Basics | Design World Motion Handbook

What is DC injection braking and how does it compare with other methods?

November 15, 2018 By Danielle Collins Leave a Comment

Many variable frequency drives (VFDs) include DC injection braking functionality.
Image credit: Galt Electric

AC induction motors excel at driving high-inertia loads at high speeds — in fans, saws, conveyors, and mills, for example — and variable frequency drives make it possible to match the motor speed to the application requirements by varying the frequency of the supplied voltage.

But when quick stops are needed — whether for safety reasons or to prevent wasted time and lost productivity waiting for the load to come to a standstill — high speeds and high load inertia make it difficult to brake the motor and stop the load.

In an AC induction motor, three-phase AC power is fed to the stator windings, creating a magnetic field that rotates. This magnetic field induces a current and a corresponding magnetic field in the rotor. The interacting magnetic fields of the rotor and stator cause the motor to turn.

DC injection braking works just as its name implies — by injecting DC voltage into the motor windings. Before DC injection braking can begin, the AC power to the motor stator must be disconnected — typically by the opening of a relay. In turn, a relay controlling motor braking closes, allowing DC voltage — sourced by the DC bus — to be applied to the windings. To prevent the braking current from exceeding the drive and motor ratings, circuitry in the VFD or in an external braking module controls the amount of voltage applied.

DC injection braking begins when AC voltage is removed (S1 opens) and DC voltage is injected into the windings (S2 closes).
Image credit: KB Electronics, Inc.

When DC current is applied to the motor windings, it creates a fixed (rather than rotating) magnetic field. Braking action is produced by the rotor working to align to this stationary field. The higher the DC current, the stronger the braking force.

The energy generated during braking is dissipated as heat by the motor (particularly the rotor) and the controller, so thermal limitations of these components dictate how much braking current can be applied and the amount of time for which it can be applied before overheating occurs. If DC injection braking is used frequently, the additional heating created during braking must be taken into account when sizing the motor.

Although DC injection braking can be used to hold a load stationary, it’s not generally recommended due to heat generated in the motor. And because DC injection brakes require a constant power supply, they’re not considered fail-safe devices.

DC injection braking is just one of several electrical methods of bringing an AC induction motor to a stop. Two other forms of braking — dynamic braking and regenerative braking — convert mechanical energy generated as the rotor slows down into electrical energy.

During motor starts and stops, or when the load is overhauling, an AC induction motor functions as a generator and produces electrical energy from mechanical energy. This electrical energy can be fed back to the AC power source.
Image credit: Bonitron

In a dynamic braking system, this electrical energy is released as heat through a voltage regulated resistor. In regenerative braking, this electrical energy is fed back to the power supply or to a common DC bus. Dynamic braking is best for applications that require only periodic braking without high heat dissipation demands, while regenerative braking is best for applications with frequent stops and starts or overhauling loads.

A drawback of both dynamic braking and regenerative braking is that the braking force decreases as the motor (rotor) slows down. DC injection brakes are often used in conjunction with dynamic or regenerative braking systems to provide the final braking power required to fully stop the motor and load. And because DC injection brakes can cause thermal problems for the motor and drive, the use of dynamic or regenerative braking in the beginning, together with DC injection braking at the end of the stop, reduces the risk of overheating while providing fast, complete stopping for high-inertia, high-speed AC induction motor applications.

Geared stepper motors drive movements of Breakfast Brixel moving architectural wall

November 13, 2018 By Lisa Eitel Leave a Comment

A new architectural wall called the Breakfast Brixel is making some installations more dynamic. The system is built from dozens of digitally controlled rotating bricks that act as pixels to create waves, stripes, and undulating shapes on the moving walls. Brixel installations can even run customizable artwork and facades to evolve in response to real-time data and software commands.

Breakfast engineers designed everything from scratch — PCBs included — and employ geared stepper motors to rotate the bricks.

In addition, the center support shaft of each Brixel column allows each brick to spin endlessly in either direction without tangled wires or the use of slip rings.

