Design Talk - Harsh Environments
An application does not have to be in the arctic or a desert waste to be considered operating in a harsh environment.
Instrumentation and Tough Environments
By Gordon Greathouse, Automation Products Group, www.apgsensors.com
Dealing with tough environments is a normal part of nearly any instrumentation installation. Decisions must be made regarding protection from outside environmental elements such as water, dust, extreme heat or cold. Questions must be answered concerning installations in corrosive environments, as well as hazardous locations.
Though the list of “tough environments” is long and lengthy, what follows is a brief discussion on some of the more common considerations when installing instrumentation in such environments.
Protection of electronics in wet/damp areas is vital to the life cycle of the instrumentation involved. Considerations include:
1. Determine whether the instrumentation and associated equipment (junction boxes etc…) is rated for use in a wet environment. What NEMA and IP ratings are needed?
2. Are the instruments sealed for protection? Make sure there will be no exposed electronics.
For example, to protect the electronics in a submersible pressure transmitter a desiccant cartridge would be attached to the vent tube in the cable which would draw out moisture that could cause a shift in the sensor output.
3. Determine that there are no static discharge issues that would damage electronics.
Dusty and dirty environments can exist in any instrumentation application. Contamination of electronics can alter or ruin the functionality of instruments.
1. Determine whether the instrumentation and associated equipment (junction boxes etc…) is rated for use in a dusty/dirty environment. What NEMA and IP ratings are needed to protect the electronics and wiring from dust ingress? If the dust is combustible, hazardous location approvals are needed.
2. If installing photoelectric, laser or ultrasonic instrumentation, be aware excessive dust will alter their performance.
3. Extreme temperatures are often associated with dusty/dirty environments. Electronics and bonding materials used in components are affected by temperature. Be aware of the operating temperature limitations of your equipment.
Instrumentation used in industry is often installed in corrosive environments. A corrosive environment may include gaseous contaminants like ammonia, hydrochloric and sulphuric acids to name a few. Dusty and humid areas can also be considered corrosive environments. Considerations include:
1. Corrosive chemicals usually attack instrumentation through plumbing leaks, fumes and vapors. Electronics and wiring may corrode and lead to instrument failure and loss of output information.
2. Exterior housing and any electronics must be protected from chemical exposure. Proper chemical compatibility with instrumentation materials is crucial.
3. There may be circumstances when a chemical attack on an instrument’s metal surface would cause contamination, such as in a clean room application. Instrumentation that is constructed from materials that are metal-free and resistant to the chemicals being used should be considered.
The National Electrical Code (NEC) defines hazardous locations as those “where fire or explosion hazards may exist due to flammable gases or vapor, flammable liquids, combustible dust or ignitable flyings.”
The NEC has defined hazardous locations into three classifications:
Class I: Presence of flammable gases or vapors.
Typical Class I locations: gasoline refineries, spray finishing areas, natural gas plants.
Class II: Presence of combustible dust.
Typical Class II locations: Grain elevators, flour and feed mills, plants that manufacture metal powders from magnesium or aluminum.
Class III: Easily-ignitable fibers or flyings.
Typical Class III locations: textile mills, plants that cut wood and create sawdust or flyings.
Things to consider when installing instrumentation in hazardous locations:
1. Make sure the instrumentation has the proper approvals (CSA, FM etc…) and is approved for use in the proper hazardous location class.
2. Determine under what kind of conditions are hazards present. Normal or abnormal conditions. This will determine whether the instrumentation requires a Division 1 or Division 2 classification.
Division 1: Hazards present under normal conditions.
Division 2: Hazards present under abnormal conditions.
Note: If you are unsure, choose Division 1
3. Does the instrumentation need to be explosion proof and installed with rigid conduit, or will IS (intrinsically safe) approvals and installation with a barrier be acceptable? Installations with a barrier require incendive wiring and are acceptable in both Division 1 and 2. Non-incendive wiring is acceptable in Division 2, but a current limiting device is still required.
This discussion only covered a few basic areas of tough environments and the issues to consider when choosing and installing instrumentation equipment. Each area is worthy of its own discussion, and this article is intended to provide starting point when considering electronic installations in these tough environments.
