Danielle Collins - Motion Control Tips

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)

Encoder Products absolute encoders – now with improved electronics

April 2, 2019 By Danielle Collins Leave a Comment

EPC’s line of absolute encoders offers magnetic multi-turn technology and higher resolution.

Encoder Products Company (EPC) offers a full line of absolute encoders with models in both shaft and blind hollow bore, in sizes 36 mm, 58 mm, and standard size 25 with a 2.5 inch housing. Now, EPC absolute encoders come with even more communication options and higher resolution.

Absolute Technology

An absolute encoder is a device that provides a unique code for each increment of shaft rotation, meaning that an absolute encoder indicates both that the shaft position has changed, and the absolute position of the shaft. EPC’s absolute encoders now offer a higher resolution of 16 bits for single turn resolution and 43 bits for multi-turn resolution, allowing these absolute encoders to measure smaller increments, and allowing a longer run-time before reset.

“Absolute encoders are becoming more and more prevalent in industry,” explains Sarah Walter, EPC Technical Applications Engineer. “They offer connectivity, are more resistant to electrical noise, and retain position information after a power loss.”

Absolute encoders are ideal for safe operation of systems that always need exact location information to prevent damage to equipment or bodily injury. This is especially true in the event of a loss-of-power scenario. EPC absolute encoders maintain position value without being powered, so if there is rotation during the power loss, this movement is tracked and retained. Once power is restored, the encoder can transmit the location information immediately.

As more equipment and industries rely on real-time data, absolute encoders make an excellent motion feedback solution. There is no longer a need to use additional devices and time to calculate positional data. EPC’s absolute encoders offer real-time information with SSI, CANopen, and EtherCAT communication protocols. Watch the video for more information.

EPC Absolute Encoders offer:

16 bit resolution for single turn, and 43 bit resolution for multi-turn
SSI, CANopen or EtherCAT communications
Maintenance-free and environmentally-friendly magnetic design
Energy harvesting magnetic multi-turn technology
No gears or batteries
Electronic cam switches on CANopen and EtherCAT models
Compact design with bus cover

Encoder Products Company

What features make resolvers suitable for harsh environments?

March 29, 2019 By Danielle Collins Leave a Comment

Resolvers were initially developed for military applications and are probably best known as the preferred feedback solution in harsh environments. Where optical or magnetic encoders are sensitive to dust, liquids, and magnetic fields, resolvers can operate in dirty, humid, wet, and high-radiation environments. But what is it that makes them so robust under conditions that would damage or interfere with other feedback devices? The key is separation of the electronics from the resolver itself.

A resolver is essentially a rotating transformer, consisting of a rotor with one primary winding and a stator with two secondary windings. The two secondary windings are oriented 90 degrees from each other and are referred to as the sine and cosine windings. The rotor is attached to the rotating part that is being measured — such as a motor shaft — and the stator is stationary.

An AC voltage with constant frequency (in the range of 2 kHz to 10 kHz) excites the primary winding, which induces voltage in the two secondary (sine and cosine) windings. The induced voltages will have the same frequency as the supply, or reference, voltage, but because the two secondary windings are 90 degrees out of phase, the induced voltages in the secondaries will also be 90 degrees out of phase.

The voltages induced in the sine and cosine windings are directly related to the shaft angle (θ)
Image credit: Advanced Micro Controls, Inc.

The amplitudes of the induced voltages are proportional to the angular position of the shaft. Therefore, the shaft angle can be determined by taking the arctangent of the ratio of the sine voltage to the cosine voltage (sinθ/cosθ = tanθ). And because shaft position is determined only by the voltage ratio of the secondary windings, the position information is absolute, meaning the resolver can always determine the shaft position — even if it has been powered down and restarted — without having to move to a reference or index signal.

The induced voltages (sine and cosine) are 90 degrees out of phase.
Image credit: Teledyne LeCroy

Note that no mechanical components, optics, or sensors are required for a resolver to work. This is the key to the resolver’s ruggedness. The electronics that convert the resolver’s analog output to a digital signal (sometimes referred to as an R/D converter) that the controller can accept are housed separately from the resolver itself. This means that contamination, vibration, extreme temperatures, and even radiation have little or no effect on the resolver’s operation.

In this steel mill application, a resolver is connected to a lead screw (middle) that positions side guides during the rolling process.
Image credit: Dynapar

Resolvers are often used for commutation and speed control of motors.
Image credit: Nidec-Avtron

In theory, resolvers have infinite resolution, since every rotational position of the shaft is associated with a unique sine/cosine voltage. But signal conditioning — the R/D converter — limits both accuracy (typically in the range of 3-15 arcminutes) and rotational speed (typically 5000 rpm or less).

