Danielle Collins - Motion Control Tips

What is IO-Link and how is it used in motion control applications?

June 13, 2019 By Danielle Collins Leave a Comment

Industry 4.0 and the Industrial Internet of Things (IIoT) offer significant benefits to automation and processing plants, through improved machine-to-machine communication, faster and more comprehensive data collection and sharing, and improved predictive maintenance. One of the key enablers of these smart factories is a communication protocol known as IO-Link — an open standard, point-to-point communication link for sensors and actuators. IO-Link offers benefits over traditional IO in the form of reduced cost and complexity, improved process efficiency, and increased machine availability.

The international standard IEC 61131-9 specifies a single-drop digital communication interface technology (SDCI, commonly referred to as IO-Link) for sensors and actuators. It’s important to note that IO-Link is not a fieldbus — in fact, it can be applied over various networks, fieldbuses, and backplane buses.

An IO-Link configuration consists of a master and devices such as switches, sensors, and actuators. The master can be connected to virtually any fieldbus, and information flow between the master and devices is bi-directional. A key differentiator between IO-Link and standard IO is that with IO-Link, three types of data can be transmitted between the master and devices: process data, service data, and events.

Image credit: PROFIBUS

Process data is information the device reads from and transmits to the master. Service data includes information about the sensor or device itself, such as parameter values, serial number, status, and configuration. Event data includes both standard and vendor-specific notifications, which are further qualified as errors, messages, or warnings. Process data is transmitted cyclically (automatically transmitted on a regular basis), while service and event data are transmitted acyclically (only when needed or requested), and IO-Link provides data transmission rates of 4.8 kbps, 38.4 kbps, and 230 kbps.

Another benefit of IO-Link is that it is based on standard, unshielded, three-wire cables, so it eliminates custom cabling and connectors and simplifies wiring overall, particularly for analog devices. (When power is required to the device, a standard 5-wire cable is used.) It also does away with the need for A/D (analog-to-digital) and D/A (digital-to-analog) signal conversions for analog devices, which degrade the quality of data transmission. With IO-Link, only an A/D conversion is necessary, making the signal transmission between the sensor and controller more reliable and eliminating data losses. And IO-Link addresses the multiple interfaces that different devices require — such as binary switching, analog IN/analog OUT, or RS232 — and instead uses a single interface for the communication, regardless of whether the device is a sensor, actuator, gripper, or valve.

Information flow between the IO-Link master and connected IO-Link devices is bi-directional, and the master can be connected to virtually any fieldbus.
Image credit: Pepperl+Fuchs Comtrol, Inc.

Each IO-Link device has an IO device description (IODD) which is independent of the fieldbus or controller. The IODD contains information about the device, such as manufacturer, model number, serial number, device type, and parameter information. And IO-Link allows bi-directional communication, so together with the IODD, IO-Link devices can be parameterized remotely, and parameters can be changed dynamically when needed, such as when a product changeover occurs.

The IODD also makes replacing a device almost fool-proof, since the master can reject a device that doesn’t match the one being replaced. And if the replacement is a new version of the device, the master can check for and enable backwards compatibility (if available) between the new and previous versions. And like initial parameterization, the new device can be commissioned quickly and remotely.

In addition to the hands-on benefits that users see during assembly, integration, and start-up, IO-Link supports the IIoT and Industry 4.0 initiatives by providing more data about the device, machine, and process, which allows users to perform real-time monitoring and diagnostics and make better, faster decisions.

Omron releases new proximity sensors with enhanced sensing distance and IO-Link

June 10, 2019 By Danielle Collins Leave a Comment

Omron Automation Americas, a globally recognized provider of advanced industrial automation solutions, has announced the release of over 2,500 new models of its E2E NEXT proximity sensor series featuring exceptionally long sensing distances and IoT functionality.

Highlights of the new models include an enhanced sensing distance that minimizes target contact during production and IoT capabilities for improved predictive maintenance.