“A computer runs a fully featured application in the background. It sends data to the Brixels with movement commands that include speed and direction. We can also set custom acceleration and deceleration curves at any time on the fly when the Brixels are moving. This lets us create beautiful and organic displays — such as wind animations,” explained Breakfast CTO and cofounder Mattias Gunneras.

For high-level programming of animations and interactive movements, controls were dynamically scripted was Python, OpenCV-Python, and Numpy. In addition, for Brixel Silhouette — a Brixels mode that causes the wall to rotate bricks to mirror the shapes and movements of people in front of the installation — Breakfast engineers used 3D simulation for testing through a VR headset. That let the team get a feel for the displays before a prototype physical installation was ready.

“We’re using stitched-together 3D depth cameras to create a point cloud of the scene in front of the installation,” said Gunneras. With this technique, we can create the mirroring effect to map a person … regardless of how close they stand to the installation.”

Breakfast Brixel walls are customizable to work as facades, railings, dividers, and fences. The design allows an unlimited number of rows and columns.

When there’s nobody standing in front of the installation, kinetic animations and relevant information can play across the installation in the form of letters created by positive and negative colors and space.

Brixel walls can display an array of digitally programmed visuals and movements. The bricks come in an array of sizes, shapes, materials, and colors.

A Linux controller sits at the heart of each installation to execute programs that process visual data and pass high-level data to the bricks. Data goes via RS-485 connections to controller PCBs at the bottom of each column. Each controller PCB then sends the data over serial lines up the columns to each individual brick. Brixel installations are controllable and updated via a web-based app running off the Linux controller — for access from any mobile device. Users can upload content, create playlists, and set schedules.

HMI simplifies setup of robot-assisted automation for one integrator

November 12, 2018 By Lisa Eitel Leave a Comment

Integrator Inovatech Automation recently decided they were spending too much time programming expensive HMI and PLC platforms, so contacted Pro-Face America for help.

The integrator needed an HMI capable of universal robot and I/O-Link integration, legal for Inovatech Automation branding, and easy to program.

“I was apprehensive at first because we have a lot of projects going … so timing is short,” said Inovatech director of operations Travis Buset. “More specifically, I had concerns about learning something on the fly — and feared the onboarding experience and programming were going double our time investment.”

Pro-Face America supplied its LT4000M HMI plus Control unit. This hardware reduces costs by eliminating the need for an external PLC. In addition, for Inovatech Automation the LT4000M halved engineering time for configuration; offered easy integration to a Universal robot and IO-Link network; and was delivered with a custom overlay for machine-branding purposes.

Inovatech uses Pro-Face America’s LT4000M HMI plus Control unit on its modular automation station.

Trimming the time for setup was the most important benefit, though.

“Another platform needed two different pieces of software. In contrast, Pro-Face uses one software with drag-and-drop functionality. What once took me 32 to 40 hours to program now takes eight hours on the Pro-Face.” In fact, Buset had one programmed in eight hours his first time trying.

“This a powerful unit and well worth its cost for the level of configuration it allows.” Inovatech estimates it’s now operating 75% more efficiently than before.

Conveyors with direct-drive motors offer key advantages to Pa. facility

November 11, 2018 By Lisa Eitel Leave a Comment

American Axle Manufacturing (AAM) is increasing the functionality of its St. Mary’s, Pa. manufacturing operations with conveyors sporting advanced drive technology. Small yet powerful direct-drive motors from Electric Torque Machines actuate Dorner conveyors that move small parts around the AAM facility.

AAM designs, engineers, validates, and manufactures an array of driveline, metal-forming, powertrain, and casting technologies — and is known for manufacturing large components such as axles and differentials. However, the AAM St. Mary’s facility manufactures many of the small and lightweight parts that go into AAM drivetrains. Transporting these parts from station to station and within workcells requires small and precise conveyors. The challenge is that the compactness of such conveyors is degraded by the bulk of traditional gearmotors. That’s what prompted AAM to specify conveyors driven by Dorner’s Universal Drive offerings — with direct-drive motors from Electric Torque Machines at their core.