One way to achieve higher accuracy is to use a multi-speed resolver. In this design, the number of magnetic poles is increased, so each mechanical revolution creates multiple sine and cosine curves. The downside of multi-turn designs is that they lose the ability to provide absolute feedback. But there is a way to get absolute feedback and better accuracy from resolver set-ups — by mounting, or “stacking,” a single-speed resolver on top of a multi-speed resolver.

Feature image credit: Dynapar

Omron to demonstrate advanced solutions for flexible manufacturing, traceability and more at Automate show

March 21, 2019 By Danielle Collins Leave a Comment

Industry-leading automation solutions provider, Omron Automation Americas, will demonstrate a diverse solution portfolio encompassing traceability, flexible manufacturing, human-machine collaboration and more at the upcoming Automate Show in Chicago this April.

Omron Automation Americas, a global leader in end-to-end solutions for industrial automation, will be at the upcoming Automate Show to demonstrate applications for the factory of today and that of the future. Omron’s diverse technologies on display will feature solutions for traceability and flexible manufacturing.

The biennial Automate Show gives attendees the opportunity to see many of the world’s cutting-edge manufacturing technologies in action. Taking place from April 8-11 in Chicago, it provides a key opportunity for Omron to showcase technologies that help manufacturers improve productivity, flexibility, and human-machine interaction.

In booth #8737, Omron will be presenting its Factory Harmony exhibit, a multifaceted display that provides a vision of the manufacturing floor of the future. Each demo in the exhibit is designed to illustrate ways in which machines can collaborate more effectively with humans on the factory floor.

The Factory Harmony exhibit includes a fully integrated vision-guided robotics pick-and-place demo that shows the potential for machines to manage finely detailed work with speed and accuracy as part of a flexible manufacturing system, as well as a traceability demo comprised of laser marking and robotic technologies. Flexibility and traceability are key targets of Omron solutions.

Other robotic solutions on display at Automate include Omron’s TM Series collaborative robot, a flexible solution for human-machine collaboration, and the LD Series mobile robots that move materials flexibly throughout the factory. Both solutions help manufacturers meet growing demands for customization without completely reconfiguring their production lines.

The Omron booth will also feature a demo of its most advanced machine vision technology, as well as its safety product offerings. In total, the Factory Harmony exhibit demonstrates a new standard for human-machine collaboration to help manufacturers enjoy improvements in productivity, efficiency, quality, and worker satisfaction.

Omron Automation

What is nested vector interrupt control (NVIC)?

March 20, 2019 By Danielle Collins Leave a Comment

In a microcontroller, such as those at the heart of industrial motion controllers, interrupts serve as a way to immediately divert the central processing unit from its current task to another, more important task.

An interrupt can be triggered internally from the microcontroller (MCU) or externally, by a peripheral. The interrupt alerts the central processing unit (CPU) to an occurrence such as a time-based event (a specified amount of time has elapsed or a specific time is reached, for example), a change of state, or the start or end of a process.

Interrupts can be triggered internally – from a timer, for example – or externally, from peripherals.

Another method of monitoring a timed event or change of state is referred to as “polling.” With polling, the status of a timer or state change is periodically checked. The downsides of polling are the risk of excessive latency (delay) between the actual change and its detection, the possibility of missing a change altogether, and the increased processing time and power it requires.

With polling, the CPU periodically checks for a change of state. Interrupts automatically notify the CPU when a change of state occurs, ensuring the change is not missed and its detection is not delayed.
Image credit: Renesas Electronics Corporation

When an interrupt occurs, an interrupt signal is generated, which causes the CPU to stop its current operation, save its current state, and begin the processing program — referred to as an interrupt service routine (ISR) or interrupt handler — associated with the interrupt. When the interrupt processing is complete, the CPU restores its previous state and resumes where it left off.

Nested vector interrupt control (NVIC) is a method of prioritizing interrupts, improving the MCU’s performance and reducing interrupt latency. NVIC also provides implementation schemes for handling interrupts that occur when other interrupts are being executed or when the CPU is in the process of restoring its previous state and resuming its suspended process.

The term “nested” refers to the fact that in NVIC, a number of interrupts (up to several hundred in some processors) can be defined, and each interrupt is assigned a priority, with “0” being the highest priority. In addition, the most critical interrupt can be made non-maskable, meaning it cannot be disabled (masked).

One function of NVIC is to ensure that higher priority interrupts are completed before lower-priority interrupts, even if the lower-priority interrupt is triggered first. For example, if a lower-priority interrupt is being registered* or executed and a higher-priority interrupt occurs, the CPU will stop the lower-priority interrupt and process the higher-priority one first.

