Basics | Design World Motion Handbook

Pneumatic actuators and systems taking advantage of next generation data

May 17, 2019 By Paul Heney Leave a Comment

Many industrial applications require linear motion during their operating sequence. One of the simplest and most cost-effective ways to accomplish this is with a pneumatic actuator, often referred to as an air cylinder. Actuators are typically mechanical devices that take energy and convert it into some kind of motion. That motion can be in any form, such as blocking, clamping, or ejecting, but typical motions are rotational or linear in scope.

As old-school as pneumatic actuators are, they’re definitely not components that are stuck in the past. Improved sensing and the related advancements that engineers have seen with the age of the IoT and fog computing are making meaningful improvements to this class of actuators.

For example, according to Jacob Toppen, Product Marketing Specialist for Aventics at Emerson, his company’s Smart Pneumatics Monitor (SPM), is an edge device capable of collecting data locally — totally independent of the control network.

“It uses pre-installed algorithms to analyze data, display relevant system information on a dashboard, and distribute data to other systems using popular messaging protocols,” Toppen said. “By collecting sensor data and performing cycle counting, the SPM module provides reliable information on the state of wear of actuators, valves, and non-pneumatic components, plus analyze energy efficiency. This minimizes the risk of machine downtime and significantly lowers operating costs.”

Similarly, Frank Latino, Product Manager, Business Unit Electric Automation, Festo, noted that his company has a solution using its IoT GW to do fog computing (an edge device to carry out substantial amount of computation and extend to a cloud).

“In essence, we are filtering and conditioning data from our unique pneumatic energy savings module MSE6-E2M to provide a dashboard targeting energy savings and maintenance information,” Latino said. “Changes in pressure, flow, and consumption data can be provided. A calculation of air savings is possible based on machine utilization of compressed air.”

Toppen said that he feels the Smart Pneumatics Monitor is an innovative IIoT gateway with simple operation, great flexibility, and outstanding efficiency.

“The programming software, Node-RED, comes pre-loaded on the Linux edge device. Node-RED is an open source graphical interface software developed by IBM which is easily accessible via a web browser when connected to the device,” he explained. “What’s more, Emerson’s Aventics is taking advantage of wireless connectivity in its sensors. Wireless sensors are used to transfer data to computing devices for process optimization and predictive maintenance.

Latino said that wireless communication “has a place in automation for the right application. It is best to follow standards, such as IO-Link wireless, so performance and interoperability are assured. I have yet to see power transfer technology suitable for actuators in the automation market.”

Coming soon
Latino noted that compact, inexpensive, distributed I/O blocks have been trendy due to ease of installation. Latino said that Festo plans to release a new series in this area, including pneumatic valves, later in 2019.

Nathan Irvine, Product Specialist for Aventics at Emerson, said that online pneumatic product configuration tools are gaining popularity as a self-service solution for engineers.

“They allow quick product selection, part number generation, and easy communication with manufacturers,” Irvine said. “Our product configurators allow on-the-fly generation of 2D and 3D CAD models and dimensioned drawings for immediate integration into machine designs. The customer also benefits from all details being uploaded automatically in the ERP system to speed ordering and production. Another example is easy customization of the pressure output and electrical input signal range of our electro-pneumatic pressure regulator valves.

Meanwhile, Latino said that they still see servo-pneumatic filling a space between standard pneumatic and electric drive motion. Force control, reduce weight, and simpler design are all included in servo-pneumatic.

The other hot area for Emerson is web-based sizing tools, such as Aventics’ CylinderFinder, which allows for quick product selection guidance based on application parameters and desired product features.

“The results of Aventics’ engineering tool calculations are made available to the user for validation. In most cases, the sizing tools are linked to product configurators for one-stop product sizing, selection, CAD generation, and additional support including immediate online purchase, if desired,” said Irvine.

“Fluid power continues to provide simple, cost-effective solutions for all types of actuation. Advanced pneumatic position control incorporates precise electro-pneumatic valves, low-breakaway pneumatic cylinders, and accurate position detection along the entire stroke,” said Irvine. “We offer proportional pneumatic technology and new solutions for external position detection. These permit the integration of readily-available standard components to create a basic positioning system with open-loop or closed-loop control via analog or fieldbus communication. Our Advanced Electronic System (AES)-based positioner allows precise positioning based on parameters held in a PLC. For applications not requiring fieldbus communication or a PLC, our Electro-Pneumatic Positioning System (EPPS) provides a simple solution for accurate positioning.”