Dorner Conveyor and Electric Torque Machines helped improve the material handling setup inside the Pennsylvania facility of American Axle Manufacturing.

The direct-drive motors have high continuous torque density (particularly at lower operating speeds) to eliminate the need for a bulky gearbox. Integrated into Dorner’s 2200 Universal Drive — which includes the motor, configurable motor mount, and controller — they reduce the conveyor-drive size by nearly 70% while boosting functionality and maintaining required output torque.

“We chose Dorner’s Universal Drive for its unique motor,” said AAM facilities engineer Michael Vandervort. “The motor is smaller and lighter than traditional ac motors … and can run in designated working areas unguarded, because the motor runs cool. In contrast, traditional ac motors are too hot to touch.”

ETM’s direct-drive motor imparts advanced functions to simplify automated tasks.

Vandervort also cited the wide speed and torque ranges of the Dorner Universal Drive as key benefits. “The drive lets us use the conveyors for applications such as slow pass-under vision tasks; heavy accumulation tasks; and high-speed part transfers.”

The on-conveyor inspection function is especially challenging. “At our facility, we use a lot of small conveyors to move light loads. Electric Torque Machines developed motor tuning functions to help stabilize our vision imaging on pass-under applications with very low torque requirements. We really appreciate ETM working with us and developing solutions for our specific needs,” Vandervort added.

“It was energizing to work with industry leader AAM — and encouraging to see the industry ready to adopt new technologies. AAM’s application illustrates the advantages of direct-drive motors,” added ETM V.P. of engineering Scott Reynolds.

Digital printing pairs with a machine capable of handling its varied output

November 11, 2018 By Lisa Eitel Leave a Comment

VITS International makes sheeting and finishing machines for packaging, printing, and converting. The OEM saw the digital-printing market needed finishing capable of commercial production speeds. So VITS designed and built the SPRINT Variable Data Finisher with a Bosch Rexroth drive and control platform to meet the demand.

This is a VITS Variable Data multi-web finisher.

Digital inkjet printers can execute output comparable to that of traditional web-offset printing — at near-commercial rates to 1,000 feet per minute. Digital printing also allows variable data printing to dynamically vary the number of pages and content on the pages — even handling significantly varied page dimensions. Such capabilities enable incredible customization but complicates the finishing of printed web. So finishing systems on digital printing lines must be able to quickly cut, collate, and assemble disparate pages into final readable pieces.
“We decided to develop such a finishing machine to support both offline and inline finishing,” said VITS International president and CEO Deirdre Ryder. The result is VITS’ SPRINT Variable Data Finishing System that turns variable print material into finished product at up to 1,500 fpm. The SPRINT machine works for inline systems (which get and finish a single web directly from the digital printer) and offline multi-web finishing (for processing multiple webs into a single finished signature or book).

“Our printing customers want to output larger products at faster rates of speed,” said VITS director of product applications Kim Markovich. “Particularly for direct mail, being able to finish multiple webs and ribbons with suitable register control means printers can assume more work.” SPRINT finishing machines include independently driven axes controlled by a Rexroth IndraMotion MLC motion control system. A Bosch Rexroth L45 controller employs Sercos III for deterministic synchronization of drives and fieldbus communication for I/O.

The inline machine version usually has 10 to 12 driven axes; multi-web offline versions can have up to 30. A Rexroth cross-communication card with Sercos-III-based communication helps synchronize VITS finishers with inline-mounted printing presses (also sporting Rexroth controls). Machine modules trigger various functions to convert printed web into books or mailers with Rexroth IndraDrive servo drives and IndraDyn servomotors for actuating the functions.

Offline multi-web machines interface with paper rolls on register splicers. These continuously feed web at a constant-tension infeed with precise gain and control to prevent issues.

The paper goes through a station that slits it in half and stacks the halves.
The paper halves travel to a ribbon-gathering station and onward to a station that folds the ribbons in half.
The paper web goes through a shear-slitting station for edge trimming.
Finally, the web goes through a variable-data rotary cutter that slices each page to size and collates finished product for the final binding or saddle stitching.