* A register is a special, dedicated memory circuit within the CPU that can be written and read much more quickly than regular memory. The register is used to store information such as calculation results, CPU execution states, or other critical program information.

Similarly, a handling scheme referred to as “tail-chaining” specifies that if an interrupt is pending while the ISR for another, higher-priority another interrupt completes, the processor will immediately begin the ISR for the next interrupt, without restoring its previous state.

With tail-chaining, if an interrupt is pending as the ISR for another, higher-priority interrupt completes, the processor will immediately begin the ISR for the next interrupt, without restoring its previous state.
Image credit: Arm Limited

The term “vector” in nested vector interrupt control refers to the way in which the CPU finds the program, or ISR, to be executed when an interrupt occurs. Nested vector interrupt control uses a vector table that contains the addresses of the ISRs for each interrupt. When an interrupt is triggered, the processor gets the address from the vector table.

The prioritization and handling schemes of nested vector interrupt control reduce the latency and overhead that interrupts typically introduce and ensure low power consumption, even with high interrupt loading on the controller.

Feature image credit: Arm Limited

What is Space Vector Pulse Width Modulation (SVPWM)?

March 14, 2019 By Danielle Collins Leave a Comment

One common, modern method for controlling three-phase induction motors and permanent magnet synchronous motors (aka BLAC or PMAC motors) is field oriented control, which independently controls the magnetizing and torque-producing components of the stator current. This allows the torque-producing component to be kept orthogonal to the rotor flux, maximizing torque production.

Space vector pulse width modulation (SVPWM) is a technique used in the final step of field oriented control (FOC) to determine the pulse-width modulated signals for the inverter switches in order to generate the desired 3-phase voltages to the motor.

Field oriented control with space vector pulse width modulation.
Image credit: Texas Instruments

The following is a summary of how FOC with SVPWM operates:

1) Measure two of the three motor phase currents and feed them into a Clarke transform to convert them from a three-phase system (i_a, i_b, i_c) to a two-dimensional orthogonal system (i_α, i_β). Note that it’s not necessary to measure all three currents, since the sum of the three must equal unity (0). So the third current must be the negative sum of the first two.

2) Apply a Park transform to convert the two-axis stationary system (i_α, i_β) to a two-axis rotating system (i_q, i_d), where the d axis current is aligned to the rotor flux and the d axis current (the torque-producing component) is orthogonal to the rotor flux.

3) The stator current flux and torque are controlled independently, typically by PI controllers. Voltages to be applied to the motor, V_dand V_q, are determined from the PI controllers.

4) Next, an inverse Park transform converts the two-axis rotating system (V_sqref, V_sdref) back to a two-axis stationary system (V_s_αref, V_s_βref). These are the components of the stator voltage vector and are the inputs for the SVPWM, which generates the 3-phase output voltage to the motor. (Note that the use of SVPWM eliminates the need for an inverse Clarke transform to obtain the three-phase output voltages.)

Field oriented control transforms a three-phase system time-dependent system into a two-coordinate (d and q), time-invariant system. SVPWM is used in the final step to determine the PWM signals to be applied to the motor.
Image credit: Freescale Semiconductor

The details behind SVPWM

Voltage is delivered to the motor by a three-phase inverter with six transistors (two on each leg of the output). Each of the three outputs can be in one of two states (top transistor closed and bottom transistor open, or vice-versa), giving eight (2³) total states for the output. These are referred to as base vectors.

For each leg of the inverter output, either the top transistor will be open and the bottom closed, or vice-versa. Therefore, there are eight total states (2³) for the inverter output.
Image credit: switchcraft.org

The eight base vectors are plotted on a hexagonal star diagram. Each vector makes up a spoke of the star, with 60 degrees phase difference between adjacent vectors. The two vectors (V₀ and V₇) that contain outputs which are either all plus or all minus are referred to as null vectors and are plotted at the center (origin) of the star.

Each base vector makes up one segment of a hexagonal star, with the null vectors (V₀ and V₇) plotted in the center.

The goal of SVPWM is to produce a “mean vector” during the PWM period (T_PWM) that is equal to the desired voltage vector (V_out).

The location of V_out is determined on the star diagram, and the base vectors that constrain that sector (V₁ and V₃, for example), along with one of the null vectors, are used to synthesize the desired voltage. This is done by applying V₁for a specified time (T₁), V₃for a specified time (T₃), and the null vector for the amount of time necessary (T₀) to provide a resultant vector equal to V_out.