When to use gears to change the inertia ratio of a motor-driven system

May 15, 2019 By Danielle Collins Leave a Comment

In any system where a motor drives a load to achieve precise positioning, velocity, or torque, the ratio of the load inertia to the motor inertia plays a significant role in determining the motor’s ability to effectively and efficiently control the load, especially during acceleration and deceleration portions of the motion profile.

In this context, the inertia we’re referring to is mass moment of inertia (sometimes referred to as rotational inertia), which is an object’s resistance to change in rotational speed. An object’s mass moment of inertia is determined by its mass and geometry — specifically, the radius between the object’s center of mass and the point around which the object rotates.

Adding a gear set or gearbox between the motor and the load can improve the load-motor inertia ratio.
Image credit: Omron Industrial Automation

If the inertia of the load is significantly higher than the inertia of the motor, the load will essentially try to “drive” the motor, and the motor will have a difficult time achieving the desired position (or speed or torque). In an attempt to control the load, the motor will draw higher current, which decreases efficiency and increases wear on the motor and electrical components.

On the other hand, if the inertia of the motor is significantly higher than inertia of the load, it’s likely that the motor is oversized, which has negative implications throughout the system — including higher initial cost, higher operating cost, larger footprint, and the need to oversize other components, such as mounting hardware, couplings, and cables.

If the inertia ratio is too high — that is, if the load inertia is much higher than the motor inertia, causing problems with positioning accuracy, settling time, or control of velocity or torque — the load inertia that is seen by, or reflected to, the motor can be decreased by adding a gear set or gearbox between the motor and the load.

J_L = inertia of load reflected to motor

J_M = inertia of motor

Adding a gear set or a gearbox to a motor-driven system reduces the load inertia by the inverse square of the gear ratio, meaning that even a relatively low gear ratio can have a significant effect on the inertia ratio.

J_L = inertia of load reflected to motor

J_D = inertia of drive (ball screw, belt, rack & pinion)

J_E = inertia of external (moved) load

J_C = inertia of coupling

J_G = inertia of gear set or gearbox

i = gear ratio

Note that the inertia of the gear set or gearbox (J_G) is added to the load inertia, but its effect is typically small compared to the reduction provided by the gear ratio.

In addition to optimizing the load-motor inertia ratio, gearboxes are often used in motion control applications to increase the torque delivered to the load from the motor, but they also decrease the rotational speed delivered to the driven component from the motor, by an amount equal to the gear ratio. This is why gearboxes are often referred to as “gear reducers” or “speed reducers.”

In other words, if a motor running at 1200 rpm is driving a load through a 3:1 gearbox, the rotational speed delivered to the load will be 400 rpm (1200 ÷ 3). This reduction in speed can enable the system to operate at a more favorable location on the motor’s speed-torque curve.

Although it is possible to use a gearbox or gear set that is configured to reduce torque (and increase speed), in motion control applications, a more typical solution would be to choose a smaller motor.

Feature image credit: M. M. Nazari

What are some common PAC chassis, module, controller, and I/O options?

May 15, 2019 By Miles Budimir Leave a Comment

ControlLogix PACs from Rockwell Automation feature tight integration between the programming software, controller, and I/O modules which helps reduce development time and cost.

In the field of programmable controllers, there are a number of different types of product offerings, depending on the application requirements. Traditional programmable logic controllers (PLCs) have been the industry standard for decades. They are well known and understood in terms of operation and programming and there are many PLC manufacturers to choose from.

As control demands became more complex, programmable automation controllers (PACs) came onto the scene to handle these more demanding cases requiring more complex control and coordination, involving many more axes and I/O than a typical PLC could handle. According to an industry standard definition, PACs operate on a single platform and handle multiple tasks including logic, motion, drives, and process control.

PACs stand between PLCs and PC-based controllers, combining the best elements of both. PACs are characterized by multiple processors, more memory and communication options, and more functionality beyond just the logic-based operations of traditional PLCs. They also can be programmed using ladder logic familiar to PLC programmers and other higher-level languages consistent with IEC 61131-3 standards.

