We all know that the factory floor can be a noisy place: noise-induced hearing loss is one of the most common occupational illnesses in the U.S., but not only humans are at risk. Unseen and unheard electronic noise can wreak havoc with sensors and communications systems, especially since the arrival of the Industrial Internet of Things (IIoT), also known as Factory 4.0.
Why is this electronic “hearing loss” on the rise? The IIoT is all about using Big Data to produce what industry experts call “actionable insights,” which improve operational efficiency, save money and predict failures before they happen. Gathering massive amounts of data and getting it into the cloud begins by adding thousands of sensitive sensors to monitor all aspects of an industrial process. Those sensors must work well in an environment that was never designed to accommodate them.
EMC and the Connected Factory
Wikipedia defines electromagnetic compatibility (EMC) as the “branch of electrical engineering concerned with the unintentional generation, propagation and reception of electromagnetic (EM) energy, which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage in operational equipment.” Within EMC, there are two main classes of effects: EM emissions, or the generation of unwanted EM energy, and EM susceptibility, which is the degree to which a piece of equipment is affected by incoming EM energy. We can divide each class according to the means of EM propagation — radiation or conduction — giving rise to four distinct areas of study, and four sets of problems. Figure 2 illustrates the mechanisms for EMC propagation.
What makes the factory such a challenging EMC environment? Figure 1 earlier showed a typical IIoT scenario: a wide array of wired and wireless sensors and communication networks superimposed on a factory that was designed years, or even decades, before the development of low-power low-voltage analog and digital technology. The current generation of these devices often require power supplies of 1V or less and can potentially be affected by millivolt disturbances on power and ground lines. It gets worse since the original factory designers couldn’t foresee the IIoT’s widespread adoption of low-power wireless devices, so minimizing emissions in the GHz range probably wasn’t a high priority.
The typical factory contains many machines that can cause multiple EMC problems for low-power and wireless devices. An arc welder, for example, may be a source of both radiated and conducted emissions: radiated radio-frequency (RF) energy from the arc pulses and conducted energy from voltage harmonics and fluctuations on power and ground lines. Other machines may suffer from both emission and susceptibility issues at the same time. Figure 3 shows some common sources of EMI and their frequencies.
Some Applicable EMC Standards
As you might expect, since one noisy device can affect the public at large, the governments of the world have developed standards governing EMC performance. In the U.S., the Federal Communications Commission (FCC) sets minimum compliance standards for telecom equipment. Part 15 of the FCC regulation specifies the emissions testing needed to prevent harmful RF interference.
In the European Union, R&TTE directive 99/5/EG applies to all radio-controlled products. Industry Canada has the General Requirements for Compliance of Radio Apparatus (RSS-GEN), and other countries have similar agencies.
The regulatory agencies issue standards that cover allowed levels and approved test procedures for each of the EMC categories. Government agencies typically require testing and certification to relevant standards before a company can launch a new product.
Different standards apply to different industries, under the authority of worldwide standards authorities such as the International Electrotechnical Commission (IEC). For industrial equipment, IEC 61000-6-2 covers EMC immunity with IEC61000-6-4 as the generic emissions standard. Many applications have their own set of standards: for example, IEC 60974-1 specifically applies to power supplies for arc welding robots, and IEC 60974-10 covers arc welding robot EMC requirements.
EMC and Wireless Networks
Although industrial wired networking has been around for decades and includes such standards as Ethernet and CAN, hooking up an IIoT-enabled factory is made much easier by the rise of low-cost, low-power wireless networks. Some of the reasons for using a wireless solution in industrial applications include:
- Greater mobility, and the ability to move devices and easily connect with smartphones and tablets
- Elimination of expensive cabling
- Fast and easy installation and commissioning, especially in remote or hard-to-access locations
- Greater flexibility and capability for remote updating
- Easy integration of devices into the network
During the last decade, several wireless standards have taken hold in the connected factory. The table below shows some of the main contenders and their application in IIoT:
|802.11||Wireless Local Area Network (WLAN)||WiFi|
|802.15.4||Low-Rate Personal Area Network (LR-PAN)||ZigBee, WirelessHART, 6LoWPAN|
|802.15.1||Wireless Personal Area Network (WPAN)||Bluetooth|
IEEE 802.15.4-based networks are particularly attractive to IIoT architects because they are more suited to the small data packets and low update rates of IIoT sensor nodes. An 802.11 WLAN device, on the other hand, must be able to accommodate applications such as video streaming, which increases the complexity and the power consumption dramatically.
Many wireless products can handle one or more IIoT protocols in a single device. Texas Instruments, for example, offers the CC2630 wireless microcontroller unit (MCU) with 6LoWPAN, ZigBee and TI’s own SimpleLink functionality.
The device belongs to the CC26xx family of cost-effective, ultralow power, 2.4-GHz RF devices. The CC2630 contains a 32-bit ARM Cortex-M3 processor core running at 48 MHz, plus an RF block that incorporates an ARM Cortex-M0. It also includes an ultralow power sensor controller to interface with external sensors and collect analog and digital data during system sleep mode. This feature makes the part well suited for an IIoT low-power remote sensor node application.
Designing to Minimize EMC Issues
Designing for good EMC performance requires a multi-tiered approach, paying attention to performance from the factory level, such as grounding and power distribution, down to the individual integrated circuit. The task is made more difficult by the fact that many IIoT installations are retrofits, so wholesale changes, such as rewiring the factory infrastructure, are difficult, if not impossible, to accomplish.