Contact avoidance is essential for maximizing uptime. Seventy percent of unexpected equipment downtime is caused by component failures, of which proximity sensors account for a large proportion. E2E sensors are designed to help manufacturers reduce potential causes of unplanned downtime while benefiting from the sensors’ environmental resistance.

The addition of IO-Link functionality enables the sensors to reduce recovery time by indicating the location and cause of failures. They can also detect warning signs of impending failures and notify users via the network. The combination of IoT capabilities, long sensing distance, and oil-resistant sheathing reduces the risk of sudden equipment shutdowns by a factor of three.

In summary, Omron’s new E2E proximity sensors are:

Stable. The E2E NEXT line’s long sensing distance prevents unexpected equipment downtime caused by target contact, making the continued operations more stable.
Flexible. The extra-long sensing distance makes it possible to solve size-limited applications with smaller form factor sensors.
IoT-enabled. The DC 3-wire models use IO-Link to help identify the location and cause of failures in real time.
Oil-resistant. E2E sensors are resistant to cutting oil, which accounts for approximately 30% of unexpected component failures.
Easy to use. The sensors’ user-friendly design makes it easy to confirm detection status and ensures that your facility can recover quickly without requiring advanced support.

Manufacturers interested in learning more about the features and benefits of the E2E NEXT proximity sensor series can find additional information at automation.omron.com.

Why use a gearbox with a stepper motor?

June 6, 2019 By Danielle Collins Leave a Comment

Image credit: Global Electric Motor Solutions, LLC

Stepper motors are known for their accurate positioning capabilities and high torque delivery at low speeds, but they require careful sizing to ensure the motor matches the load and application parameters, to minimize the possibility of lost steps or motor stalling. Adding a gearbox to a stepper motor system can improve the motor’s performance by decreasing the load-to-motor inertia ratio, increasing torque to the load, and reducing motor oscillations.

Decrease load-to-motor inertia ratio

One cause of missed steps in stepper motor applications is inertia. The ratio of the load inertia to the motor inertia determines how well the motor can drive, or control, the load — especially during acceleration and deceleration portions of the move profile. If the load inertia is significantly higher than the motor inertia, the motor will have a difficult time controlling the load, and overshoot (advancing more steps than commanded) or undershoot (missing steps) can occur. A very high load-to-motor inertia ratio can also cause the motor to draw excessive current and stall.

J_L = inertia of load

J_M = inertia of motor

One way to reduce the inertia ratio is to use a larger motor with higher inertia. But that means higher cost, more weight, and trickle-down effects on other parts of the system such as couplings, cables, and drive components. Instead, adding a gearbox to the system reduces the load-to-motor inertia ratio by the square of the gear ratio.

i = gear reduction

Increase torque to the load

Another reason to use a gearbox with a stepper motor is to increase the torque available to drive the load. When the load is driven by a motor-gearbox combination, the gearbox multiplies the torque from the motor by an amount proportional to the gear ratio and the efficiency of the gearbox.

T_o = torque output at gearbox shaft

T_m = torque output at motor shaft

η = gearbox efficiency

But while gearboxes multiply torque, they reduce speed. (This is why they’re sometimes referred to as “gear reducers” or “speed reducers.”) In other words, when a gearbox is attached to a motor, the motor must turn faster — by a factor equal to the gear ratio — to deliver the target speed to the load.

N_o = speed output at gearbox shaft

N_m = speed output at motor shaft

And stepper motor torque generally decreases rapidly as speed increases, due to detent torque and other losses. This inverse relationship between speed and torque means it’s only practical to increase speed by a certain amount before the motor is unable to deliver the required torque (even when multiplied by the gear ratio).

performance-curve-chart-motors-same-volume-speeds

Notice that stepper motor continuous torque (green) falls off rapidly as speed increases, unlike servo motor continuous torque (blue), which is relatively flat even at high motor speeds.