Maintaining registration for all these steps is core to ensuring all pages in all signatures are cut to the same dimension. Nothing prints perfectly, so the length of any print usually varies ±0.001 in. from page to page … which is an error that can accumulate quickly over hundreds of fast-moving pages.

“We use the built-in intelligence of Rexroth drives to dynamically maintain web registration — and keep the central controller’s processing power free,” said integrator Bruce Parks of Parks Consulting, who helped on the machine design. A virtual master holds all drives in synchronization. Tension zones between drives maintain tension during web splitting, merging, and cutting. Controls can also command groups of drives to adjust tension … and bring the web into register with the virtual master.

SPRINT Finishing machines use the Rexroth IndraMotion for Printing automation platform with independently driven axes under a central IndraMotion MLC motion controller (similar to the one here).

SPRINT engineers used the IndraMotion for Printing version of Rexroth’s MLC system to automate tasks via IEC 61131-compliant motion logic and PLCopen function blocks — and software libraries specific to printing and converting. “IndraMotion for Printing includes tools for most web-handling tasks. These and Rexroth’s PLCopen function blocks helped us build functions and camming profiles for our proprietary Clear Channel registration,” said Parks. Clear Channel registration control allows the cutting thousands of pages per hour while maintaining precise page registration impossible with legacy finishing systems. It enables faster size changes as well as cut tolerances impossible with standard finishing systems.

Another strength of the SPRINT machine is its rotary cutter … capable of handling images of 5 to 25 in. and configurable to cut different width chips — blank spaces between pages on the paper. Machine operators trigger the change with a push of a button. At the cutter station, twin knives slice the chip out; the knives are spaced by the width of the chip … and cutting must be synchronized with the speed of the web through the station. “Our camming lets users cut output of variable size with multiple knives and still maintain chip size — because the camming always synchronizes with the web speed through the cutting zone,” said VITS director of new product development John Salamone.

This is the first machine series VITS built on a Bosch Rexroth control and drive platform. “We had a great relationship with the previous supplier. Bosch Rexroth gave us similar commitment … and Rexroth technology let us quickly develop equipment with accuracy no competitors have. So now Rexroth-equipped machines of ours working flawlessly all over the world.

PC-based control on industrial sheet-metal presses — a Casebook example

November 11, 2018 By Lisa Eitel Leave a Comment

Sheet-metal workcells can deliver higher-quality parts in greater quantities than traditional builds when designed with centralized PC-based control and EtherCAT connectivity. That’s especially true of deep-drawing press applications (shown below) where modular hardware and software from Beckhoff maximize throughput and minimize tolerances.

To review, deep drawing is the process by which sheet metal is placed between two halves of a forming die and mechanically pressed via punch into a shape. Such drawing is deep drawing where the new shape’s depth is more than its diameter.

Sheet-metal workcells (such as the deep-drawing presses shown here) get higher throughput with centralized PC-based control and EtherCAT connectivity. Modular hardware and software from Beckhoff also help minimize tolerances. Press-specific software tools include a Beckhoff TwinCAT PLC Hydraulic Positioning library for implementing efficient servo hydraulics. TwinCAT Analytics leverages comprehensive data acquisition to support condition monitoring for minimal machine downtime.

Press-specific software tools include a Beckhoff TwinCAT PLC Hydraulic Positioning library for implementing efficient servo hydraulics. In addition, TwinCAT Analytics leverages comprehensive data acquisition to support condition monitoring for minimal machine downtime. The software supports OPC UA, MQTT, and AMQP standards for IIoT scalability and secure Cloud communication. Avoidance of data incompatibility and latencies (as those in communications between different systems) allows synchronization of press axes and processes — and trims the costs associated with extraneous hardware and engineering.

