely on the communication ports or Ethernet connections to monitor and control the I/O. The application software typically allows a programmer to develop a graphical interface that gives an operator interaction with the equipment or process. With some research and experience, you will learn how to determine how much "decision making" ability your control system requires. Cost restraints may require you to compare implementing the control system with either a PLC, PC-based control, or simple hardwired relay logic. But don't forget the fact that a PLC or PC-based control system allows easier changes and future expansion2.
Figure 3
Other device types
Another area of inputs and outputs involve the use of analog signals in a control system. Analog signals are variable and can represent a range of values. As a quick example, we may want to monitor the level of a liquid in a tank that is 100 feet tall. We can use a sensor that will produce a signal that is represented by a voltage range of 0 to 10 volts DC, with 0 feet being equal to 0 VDC and 100 feet being equal to 10 VDC. Analog signals are typically linear, so a 5 VDC signal would tell us the tank level is at 50 feet. The analog signal could be wired into a PLC analog input module, and in the ladder program we could compare the actual level to a set point and produce a discrete signal that would cause an output point to start a pump to raise or lower the level.
Inductive devices
When selecting a device to control a prime mover, such as an industrial motor to power a conveyor, or a valve to control a hydraulic cylinder, you will need to determine the ratings of the equipment:
• What is the operating voltage?
• What is the maximum current draw?
• What type of environment is it being used in?
An industrial induction motor may have ratings such as 230/460 VAC, 3-phase, 1725 RPM, a FLA (full load ampere) of 10.5 amps at 460 VAC, etc. This information can be obtained from the manufacturer's catalog or directly from the motor nameplate. In the case of a motor, you will need the ratings to choose the motor starter or possibly a
variable frequency drive for either start/stop control or speed control of the motor.
Other considerations
There are other points to consider in the specification of devices being used in your automated control system – duty cycle, surge suppression, enclosure type, heating and cooling, power, environment, static electricity and agency approvals.
Duty Cycle
When using a solenoid valve, you will want to know its operating voltage, nominal current draw and current inrush to help select the type of output device required to control its operation. It is also important to have an understanding of the solenoid valve's duty cycle (time on vs. time off). We would not want to operate a solenoid valve rated at 50% duty cycle in a continuous mode with an on time of 10 seconds and an off time of only two seconds. The short off time would not allow for the solenoid to properly cool down.
Surge Suppression
Solenoid valves, motor starters, etc. make use of an inductive coil for their operation and the coil can produce high voltage spikes that can damage output devices and nearby electronic equipment. It is always recommended to use some form of surge suppression to eliminate these voltage spikes3.
Enclosures
Selecting a proper enclosure is important to ensure safe and proper operation of your equipment. The minimum considerations for enclosures should include:
• Conformance to electrical standards (Reference NEC)4
• Protection from the elements in an industrial
environment (Reference NEMA)4
• Common ground reference (Reference NEC)
• Access to the equipment (Reference OSHA)5
• Security or restricted access (Reference OSHA)
• Sufficient space for proper installation and
maintenance of equipment
Heating/Cooling
Ensure that the devices used in your control system aren't subject to overheating, or if installed in a colder climate, the devices aren't being used below the listed low temperature operating range. Your control system, because of its physical location, may require you to have both a cooling system, such as an A/C unit, and a small heating unit as part of the same enclosure. This will ensure the devices are always operating within their temperature specifications. Basic thermal management is not difficult for most automated control systems. Investing a little thought during the specification stage can save you a great deal of redesign down the road.
DC Power
If using DC voltage from a power supply in your control system, consider using a power supply rated for at least twice the calculated load. This should satisfy one of the requirements if you need to have your control system UL 508 approved and will allow the power supply to operate at a lower temperature, thus increasing its life.
Environmental Specifications
The following table is an example of NEMA's common environmental specifications that generally apply to automation equipment. IEC also has a list of common environmental specification designations for enclosures and equipment4.
Static Electricity
Most equipment and devices will operate down to 5% relative humidity. However, static electricity problems occur much more frequently at humidity levels below 30%. Make sure you take adequate precautions when you touch the equipment. Consider using ground straps, anti-static floor coverings, etc. if you use the equipment in low-
humidity environments.
Agency Approvals
Some applications require agency approvals for particular components. Some of these required approvals are:
• UL (Underwriters' Laboratories, Inc.) 6
• CUL (Canadian Underwriters' Laboratories, Inc.)
• CE (European Economic Union)
The requirements for any of these agency approvals need to be part of your specification and will determine the selection of most of your controlling devices.
Enclosure Lighting and Convenience Receptacle
It is always a good idea to include interior lighting for your control system enclosure or cabinet to help during routine maintenance to the control system. Provide a convenience receptacle inside the control system enclosure to supply power to test equipment, calibration equipment, etc.
Product Selection
Suppliers’ literature and Web sites are an excellent resource for evalulating product specifications. For industrial control product selection information for AutomationDirect products, refer to:
http://www.automationdirect.com/static/specs/productselection.html
Watch for Part 3, Automation Control System Design and Build, in our next issue.
Footnotes:
1 For "Sinking and Sourcing Concepts" refer to
http://www.automationdirect.com/static/specs/sinksrc.pdf
2 For "Considerations for Choosing a PLC" refer to
http://support.automationdirect.com/docs/worksheet_guide-lines.html
3 For more information on surge suppression for outputs, see chapter 2 of any PLC user manual, available on our Web site.
4 Information for the National Electrical Manufacturer's Association (NEMA) can be found at their Web site at: http://www.nema.org. NEMA is also being harmonized with the International Electrotechnical Commission (IEC) (Web site: www.iec.ch/) and other European standards. Additional information can be found at Global Engineering Documents' Web site at: www.global.ihs.com. Global Engineering Documents is also the source for obtaining NEMA, IEC and CE documents.
5 Additional information can be found on OSHA's Web site at:
http://www.osha.gov/SLTC/controlhazardousenergy/
6 Additional UL information can be found at:
http://www.ul.com/controlequipment/devices.html
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