Because PACs are similar to PLCs both in function and form, many of the options ordinarily available for PLCs are the same for PACs. So for instance, common component options include the controller, chassis, modules, and I/O.

Analog I/O modules from AutomationDirect are available in current, voltage, and temperature models with LCD displays. They can handle many standard analog I/O signal types including 0-20 mA, 4-20 mA and 0-10 Vdc.

Controller – the controller contains the central processing unit (CPU), memory, and the main networking and communication ports such as Ethernet and USB.

Chassis – the physical enclosure housing all the PAC components such as the controller/CPU unit, power supply, and modules and I/O. Options include rack-based systems (mountable to backplanes inside equipment racks via DIN rails) or mounted directly to machines.

Module – modules exist for all manner of applications and uses, designed to perform some specific function. So for instance, there are modules designed specifically to accept either analog or digital signal inputs and designed for specific uses. Examples include relay modules, counter modules, serial modules, servo or stepper controller modules, timer modules, and data acquisition modules, among others, including I/O modules (see below).

I/O – Just as with PLCs, I/O type can include voltage or current analog, digital, or mixed analog/digital. Common physical variable inputs to I/O modules can be pressure, force, or temperature data from thermocouples, among others.

What is disturbance rejection in motion control?

May 13, 2019 By Danielle Collins Leave a Comment

One of the key functions of a motion controller is to create the trajectories the motor follows in order to reach the target position (or velocity or torque). As the motor turns, the controller processes feedback from the encoder and compares the actual position of the motor with the desired position. If there are any deviations between the actual position and the desired position, the controller issues commands to correct the error.

Deviations can be caused by inaccuracies in the mechanical components (backlash in screws, compliance in connecting elements, or errors in mounting surfaces) or by unknown disturbances — often in the form of unexpected forces on the system that cause the position, velocity, or torque to rise or fall sharply and unpredictably.

The motor’s ability to follow the given trajectory and achieve the desired position is tracked and managed by the controller through a function known as command tracking or reference tracking. (When the controller is monitoring a process variable, such as temperature or fluid level, rather than the position, speed, or torque of a motion control system, this function is referred to as setpoint tracking.)

Each gain in the PI (proportional-integral) or PID (proportional-integral-derivative) control loop affects the system’s reference tracking. In addition, feedforward control, a proactive control method that predicts the system’s error and injects commands into the control loop to minimize the error, is often used to improve reference tracking and further minimize the deviation between the commanded position and the actual position of the motor.

To address unexpected forces that cause the motor to move away from the target value, the controller uses a function known as disturbance rejection, which processes the disturbance and provides commands that correct for these unknown forces or conditions.

Block diagram for a PID controller with an input disturbance (T_d).
Image credit: Quanser Inc.

In control tuning, the proportional gain (k_p) determines the amount of disturbance rejection. This makes sense because the proportional gain issues an error correction signal that is directly proportional to the error. So when a disturbance occurs, causing the motor to deviate from the desired state, the proportional gain outputs a correction signal to bring the motor back toward the desired state. In theory, the higher the proportional gain, the better the disturbance rejection. But in real-world applications, the maximum value of proportional gain that can be used without causing instability is limited by the system bandwidth and phase margin.

A controller can be tuned to balance the reference tracking and disturbance rejection functions, as shown here, or to favor one or the other.
Image credit: MathWorks Inc.

Motion controllers often use both functions — reference tracking and disturbance rejection — to provide accurate, steady-state performance and to reduce sensitivity to changing or unexpected loads. However, there are tradeoffs when both functions are used: the lower the overshoot and settling time for reference tracking, the more sluggish the disturbance rejection will be, and vice-versa. Tuning the controller involves finding the appropriate balance between the overshoot and settling time of the reference tracking function and the response time of the disturbance rejection function.

AC induction motor inertia: What’s the difference between WK2 and WR2?

April 29, 2019 By Danielle Collins Leave a Comment

In any motor-driven system, the difference between the motor inertia and the load inertia will affect the system’s performance — especially its ability to accelerate the load quickly and efficiently.

While servo motors are the preferred choice for applications that specify a precise move profile, AC induction motors are often the better choice for applications where a rotating device (such as a fan, blower, or gearbox) needs to achieve a given speed within a certain amount of time and maintain that speed regardless of changes in the load.