On the hardware side, Beckhoff Compact C6015 and C6017 Industrial PCs serve as universal control and visualization platforms or (in some setups) as IoT edge devices. (Precision synchronization makes for better workpiece quality with fewer reject parts.) Within the press itself, actuation with servo-electric hybrid axes leverages the fast motion cycles and simple mounting requirements of hydraulics while improving on their baseline efficiencies and need for maintenance. Beckhoff’s dynamic servo-drive solution consists of the AX5000 servo drive and AM8000 servomotor with One Cable Technology — controlled via the aforementioned TwinCAT PLC Hydraulic Positioning software.

For material handling into the press and machine tending, AMP8000 distributed servo drives integrate amplifier electronics directly into servomotors, allowing engineers to build modular workcells. Putting power electronics in the machine makes control cabinets smaller — to a single coupling module for supplying the workcell’s servo drives with one cable via a distribution module.

How to select gear ratio for a gear reducer?

November 9, 2018 By Miles Budimir Leave a Comment

This is the first of a multi-part series on gear reducers. In this first part we look at selecting the proper gear ratio for a gear reducer.

The information here is supplied by Bodine Electric Company.

Gear reducers are an essential part of mechanical power transmission systems, being used for speed and torque conversion. Selecting the right one for an application is a fairly straightforward process. The first step in selecting a gear reducer is to know the required torque and speed as well as the most suitable type of motor to use. Then, it can be determined if a gear reducer is needed in the particular case. If so, then the next step is to select the proper type and ratio.

A cutaway of a gear motor showing the internal worm gearing. (Image via Bodine Electric Company)

So what does a gear reducer do? The most common benefits are that they can reduce speed, multiply torque, and solve inertia mismatching issues. For instance, to take speed reduction, the amount needed will depend on the type of motor used. That’s because some motors can operate at low speeds (less than 1,000 rpm, for instance) without the need for a gear reducer, while others cannot.

For speed reduction the gear ratio is calculated using the equation:

G = N_MOTOR / N_LOAD

where G is the gear ratio, and N is the angular velocity of the motor and the load.

The way gear reducers work is that they essentially multiply the output torque of the motor. So for instance, a small motor with a gear reducer may be less expensive and smaller than a larger motor without a gear reducer producing an equal amount of torque.

To calculate the required gear ratio when torque multiplication is needed, use the equation:

G = T_LOAD/ [ T_MOTOR x e_GEARS ]

where G is the gear ratio, T is torque and e is efficiency.

Another function of gear reducers is to reduce the reflected load inertia to the motor by a factor of the square of the gear ratio. For most motion applications but especially high performance ones, the ideal condition is for the reflected load inertia to equal the motor inertia.

To calculate the gear ratio when inertia matching is the main concern, take the square root of the ratio of the load inertia to the motor inertia as in the equation:

G= sqroot [ J_LOAD / J_MOTOR ]

where G is the gear ratio and J is the inertia of the motor and load.

In part 2, we’ll look at the different types of gear reducers and how to select the best one.

What do IP ratings really mean, and how do they differ from NEMA ratings?

November 9, 2018 By Danielle Collins Leave a Comment

Image credit: Kollmorgen

Electrical enclosures, such as those for motors, sensors, and HMIs, are often given what is known as an IP rating, consisting of the two letters, “IP,” followed by two numeric digits — IP65, for example. This IP rating indicates the amount of protection the enclosure provides against the ingress of solid particles and liquids.

Unlike other designations that are often applied to industrial equipment, such as “high torque,” or “waterproof,” IP ratings are specific and internationally accepted. Both the rating system and the testing process are defined by the International Electrotechnical Commission in standard IEC 60529.

The initials “IP” are often taken to mean “Ingress Protection,” but according to IEC 60529, the correct term is “International Protection.”

Decoding the IP rating system

The first numeric digit in the IP rating specifies the protection level against foreign objects, based on the size of the object (12 mm versus 1 mm, for example). The higher the number, the better the protection, with the maximum rating being 6, which indicates a dust-tight enclosure. An enclosure that meets a stated degree of protection against solid objects must also meet all the lower degrees of protection. For example, an enclosure rated IP4x must also meet IP3x, IP2x, IP1x, and IP0x requirements.