Confusion and Misconceptions about NFPA-79
A Few of the Fables
“AWM is strictly forbidden. If AWM is marked on the cable, I cannot use it.”
In another notable exception, it is permissible to use cable that has multiple listings and recognitions—listed type TC (Tray Cable) and AWM-recognized cable, for example. In this instance, the printed legend on AWM may be ignored in deference to the cable's listing type, TC. UL (or other agency) listed cables are required.
“The cable must be MTW.“
“The cable must be 600V.”
“The smallest conductor size allowed is 22 AWG.”
“Cords must be 600V.”
ITC is defined in NEC article 727, which classifies cable for remote instrumentation and controls in industrial environments under certain conditions. Most notably, article 727 specifies 300V insulation-rated cable, limited to applications of 150V or less with a 5-Amp maximum current. The conductor is limited to sizes not smaller than AWG 22 and not larger than AWG 12.
A Bonus: Exposed Run
Knowledge Is Power
Tough Environments Demand Tough Cables and Components
Electronic cables and connectivity components used in Ethernet networks in manufacturing, processing and utility plants, face environmental challenges unheard of in commercial office settings. On the plant floor, mission-critical communications systems are routinely exposed to dust, moisture, oil and corrosive chemicals, as well as extreme temperatures, machine vibration and, often, high levels of EMI/RFI interference.
Commercial-grade components are not tough enough to withstand these harsh conditions on a sustained basis. In fact, using commercial off-the-shelf (COTS) Ethernet products in these harsh environments poses a risk of physical layer damage, which can result in incremental performance degradation, intermittent operation, and even catastrophic network failure.
Only ruggedly constructed industrial-grade cabling, switches and connectivity components can provide the protection needed to ensure the reliable performance of the communications infrastructure over time, averting signal transmission problems that can lead to excessive downtime, costly repairs, lost productivity and reduced safety.
For Industrial Ethernet systems, designers should select:
• Heavy-duty, all dielectric, indoor/outdoor-rated optical fiber cabling in single-mode and multimode constructions.
In industrial plants, maximum productivity with minimal downtime is always a key goal, and 24/7 network performance and reliability are critical to achieving that goal. If a switch or cable fails, the cost of its replacement and repair represents only a tiny fraction of the overall costs associated with production outages and downtime. So if you are responsible for designing and specifying a plant floor control system, remember to ask: Are my Ethernet cables and connectivity hardware built tough enough?
Challenge: Data recording in Harsh Environments
In high speed, long duration data recording applications, traditional hard disk drives always become problematic when the project calls for ruggedization of the solution.
SSDs are based on DRAM volatile memory or NAND flash non-volatile memory. Many SSD manufacturers use non-volatile flash memory to create more rugged and compact devices for the consumer market. These flash memory-based SSDs, also known as flash drives, do not need batteries. They are available in standard disk drive form factors (1.8-inch, 2.5-inch, and 3.5-inch). In addition, the non-volatility of flash SSDs means content retention even during sudden power outages, ensuring data persistence. Flash SSDs are slower than DRAM and some designs can be slower than traditional HDDs on large files. But flash SSDs have no moving parts and thus long seek times and other delays inherent in conventional electro-mechanical disks are not a consideration. SSDs based on volatile memory, such as DRAM, are characterized by ultra fast data access, typically less than 0.01 milliseconds. They are used primarily to accelerate applications that are held back by the latency of Flash SSDs or traditional HDDs. DRAM-based SSDs usually incorporate internal battery and backup storage systems to ensure data preservation during power failure.
Flash based systems are leading the way in market share and price performance. There are two types of memory: SLC and MLC. MLC stores twice the bits per cell and is used more commonly due to its inherent cost savings. However, for the applications that utilize “circular buffer recording” (highly repetitive recording), SLC is recommended due to superior read/write cycle reliability.
SSDs are here to stay and their use in a myriad of applications is growing each quarter. At Conduant Corporation, more of the custom high performance recording systems supplied to scientific and defense related clients require the stability and ruggedness SSDs provide. Try SSDs on your next project. You won’t be sorry.