In servo-driven motion control applications, we typically look at the inertia ratio — the ratio of the load inertia to the motor inertia — to give us an idea of how the motor will perform for the type of application and performance requirements (high acceleration, fast settling time, accurate positioning). Although inertia matching is less critical for induction motor applications than for servo-driven systems, inertia still plays an important role in the sizing and selection of an AC induction motor. This is because with rotating loads, the required motor torque is directly related to the inertia of the load.

T = torque required by motor

J = inertia of rotating load

α = angular acceleration of load

Note that inertia includes the inertia of the load (a fan, for example) plus the inertia of the motor and any other rotating components (such as couplings or pulleys) between the motor and the load.

Two definitions of inertia for AC induction motors

In motion control applications, inertia is generally given as J (as shown in the equation above) and is defined as “mass times radius squared.” However, in many rotating equipment applications, inertia is given as either WR² or WK² and is defined as “weight times radius squared.”

Notice that the inertia equations for induction motors use weight (which is a force) rather than mass. Also notice that the distance (radius) can be specified as either “R” (WR²) or “K” (WK²). R is the radius of the object, from its center to its outer edge. K is the radius of gyration, which is the distance from the point around which the object rotates, to the point at which the mass can be considered to be concentrated.

These expressions for inertia — WR² and WK² — are often used interchangeably, although R (radius) and K (radius of gyration) are rarely the same values. Here’s why…

If a hollow cylinder with extremely thin walls rotates around its axis, it’s reasonable to assume that the cylinder’s mass (weight) is concentrated around the outside circumference. So the radius of gyration for the thin-walled cylinder will be measured from the center to the outside edge of the cylinder. Thus the radius of gyration, K, is equal to the radius of the cylinder, R.

If a hollow cylinder has very thin walls, the mass can be assumed to be concentrated at the outer circumference of the cylinder.

However, if the cylinder is solid and uniform, its mass (weight) can be assumed to be concentrated at a distance of 1/2 the radius. In this case, the radius of gyration is ½ of the radius (K = ½R).

If the cylinder is solid and uniform, the mass is assumed to be concentrated at 1/2 the radius, so the radius of gyration, K, equals 1/2*R.

Using inertia to determine torque and acceleration

The distinction between these two inertia equations, WR² and WK², is important because in AC induction motor applications, inertia is used to determine the motor torque required to achieve a desired speed within a given time.

T = acceleration torque (lb-ft)

W = weight of load to be accelerated (lb)

R = radius (or K, radius of gyration) (ft)

N = change in speed (rpm)

t = time to accelerate (s)

Note: 308 is a constant that incorporates the conversions from lbf to lbm, and from rotations per minute to radians per second

Inertia can also be used to determine the time the motor will take to reach a given speed with a defined acceleration torque.

This is important because induction motors draw very high current during start-up and acceleration, and high current for an extended period of time can damage the motor.

What are ultrasonic proximity sensors?

April 19, 2019 By Miles Budimir Leave a Comment

Ultrasonic proximity sensors are a common type of proximity sensor used in many manufacturing and automation applications. Mainly for object detection and distance measurement, they’re commonly used in food and beverage processing and various packaging applications. Ultrasonic sensors work by using sound frequencies higher than the audible limit of human hearing (around 20 kHz), which is typically in the range of 25 to 50 kHz.

The basic physical principle of ultrasonic sensing is that the sensor sends out an ultrasonic pulse and receives a pulse back. Using the time difference between the sent and received signal, the distance to the object can be determined. A common design is to build both the transmitter and the receiver into the same physical housing, although they can also be housed in separate units like certain photoelectric sensors with separate emitters and detectors. Housing the transmitter and receiver in the same unit simplifies installation and cabling.

An ultrasonic proximity sensor with both the transmitter and receiver integrated into one housing, showing both the emitted sound wave and the reflected wave from the detected object. (Image via Baumer)

Because ultrasonic proximity sensors use sound rather than light, they can be used where photoelectric sensors have difficulty, such as in detecting clear plastic objects and labels, highly reflective surfaces that throw off optical sensors, or even liquid levels. They’re also immune to common contaminants such as dust, moisture, and ambient light.

Depending on the application requirement, ultrasonic proximity sensors can be installed and operated in a number of different ways. In fact, because the sensing itself is based on emission of waves and their detection, the ways they can be installed parallel those of photoelectric sensors. That is, setups can include simple reflection of sound waves (as in retro-reflective modes) or they can be setup for through-beam type sensing or a diffuse mode.