The second numeric digit specifies the protection level against the ingress of liquids, based on the amount of liquid and the direction of its approach to the object. The test to determine protection against liquids is time-based, and where applicable, includes specification of nozzle size, distance from object, and delivery pressure.

Image credit: Techathlon

It’s important to note that for protection against liquids, an enclosure must meet all the protection requirements below its rated designation, up to and including level 6 protection. Enclosures rated IP-7 and IP-8 do not have to comply with IP-5 and IP-6 requirements for water jet exposure.

If a device has not been tested or rated for one of the criteria, that digit is replaced with an “X.” For example, IPX4.

An important extension of the IP ratings comes from the DIN standard 40050-9 (which has been rolled into the ISO standard 20653:2013). This DIN standard adds an additional letter, “K,” to indicate environmental sealing. This additional specification applies only to IP6- ratings, which means the device must be dust-tight before the environmental sealing (“K”) designation can be applied.

An IP6-K rating, such as IP69K, means the device can withstand high-pressure cleaning and steam cleaning, with the test consisting of 80° C water delivered at pressures of 8 to 10 MPa (80 to 100 bar) at a rate of 14 to 16 L/min, with the nozzle located 10 to 15 cm from the device at angles of 0, 30, 60, and 90 degrees for 30 seconds each. Food processing and pharmaceutical applications often require electrical equipment that meets IP69K standards, due to the use of high-temperature, high-pressure liquids and steam for cleaning.

How are NEMA enclosure standards different?

NEMA — the National Electrical Manufacturers Association — also provides a standard, referred to as NEMA 250-2014, that defines an enclosure’s ability to provide environmental protection.

While the NEMA and IEC standards have the same general purpose, the protection requirements and test standards specified by NEMA 250 are, in some cases, more rigorous. For example, the NEMA 250 standard addresses environmental conditions such as corrosion, icing, oil, and coolants — none of which are addressed by the IEC 60529 standard.

Protection ratings in the NEMA 250 standard are categorized by whether the equipment is constructed for indoor or outdoor use, and whether it is applied in non-hazardous or hazardous locations. For example, NEMA enclosure Types 1, 2, 4, 5, 6, 12, and 13 are applicable to enclosures constructed for indoor use in non-hazardous locations. Types 3, 4, and 6 apply to enclosures constructed for outdoor use in non-hazardous locations. (Note that there is some overlap between indoor and outdoor use in Types 4 and 6.) Types 7, 8, 9, and 10 apply to enclosures in hazardous locations.

The table below is a simplified description of the NEMA enclosure ratings. For the full definitions and specifications, see the NEMA Enclosure Types document.

NEMA Enclosure Types Guide
Image credit: Marshall Wolf Automation

Can IP and NEMA enclosure ratings be cross-referenced?

In some cases, it is possible to convert NEMA Type ratings to IEC IP ratings, but because NEMA ratings include requirements not covered by IEC, it’s not possible to convert an IP rating to a NEMA Type rating, as the IP rating would not cover the same conditions as the NEMA rating.

The chart below, provided by NEMA, can be used as a guideline for converting NEMA Type ratings to IEC IP ratings.

A shaded block in the “A” column indicates that the NEMA Enclosure Type exceeds the requirements for the respective IEC 60529 IP First Character Designation. The IP First Character Designation is the protection against access to hazardous parts and solid foreign objects.

A shaded block in the “B” column indicates that the NEMA Enclosure Type exceeds the requirements for the respective IEC 60529 IP Second Character Designation. The IP Second Character Designation is the protection against the ingress of water.

Feature image credit: Unmanned Systems Source

What are precision-link conveyors? Summary of one type of transfer system

November 6, 2018 By Lisa Eitel Leave a Comment

Precision-link conveyors index components with high accuracy, speed, and quality to give manufacturers a way to make assemble, mark, weld, and manufacturing operations more efficient.