Designing for EMC: Factory Level
At the factory level, good EMC performance begins with the design of the power distribution system. The typical factory uses high-voltage AC and DC systems that can give rise to many EMC-related events, such as transients from power network operations or power factor capacitor switching, fast transients from arcing contacts and collapsing magnetic fields in high-power contactor coils. Natural events such as direct or indirect lightning strikes can also induce voltage transients in factory equipment.
On the shop floor, rotating machines are used in almost all situations, from CNC machines to pumps or industrial robots. Unfortunately, they are also a major cause of EMI, particularly brushed DC motors with their “arc and spark” brushes; even brushless motors (BLDC) have PWM controls that generate high-speed switching transients. A site survey can help identify sources of RF noise and separate the main offenders with isolation and shielding.
Modern factories also include miles of wiring routed through conduits, under floors and inside walls and ceilings. In an older installation, these wires may have been installed over many years in a haphazard fashion with the result looking something like Figure 4. The wires carry everything from high-voltage, high-current power signals to low-level sensor and transducer inputs and outputs. At the factory level, the wiring can act as a large antenna that can transmit and receive radiated electrical noise from both internal and external sources. Minimizing the area of the loop formed by long wire runs can help to reduce the tendency to pick up noise.
Cabling can propagate conducted noise as well. For example, inductively coupling noise from one wire to the next in a bundle. Protecting noise-sensitive signals with shielded cabling is an important element in reducing such noise. Although the initial cost is higher, isolating and solving EMC issues after installation can be a lot more expensive. Belden’s 9536 cable, for example, contains six 24 AWG stranded tinned copper conductors, semi-rigid PVC insulation, and a shield plus drain wire that provides 100% coverage. For cable tray use in Category 5e 100BaseTX networked communications, Belden’s 796x cable offers four conductors and a braided shield with an industrial-grade sunlight and oil-resistant PVC outer layer.
Other suppliers such as Glenair offer a range of flexible braided tubing, conduit, and fittings to provide EMI protection. Glenair’s Armorlite, for example, is nickel-clad stainless steel EMI/RFI braided shielding designed for high-temperature applications up to 260°C.
Designing for EMC: System Level
Figure 5 shows the block diagram of a typical IIoT node-level device. It contains a broad variety of components that can contribute to EMC problems: conducted noise can enter or leave via the power supplies or wired communication lines, and the wireless interface can receive or emit radiated noise.
For reducing conducted susceptibility or emissions, stopping the noise at the point at which it enters or leaves the board — at the connector — is a very effective approach.
A filtered connector combines a standard connector with EMI/RFI suppression components to help solve EMC problems. The filter elements are housed within the connector body, maximizing the available PCB area, and saving weight compared to a standard connector and discrete filtering components.
For example, Harting offers a line of D-sub connectors that include a ferrite filter block to block high frequencies. The D-sub form factor is widely used in industry, and the Harting connectors come in 9, 15, 25, and 37 contact versions.
Often, it’s necessary to provide off-board EMC filtering for high-voltage DC power lines. The API 51F-726-002 EMC feed-through filters shown in Figure 6 are designed for tapped hole or through-hole mounting. Resin seals at both ends provide protection in a harsh industrial environment. With a choice of C, L, or Pi filters, they are effective at filtering out noise on DC power lines and can handle voltages up to 500 V DC/220 V AC (400 Hz).
Integrated circuit solutions are also available to help cut down on EMC issues. Texas Instruments’ TPDxF003 family is a line of filtering devices designed to reduce EMI emissions as well as provide system-level electrostatic discharge (ESD) protection. Each device can dissipate ESD strikes above the maximum level specified by the IEC 61000-4-2 international standard. The filtering structure reduces EM emissions by providing high-frequency roll-off: the device features a -3 dB bandwidth of 200 MHz and attenuates the signal by more than 25 dB at 1 GHz. Four-, six- or eight-channel devices are available.
To stop incoming radiated EMI, many designs shield the internal circuitry with a grounded enclosure that forms a Faraday cage: this can also block internally-generated emissions from leaking into the external environment.
Designing for EMC: Device Level
Drilling down even further to the board and device level, radiated or conducted emissions in one area of a PCB can cause problems in another area of the same board. For example, high-spread switching transients from digital clock pulses or switching power supplies can cause errors in low-level analog measurements.
Within a circuit block, good layout and design techniques are key to ensure that signals from one circuit do not capacitively or inductively couple into another circuit. Some of these techniques are:
· Slow voltage and current rise and fall times to minimize sharp transitions and reduce high-frequency content
· Decrease the surface area of magnetic loops on the circuit board
· Separate high-current grounds from digital, and especially analog grounds, with a “star ground” arrangement
· Run power and ground traces directly over each other to minimize loop area and reduce impedance
· Use a clock with a dither feature to spread the frequency spectrum and reduce radiated EMI
· Use ground planes or layers under noisy components such as microcontrollers
Many factors must be considered, including component placement and packaging constraints, so several design iterations may be needed to arrive at an acceptable solution.
This article is just a brief overview of a complex topic. To find out more, click on the links within this document, or check out these resources from some of our suppliers.
Texas Instruments has a helpful application note on PCB design guidelines for reduced EMI. TDK offers a “Guide Book For EMC” that discusses the role of passive components such as ferrites, capacitors, common-mode filters and varistors in achieving good EMC performance. And Analog Devices, another Mouser supplier, has a tutorial on EMI, RFI and shielding techniques to help protect sensitive analog circuitry.
The Industrial Internet of Things, also known as Factory 4.0, involves the integration of low-level, low-power, analog digital and wireless functionality into an electromagnetically hostile environment. Designing for good EMC performance requires attention to detail at all levels, from the factory building itself to the individual board layouts.