The UT/TU Series of ultrasonic proximity sensors from AutomationDirect feature a 30-mm diameter face in a plastic body. Wide beam sensing produces a sensing range of up to 6 m. Discrete output models are available with adjustable sensitivity range.

For most sensing applications using ultrasonic sensors, it’s desirable to have a fairly narrow output beam in order to avoid reflections that could produce inaccurate readings. A wider beam will disperse over a greater area and may cause interference patterns that could cause inaccurate readings.

Besides beam angle, consider other parameters such as the optimal sensing mode for the application, the required measurement range, the output type (analog or switch/relay output), as well as the size, shape, and material of the housing.

What is droop control in the context of AC motors and drives?

April 17, 2019 By Danielle Collins Leave a Comment

One of the fundamental characteristics of an AC induction motor is that the rotor spins at a slower rate than that of the stator’s rotating magnetic field. This difference in speed between the rotor and the stator’s magnetic field is known as slip.

Slip allows the motor to produce torque — the higher the slip, the higher the torque production, but also, the lower the motor’s efficiency (in most cases). But in load-sharing applications, motor slip can be used to prevent one motor from taking a disproportionate share of the load.

N_s = synchronous speed (rpm)

N = rotor speed (rpm)

Load-sharing, in this context, refers to two or more AC motors connected to and driving the same load. Theoretically, each motor in a load-sharing set-up would experience an equal percentage of the load. But in real-world applications, numerous factors, such as variations in load along the length of a conveyor, or inconsistencies in the driven process, can cause the load to be distributed unevenly among the motors.

“Natural” load balancing

Because slip is inherent in AC motors, when two or more motors are connected to one drive, their natural slip will work to achieve balanced load sharing. As one motor experiences increased load, its speed (i.e. rotor speed) slows down, increasing slip between the rotor and stator. The second (and subsequent) motors, which are connected to the same drive, will also slow, allowing them to take on the additional load. While using the motors’ natural slip is a simple method of load-sharing, it is very imprecise and not reliable at high loads.

Droop control

For applications where load-sharing needs to be precise and controllable, some AC drives include a function — typically referred to as droop control — that manipulates motor slip to better ensure equal load-sharing. Here’s how it works:

Droop control effectively shifts the motor’s torque vs frequency curve so that an increase in torque causes a decrease in the drive’s output frequency.
Image credit: Schneider Electric

The drive monitors the motor torque (via current draw), and if the torque increases beyond a set value (indicating that the motor is taking on too much load), the droop control function decreases the output frequency of the drive.

The rotor speed has slowed due to the increased load, and the decreased output frequency from the drive causes the stator magnetic field to also decrease in speed. Hence, there’s a smaller difference between stator speed and rotor speed. In other words, slip is reduced, which reduces the motor’s torque production, allowing other motors to take more of the load.

Droop control can work both ways — reducing the output frequency of the drive when the load increases (to decrease the amount of load on an overloaded motor), or increasing the output frequency of the driven when the load decreases (to increase the amount of load on a non-drooping motor). This two-way method of controlling load sharing is sometimes referred to as “bipolar” droop control.

Droop control is often used to increase slip in an overloaded motor, allowing other motors connected to the same load to take a higher share of the load. One application where droop control is useful is a conveyor with multiple driven rolls. If one section of the conveyor (and, therefore, one motor) sees an increase in load, droop control prevents the motor from sustaining this disproportionate share of the load, preventing damage to the motor and, potentially, to the system.
Image credit: Rockwell International Corporation

What’s the difference between a dashpot and a snubber?

April 12, 2019 By Danielle Collins Leave a Comment

Dashpots and snubbers are two types of pneumatic device used for controlling the movement of a load — typically for the purpose of controlled deceleration or motion damping. Although they can be used to control rotary motion, the more common uses for dashpots and snubbers in industrial applications involve the control of linear motion mechanisms, such as solenoids or spring-loaded devices.

Both dashpots and snubbers contain two primary parts: a glass cylinder with a polished bore and a precision piston, often made of a low-friction material such as graphite. And both devices operate by forcing ambient air through an adjustable orifice at a controlled rate. But despite these similarities, dashpots and snubbers are designed for different applications. Case in point — a dashpot is best for applications that require accurately controlled force or velocity, whereas a snubber is best used for end-of-stroke damping where accurate control of impact is required.