By Marc Halliburton • Engineering manager | Motion Index Drives Inc.

Conveyor designs abound, and the last decade has brought more use of application-specific offerings — especially for the subcategory that industry calls transfer systems. One such conveyor is the precision-link conveyor — for assembly, marking, welding, as well as mechanical and optical inspection of parts. Standard precision-link conveyors deliver standard link-positioning accuracy of ±0.08 mm (±0.003 in.) or better.

Motion Index Drives (MID) offers many variations of its LFA Precision Link Indexing Conveyors. The conveyors come with any number of different fixed strokes; completely programmable servo-driven versions are also possible. For highly complex precision-link indexing configurations, MID works with design engineers to devise custom arrangements.

While there are belt-driven versions, many of these precision-link conveyors consist of one continuous run of high-precision drive chain. Then each chain link (often aluminum) contains four cam followers that guide the link along hardened and fine-milled guide rails. In some cases, the cam followers are sealed and lubed for life.

Dowels and bearings complete the chain assembly: The cam followers are pressed into needle bearings at the link joints to eliminate wear and prevent moving parts from contacting the links themselves.

Shown here is a link and section of track.

Each chain link also includes tapped holes and dowel holes to accommodate individual pieces of product to be transported and otherwise machined or worked. Link fixtures can either interface with station tooling installed inboard or externally.

Typical precision-link conveyor frames are made of extruded aluminum frame and steel plates; the link track is machined, hardened, and ground steel. Some advantages of aluminum-extrusion frames is that they are very straight over long distances, are easily machined, and allow for easy mounting of accessories.

More on conveyor versions with aluminum components

Precision-link conveyors with an inner structure of aluminum extrusion offer distinct benefits. The extrusions allow long straight conveyor runs that maintain rigidity. Such structure also allows easy customization and mounting of accessories.

Links made of aircraft-grade aluminum also offer benefits. Aluminum links have a mass that’s a third of that of steel — for inertia that’s also a third that of steel links. That lets engineers use a smaller drive unit (whether index drive or high-accuracy gearmotor) or add heavier tooling without having to oversize the drive unit or compromise on indexing speeds.

Precision-link conveyor drives and customization

Driving the chain on a precision-link conveyor is either a customized drive based on a servomotor or the hardened cam wheel of a standard indexer. If the latter, one indexer cycle can spur any number of link stroke lengths … ranging from a few millimeters to a meter. No matter the drive, most precision-link conveyors are setup for long dwell times and quick indexing.

At the non-drive end of the conveyor circuit, a hardened and preloaded cam guides the chain and provides takeup to prevent backlash at the link connections. Common precision-link conveyor arrangements make runs that include two straight runs with 180° turns at the cam guide end and cam wheel end (if the drive is based on an indexer design). That said, variations include kidney-bean-shaped tracks, round tracks, and vertical variations — those that recirculate links with empty fixtures back to a start position on the underside of the conveyor as a way to minimize overall footprint.

This Motion Index Drives (MID) custom conveyor has a unique kidney-bean geometry and 37.5-mm pitch.

Customized precision-link conveyors are manufactured to end-user requirements. With some manufacturers, nonstandard requests receive support beginning with the project design phase — for final conveyors that may include specialty drive units, tailored frame shapes, custom pitches and strokes, tapped holes, mounting plates, through holes in the chassis, and custom coatings on the components. Here, another option is custom link cover plates to prevent particulate ingress into the track and eliminate pinch points around curved conveyor ends.

Aluminum-extrusion frames simplify the mounting of accessories on precision-link conveyor systems.

Where precision-link conveyors excel

Precision-link indexing conveyors are most suitable for high-speed assembly of components — especially when there are high part counts or complicated assembly processes. Work processes are usually performed along the conveyor’s linear sections. Most useful for manufacturing tasks for aerospace, solar and wind energy, automotive, defense, electronics, consumer goods, and medical devices, precision-link conveyors are fast and accurate in positioning to allow assembly (and other tasks) on up to 150 parts per minute. That’s because whether driven by a servomotor or cam-controlled indexer, precision-link conveyors advance the fixtures precisely and without the need for additional positioning systems to locate fixtured parts before processes are initiated.