Reduce resonance and vibration

But speeding up the motor does have a benefit. The additional speed required by the motor when a gearbox is installed means the motor operates outside its resonant frequency range, where oscillations and vibrations can cause the motor to lose steps or even stall.

The stepper motor with a gearbox (top) shows significantly less vibration than the non-geared stepper motor (bottom), especially at low speeds in the resonant frequency range.
Image credit: Oriental Motor USA Corp.

Image credit: Oriental Motor USA Corp.

In addition to ensuring the gearbox has the correct torque, speed, and inertia values, it’s important to choose a high-precision, low-backlash gearbox — especially when connecting the gearbox to a stepper motor.

Recall that stepper motors operate in an open-loop system, and backlash in the gearbox degrades the system’s positioning accuracy, with no feedback to monitor or correct for positioning errors. This is why stepper applications often use high-precision planetary gearboxes, with backlash as low as 2 to 3 arcminutes. And some manufacturers offer stepper motors with harmonic gears that can exhibit zero backlash under most application conditions.

What are negative-stiffness vibration isolators?

May 30, 2019 By Danielle Collins Leave a Comment

Protecting sensitive equipment from harmful vibrations can be done with either passive or active isolation systems, with the choice often being made on price, simplicity, and the level of protection required. Although passive systems are typically simpler and lower cost, most passive designs can’t protect against the low-frequency vibrations caused by structural resonances. And active systems use actuators and sensors — which require external power — to counter vibrations, meaning they’re more expensive, more complex, and more prone to failures.

But one type of passive system, known as a negative-stiffness vibration isolator, can protect even the most sensitive equipment used in nanotechnology, microscopy, optical, and semiconductor applications from a wide range of vibration frequencies, including the very low-vibration disturbances caused by building structures, nearby traffic, and other ambient sources.

Negative-stiffness vibration isolators are classified as passive systems because they are purely mechanical, using a combination of springs and beam-columns to provide isolation from both vertical and horizontal motions caused by vibrations.

Q: What is a negative-stiffness mechanism (NSM)?

A: In a typical structure, an increase in force causes an increase in displacement. Negative-stiffness mechanisms are those that can, during some region of their force-displacement relationship, exhibit increasing displacement with decreasing force.

An example of a negative-stiffness mechanism is a two-bar structure under compression, as shown below.

Force (P/EAκ³) versus Displacement (δ) of a two-bar structure under compression.
Images credit: Comsol Inc.

Notice that between points A and B, displacement is increasing while force is decreasing. Thus, the structure’s stiffness is negative in that region.

Negative-stiffness vibration isolators consist of a horizontal isolator and a vertical isolator connected in series. To counter motions that involve rotation (pitch and roll), a tilt motion pad can also be connected in series with the horizontal and vertical isolators.

The horizontal isolator consists of two fixed-free vertical beams (columns) supporting a weight. The weight imparts both eccentric (off-axis) axial compressive load and a transverse bending load. This phenomenon is referred to as the “beam-column effect” and causes the lateral bending stiffness of the beams to decrease. In effect, the isolator is acting as a horizontal spring with a negative-stiffness mechanism.

The horizontal isolator is designed to take advantage of the beam-column effect, allowing it to act like a negative-stiffness mechanism.
Image credit: Minus K Technology

Vertical motion is addressed using two horizontal flexures loaded in compression, which form a negative-stiffness mechanism. The flexures are supported at their outer ends and connected to a stiff spring at their inner ends. The stiffness of the isolator is determined by the design of the flexures and by their compressive load.

Two flexures, fixed at their outer ends and connected to a spring at their inner ends, form a negative-stiffness mechanism that isolates equipment from vertical motion due to vibrations.
Image credit: Minus K Technology

The performance of an isolator is quantified by its transmissibility, which is a measure of how much vibration is transmitted through the isolator to the equipment compared to the amount of input vibration. (The inverse of transmissibility is isolation efficiency.) Negative-stiffness vibration isolators exhibit much better transmissibility than that of other passive systems, such as air-based isolation methods, and their transmissibility even exceeds that of most active isolation methods.