Dashpots

Image credit: Automotion Components Ltd.

A dashpot has a connecting rod that joins the load to the piston and provides control throughout the stroke, either by extending the connecting rod and piston (“pull” mode) or by causing the connecting rod and piston to retract into the cylinder (“push” mode). Dashpots can also control motion in both directions.

Both operating modes — push and pull — rely on the change in air pressure inside the cylinder. In push mode, as the piston moves farther into the cylinder, the air inside the cylinder is compressed, causing the pressure to rise. In pull mode, as the piston retracts out of the cylinder, the pressure inside the cylinder falls and creates a partial vacuum.

Dashpots in “pull” mode work best when the movement needs to be controlled for the entire stroke. This is because the air column is short when the motion begins and the damping force increases quickly, providing controlled motion after just a short amount of travel. It’s also important that a dashpot used in “pull” mode is returned to the starting position — otherwise, if the piston isn’t fully retracted, the air column inside the cylinder will be relatively long and the damping force will be slow to increase.

A dashpot being used in “pull” mode to control the velocity of a falling load.
Image credit: Airpot Corp.

A dashpot in “push” mode works best for applications that require a reduced impact at the end of travel. In this mode, the pressure inside the cylinder rises as the column of air becomes shorter. With push damping, the damping effect is lower at the beginning of the movement. This is because the column of air is relatively long and requires some amount of movement to create sufficient pressure to provide damping. With “push” damping, there can be a noticeable effect of the load “bouncing” on the air column (sometimes referred to as an “air spring”) mid-way through the stroke, as the pressure rises and begins to dampen the movement.

Snubbers

Image credit: Automotion Components Ltd.

Also referred to as pneumatic shock absorbers, snubbers differ from dashpots in two ways: The piston is not attached to the load being controlled, and damping occurs in compression only (i.e. push damping).

When the load contacts the piston rod of a snubber, the force of the load causes the piston to move, compressing the air inside the cylinder. This compression of air provides a controlled deceleration, with the amount of deceleration depending on the magnitude and speed of the load and the adjustment of the orifice.

If the damping is too high, the load will bounce on the air column (air spring effect) or bounce on landing. If the damping is too low, the load will land with too much force, causing damage to it or to other equipment. Although snubbers provide only compression damping, their accuracy in controlling movement and reducing shock is better than that of dashpots.

A snubber provides controlled deceleration at the end of stroke for a linear slide.
Image credit: Airpot Corp.

Feature image credit: Airpot Corp.

What’s the difference between PL and SIL machine safety standards?

April 5, 2019 By Danielle Collins Leave a Comment

Machine safety is governed by two standards: EN/ISO 13849-1 and EN/IEC 62061. Both standards are harmonized to the EU Machinery Directive 2006/42/EC, which defines the Essential Health and Safety Requirements (EHSR) for machinery. Although their methods for performing risk assessment are different, both standards — EN ISO 13849-1 and EN 62061 — when correctly applied, achieve the same result.

The EU Machinery Directive requires that machine manufacturers eliminate or minimize hazards as much as reasonable, apply necessary protective measures against hazards that cannot be eliminated, and inform users of the risks that remain and requirements for training or personal protective equipment. Although this directive is specific to the European Union (EU), it is recognized and followed in other regions around the world, to better facilitate equipment shipments outside the EU.

The EN/ISO 13849-1 machine safety standard uses a qualitative risk graph, or flow chart, to assign a performance level (PL), based on three criteria:

severity of injury
frequency and/or exposure time to the hazard
possibility of avoiding the hazard or limiting the harm

The performance level (PL) is designated by an alphabetic character, a thru e, with PLe being the highest risk level.

EN/ISO 13849-1 assigns a performance level (PL) rating from a to e, with PLe being the highest risk.
Image credit: TUV

Once the performance level has been determined, the architecture that facilitates the defined performance level is classified into one of six categories (“B” and 1 thru 5, with B being the least safe and 5 being the most safe). The architecture category is determined by combining the performance level (PL) with quantitative measures of diagnostic coverage (DC) and mean time to dangerous failure (MTTF_d).