MEMS Accelerometer for Shock Measurements
MEMS accelerometers have been used in diverse applications for shock and vibration measurement for more than two decades. It has become increasingly apparent to many engineers that wide dynamic range and built-in damping in the MEMS element design can play a critical role in the fidelity of the measurements. In practice, it is very common to encounter over 1,000 g of acceleration during routine measurements, such as car collision or product drop testing.
To accurately characterize the physical behavior of the object under high acceleration impact, an accelerometer should have sufficient headroom (dynamic range) in its measurement range. Although many of the piezoelectric (ceramic) accelerometers on the market offer ample of dynamic range for the task, piezoelectric-based sensors cannot provide DC response which is necessary for deriving velocity and displacement information accurately from the acceleration data. MEMS accelerometers, on the other hand, offer DC response needed for these critical measurements. Unfortunately, most common capacitive MEMS accelerometers one can buy off the internet today are limited to 200g full scale range - too low to be useful under most shock testing conditions.
Piezoresistive MEMS accelerometers have been used for auto safety testing for many years. In auto safety testing, crashing of automobiles generate impacts that are both high in g level and rich in frequency content (think of the dummy head hitting the windshield), making it one of the most difficult measurement environments for any accelerometer. It is in these kinds of environments that a capacitive MEMS design, with its limited dynamic range and bandwidth, has been proven inadequate in capturing the total event without compromising signal fidelity. When engineers turned to piezoresistive MEMS accelerometer in the late ’90 for its higher dynamic range and bandwidth capabilities, however, they soon discovered that the undamped nature of nearly all piezoresistive design has created a different set of problems in their measurements and analyses.
An accelerometer with insufficient internal damping may set itself into resonance (ringing) when exposed to the high acceleration impacts during the course of the measurement. The primary (direct) and secondary (indirect) effects of the resonance eventually manifest themselves in the acceleration output signals. The primary effect of the ringing may result in sensor output non-linearity at frequencies below the resonance, which is sometime difficult to characterize. The secondary effect of ringing may result in non-linearity behaviors in the subsequent electronic stages due to high (unexpected) input signal amplitude. Even when the signal conditioning electronics is set to accommodate the high input signal level, it robs the measurement chain of its useful dynamic range. Modern capacitive type MEMS accelerometers with built-in gas damping seem to offer the perfect solution. But these capacitive MEMS products are unfortunately available only in very low measurement range (<200g) and provide limited linear frequency response (< 100 Hz).
The current third generation of piezoresisitive MEMS accelerometer elements have gone through many iterations in FEA modeling and structural refinements to fine tune their dynamic behaviors. The latest generation of die design offers up to 6000 g in measurement range. To avoid overrange in the MEMS structure, special mechanical features have been incorporated in the caps to act as over-travel stops. These stops limit stress at the hinges where the piezoresistors are located to prevent premature structural failure. Damping has been dialed in to provide the maximum effect at its resonance. This new generation of accelerometers is now available in SMD package, with full scale G range at 50g, 100g, 200g, 500g, 2000g, and 6000g. Housed in a hermetically sealed LCC package, these SMD accelerometers seem ideal for a variety of applications including on-board shock and vibration monitoring in heavy equipments, off-road vehicles, and light/heavy weapons where metal-to-metal impacts are common occurrences.
Another performance advance in high-G MEMS accelerometer relates to its temperature response improvement. In applications such as engine testing and weapons development where the external temperature can change drastically from moment to moment. The accelerometer can be exposed to unexpected thermal transients that have an effect on the acceleration output performance. To compensate for the thermal non-linearity of the MEMS sensor, a custom digital ASIC has been integrated in a similar LTCC package design. This accelerometer (Figure 3) is capable of operating in temperature environment from -55°C to +125°C with very low thermal sensitivity shift and thermal zero shift. The improvement in output accuracy over a wide temperature range allows the sensor to be used in hostile environment where typical MEMS accelerometer may have problem dealing with the temperature gradient of the application, such as firing of rocket engine or gun fire. This accelerometer also features a patented design allowing the sensor to be mounted in vertical (in plane) or in the transverse (parallel) direction. This feature offers the user the flexibility of arranging multiple sensors on the same PCB to measure shock impact in all three orthogonal directions. This is particularly useful in applications where space for mounting the MEMS accelerometer is at a premium.