Negative-stiffness vibration isolators are typically chosen for their effectiveness at very low frequencies, but they offer other benefits as well. Because they’re based on flexures and springs, they don’t exhibit wear, and they can be made from a variety of metals for harsh or challenging environments, such as cleanrooms and vacuum chambers. And unlike other passive vibration isolators, negative-stiffness designs don’t generate heat, which can have detrimental effects for sensitive equipment and environments.

Vibration damping: What’s the difference between passive and active methods?

May 22, 2019 By Danielle Collins Leave a Comment

Unlike typical static or dynamic loads, forces due to vibrations are difficult to predict, model, and account for when designing motion control systems. Some manufacturers recommend adding a safety factor to account for vibrations when sizing and selecting components — especially those that involve rolling or sliding motion, such as linear guides and screws.

Of course, the ideal solution would be to avoid or remove vibrations altogether, eliminating the risk of damage to the equipment or interference with the process being performed. But avoiding vibrations is difficult since they can be caused not only by the machine or the nature of the process itself but also by structural resonances (vibrations of the building or supporting structure close to its natural frequency). For vibrations in machinery that cannot be avoided or eliminated, there are two methods to reduce their effects — vibration damping and vibration isolation.

Even in an environment as benign as a lab, a variety of sources cause vibrations that can affect the equipment or process.
Image credit: Thorlabs Inc.

Damping vs. isolation: Although the terms “vibration damping” and “vibration isolation” are often used interchangeably, they refer to different methods of vibration mitigation. Vibration damping is the process of absorbing or changing vibration energy to reduce the amount of energy transmitted to the equipment or machinery, while vibration isolation prevents energy from entering machinery.

Regardless of whether you’re discussing damping or isolation, the systems used to achieve the end result can be classified as either passive or active. Passive and active methods of vibration damping (or vibration isolation) differ in the way they respond to and manage vibrations. An easy way to distinguish between the two is that passive systems use simple mechanical devices, fluids, or elastomeric materials, whereas active vibration damping relies on a closed-loop system with feedback.

Passive vibration damping systems often employ a mechanical device or a fluid to reduce vibration, but passive damping can also be achieved with viscoelastic materials. In either case, the kinetic energy of the vibration is converted to heat. Because many types of damping systems have inherent resonances, they are primarily effective at frequencies higher than 4 Hz. Passive vibration dampers also tend to have trouble compensating for vibrations that occur in more than one direction — in other words, they are most effective for vibrations that occur only in the X, Y, or Z direction, and less so for vibrations that occur in multiple directions simultaneously.

Shown here is an example of a passive vibration damping system, consisting of a simple harmonic oscillator (a rigid mass and a spring) with an added damper. The damper removes mechanical energy from the system in the form of heat.
Image credit: Newport Corp.

Active vibration damping systems use added power in the form of electronically controlled sensors and actuators in a closed-loop system to counter vibrations and end oscillations. Unlike passive systems, active vibration damping systems can be tuned to remove resonances, allowing them to work at lower frequencies caused by structural resonances. They can also provide damping for vibrations and interference caused by the equipment itself, such as noise from cabling or acoustic noise.

Active vibration damping systems use electronically controlled sensors and actuators in a closed-loop system.

Active systems can also provide proactive (rather than merely reactive) damping response by using feed-forward control. Where standard closed-loop configurations provide equal but inverse forces to cancel vibrations that enter the system, the addition of feed-forward allows the system to proactively generate and send canceling signals based on the expected vibrations. Feed-forward systems are especially useful for extremely low frequencies.

The choice between a passive or active vibration damping system should take into consideration how sensitive the machine or process is to vibrations — for example, vibrations are more detrimental to test and measurement equipment than to industrial production machinery — and the types of vibration that could occur.

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 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 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.