The values of T1, T3, and T₀ can be determined with the following equations:

The simulation of V_out can then be expressed as:

Compared to standard field oriented control, using FOC with space vector pulse width modulation enables more efficient use of the DC supply voltage, provides lower harmonic distortion — which improves power factor — and reduces torque ripple.

Dorner Latin America Now Manufacturing at New Facility in Mexico

March 11, 2019 By Danielle Collins Leave a Comment

Dorner’s new manufacturing facility in Guadalajara, Mexico (Dorner Latin America) enables the company to build and deliver belt and flexible chain conveyor systems more quickly and efficiently for customers throughout Latin America.

Dorner is an industry leader in the design, application, manufacturing and integration of precision industrial and sanitary conveyor systems.

The new facility, staffed with experienced employees, is manufacturing Dorner’s popular 2200 Series low profile belt conveyor, which is ideal for assembly, light manufacturing, and other industrial applications. Additionally, the facility is also manufacturing FlexMove flexible chain conveyors, which are designed for pharmaceutical, paper converting, and consumer goods packaging applications. All Dorner conveyors and equipment will be supported through the Guadalajara customer service and engineering teams.

“We have seen tremendous growth in Mexico and Latin America. We’re excited to open a new manufacturing facility and continue expanding our team in Guadalajara so we can provide better and faster support for our customers,” said Dan Nasato, Vice President – International, Dorner.

Dorner – Latin America is located at:

Dorner Latin America, S. de R.L. de C.V.

Carretera a Nogales #5297, Nave 11

Fracc. Parque Industrial Nogales

Zapopan, Jalisco C.P. 45222 México

To contact Dorner – Latin America, call +52.33.30037400 or email: info.latinamerica@dorner.com.

What are pallet conveyors? (Not to be confused with pallet-moving conveyors)

March 8, 2019 By Danielle Collins Leave a Comment

Pallet conveyors transfer discreet products on carriers referred to as “pallets,” typically transported by belt, roller chain, flat-top chain, or for extremely high loads, powered rollers.

For the heaviest loads, in applications such as appliance assembly and automotive manufacturing, pallets are transported by rollers.
Image credit: Bosch Rexroth Corporation

Although they’re typically offered in standard sizes, pallets can be tooled with custom fixtures to locate and secure the product on the pallet.
Image credit: Glide-Line

Pallet sizes are, for the most part, standardized among manufacturers in metric dimensions (240 x 240 mm, for example), although some manufacturers offer pallets in inch or non-standard metric dimensions. In many cases, conveyor pallets are tooled with custom fixtures to locate and secure the product on the pallet. So not only can the pallet be accurately positioned on the conveyor, but the product can be precisely located on the pallet. This makes pallet-based conveyors the ideal solution for high-precision assembly, machining, inspection, and positioning tasks.

Note that the pallets discussed here are different from the pallets used for storing and handling bulk goods, which are often made of wood or plastic. Similarly, some roller- and chain-driven conveyors are marketed as “pallet conveyors,” for the transportation of these wooden or plastic pallets through a factory or warehouse.

Pallets can be transported around corners without changing the pallet orientation.
Image credit: Dorner Mfg. Corp.

Pallet conveyors are suitable for transporting products in two dimensions, but can accommodate small elevation changes. Curved sections — typically from 15 to 180 degrees — allow conveyors to change the direction of transport while maintaining pallet orientation. Conveyor modules referred to as “lift and rotate” units allow the orientation of the pallet to be changed while maintaining the direct of transport. Pallets can also be lifted and precisely located, for positioning at a workstation or inspection area, where pallet conveyors allow access to the workpiece from more than one side. For access to the bottom of the workpiece, pallets can be made with an open center.

The use of pallets to move individual products allows non-synchronous movement, so each product can be transported and routed independently. Because the pallets are precisely machined, they can be accurately positioned and located via sensors, and RFID tags or other data carriers can be attached to each pallet to track and document individual product movements. Pallets can be diverted and merged for offline operations, and accumulation allows products to be buffered, smoothing the flow of downstream operations.

Here, pneumatic cylinders are used to stop and lift the pallet. Precise locating is achieved by pins that mate to bushings on the pallet.
Image credit: mk North America, Inc.

Most pallet conveyors — especially those used in automation applications — are modular, based on standard conveyor sections, legs, and lift/transfer/rotate modules that can be easily reconfigured or expanded as needs change. And because components are standardized, pallet-based conveyors can be configured to meet a wide range of environmental requirements, including cleanroom, dry room, washdown, and ESD-compatible applications.

Feature image credit: Dorner Mfg. Corp.