This chart shows the relationship between Category, Diagnostic Coverage, and Mean Time to Dangerous Failure for PL levels under EN/ISO 13849-1. Note also the correlation with probability of dangerous failure per hour (PFH_d) rates.
Image credit: ABB

The EN/IEC 62061 machine safety standard (often written as just EN 62061) assigns a safety integrity level (SIL) to each function based on the severity of the potential harm (Se) and the probability of the harm occurring.

The severity of potential harm is given a score from 1 to 4, with 4 being the most severe. The probability of harm occurring is broken down into three parameters:

frequency and duration of exposure (Fr)
probability of an event occurring (Pr)
probability of avoiding or limiting the harm (Av)

EN 62061 assigns a safety integrity level (SIL) from 1 to 3 based on the severity of potential harm and the probability of the harm occurring.
Image credit: TUV

Each of these parameters is scored from 1 to 5, with 5 being the “worst,” or least safe situation, and their scores are summed to determine a class (Cl). The SIL rating is then chosen from a matrix that plots the severity scores (Se) and classes (Cl).

SIL ratings are determined by a matrix that ranks the severity of injury and the injury classification.

Once the safety integrity level (SIL) has been assigned, the system is broken into subsystems, whose architectures are classified as A, B, C, or D, with D being the “highest,” or safest. Each architecture is associated with a formula to determine the probability of dangerous failure per hour (PFH_d) of the subsystem.

Note that performance level (PL) ratings under EN/ISO 13849-1 are also correlated with probability of dangerous failures per hour (PFH_d) values, so direct comparisons can be made between EN/ISO 13849-1 performance levels and EN 62061 safety integrity levels.

There is no strict guideline regarding the use of machine safety standards for particular applications, but the choice may be influenced by:

Prior experience with one standard or risk assessment methodology
The use of safety-related controls that are not based on electrical, electronic, or programmable electronic systems (use EN/ISO 13849-1)
A requirement to use SIL ratings to demonstrate safety integrity (use IEC 62061)
Use of equipment in process industries where other safety-related systems are characterized in terms of SIL (use IEC 62061)

What are inductive proximity sensors?

April 5, 2019 By Miles Budimir Leave a Comment

Proximity sensors are commonly used in many automation applications. They’re used to sense the presence of objects and don’t require physical contact with the target or object being sensed, which is why they’re often referred to as non-contact sensors. Common proximity sensor types include photoelectric, capacitive, and inductive sensors.

The operation of a typical inductive proximity sensor is shown here. The oscillator generates an electromagnetic field that radiates out from the sensing face, inducing eddy currents in nearby metallic objects. This causes a change in the oscillation amplitude that triggers a change in the output state. (Illustration via Baumer)

Inductive sensors operate on the basis of Faraday’s Law. One way to state Faraday’s Law is that a change in magnetic flux in a coil of wire will induce a voltage in a nearby coil. This is applied in inductive proximity sensors in the following way: The sensor itself contains an oscillator circuit and a coil from which an electromagnetic field radiates out and induces eddy currents in any nearby metallic objects. The eddy currents have the effect of attenuating the oscillations from the amplifier. This reduction in oscillations is registered as the presence of a metallic object.

Because only metallic objects have inductive properties, inductive sensors can’t be used to detect plastic or cardboard or other non-metallic objects. However, different metals have different inductive properties and the type of metal being sensed will influence the sensing distance. For instance, ferromagnetic materials like steel generally have the longest sensing distance, while non-ferrous metals such as aluminum or copper have much shorter sensing distances. In general, inductive proximity sensors are well suited to shorter-range applications as the inductive effect wears off with growing distance between the sensor and object to be detected.

Miniature inductive proximity sensors, like the IFRM 03 Short family from Baumer, feature a 3-mm diameter and come in 12 or 16-mm lengths. The sensors are designed for tight spaces where standard proximity sensors can’t fit.

Inductive proximity sensors hold up well in dirty environments where contaminants don’t interfere with the sensor’s ability to detect metallic objects. For example, they’re resistant to dirt, dust, and smoke in the environment between the sensor and the object to be detected. As for build-up of contaminants on the sensor face such as dirt and dust, oil, grease or soot, these don’t effect the inductive sensing. However, metallic contaminants such as metal chips in machining applications will impact sensor operation. The key is to be sure to understand what type of contaminants an application contains in order to select the correct type of sensor that can handle them and operate effectively.