For example, these conveyors speed the manufacturing of dental floss and razor blades and other consumer goods with intricate assemblies. They also help make printing and marking applications faster (while maintaining image quality).

Typical precision-link conveyor operation

Precision-link conveyors allow automated processes right on the conveyor. After each indexed stroke (to advance the links) equipment mounted around the conveyor executes parallel and serial tasks. Most installations include multiple work stations along one or both long conveyor axes.

Cover plates shield the conveyor from particulate ingress and boost safety by enclosing pinch points.

So unfinished products are mounted to the conveyor one time and then removed from the end of the conveyor — usually as completed product. In other words, precision-link conveyors are usually designed and engineered to be core product-positioning chassis of machines executing critical functions.

Precision link conveyors are designed to run high-volume manufacturing to millions of cycles without intensive maintenance. Typical maintenance involves tension checks and adjustments of the chain … and cleaning of the conveyor if it runs in a dirty environment.

Precision-link conveyors are unsuitable for feeding parts to a line, where a process is not being done on them. This is because their accuracy is not required. Similarly, due to their cost, they’re also inappropriate where application accuracy isn’t critical.

Motion Index Drives | www.motionindexdrives.com

Link fixtures often include geometry to accommodate specific products.

What is an LVDT (linear variable differential transformer)?

November 3, 2018 By Danielle Collins Leave a Comment

A linear variable differential transformer (LVDT) is an absolute measuring device that converts linear displacement into an electrical signal through the principle of mutual induction. Its design and operation are relatively simple, providing extremely high resolution in a device suitable for a wide range of applications and environments.

The main components of an LVDT are a transformer and a core. The transformer consists of three coils — a primary and two secondaries (S1 and S2) — wound on a hollow form, which is typically made of glass-reinforced polymer. The coils are arranged such that the primary coil is located between the two secondary coils, which are symmetrical and wound in series but in opposite directions (referred to as series-opposed winding).

The core is made of magnetically permeable material and moves freely inside the bore of the transformer. A non-ferromagnetic shaft, or push rod, is coupled to the core and connects to the object being measured.

Image credit: Honeywell International

When voltage is applied to the primary winding, the resulting flux is coupled to the secondary windings by the core. This induces voltages (often denoted E1 and E2) in each of the secondary windings. The differential voltage in the secondary windings determines the distance moved, and the phase of the voltage indicates the direction of movement.

Here’s how it works…

The series-opposed winding of the secondary coils means that when the core is at the center of the transformer (equidistant between the two secondary coils), the induced voltages have equal amplitude but are out of phase by 180 degrees. Thus, the induced voltages cancel each other, and the output voltage is zero. (This is often referred to as the null point.)

When the core moves to one side — toward S1 for example — the secondary coil on that side becomes more strongly coupled to the core, causing the induced voltage (E1) of that secondary to be higher than the induced voltage (E2) of the opposite secondary (S2). The differential voltage output (E1 – E2) determines the amount of movement.

The amplitude of the differential output voltage determines the distance traveled, and the phase of the output voltage indicates the direction traveled.
Image credit: TE Connectivity

The induced voltage (E1) of the first secondary coil is in-phase with the primary voltage, indicating the direction of movement. Conversely, when the core moves to the other side of the transformer, the induced voltage (E2) of that secondary coil is out of phase with the primary voltage, indicating movement in the opposite direction.

The output of an LVDT is a direct and linear function of the input across its specified measuring range, although linearity falls off as the core reaches or exceeds the ends of its range. However, it is possible to use an LVDT beyond its specified range, in some cases, with a pre-defined table or polynomial function to compensate for the nonlinearity.

LVDT output is a direct, linear function of the input voltage up to the device’s specified measuring range. Beyond this point, linearity begins to fall off.
Image credit: TE Connectivity

An LVDT contains no electronics, but external electronics — referred to as a signal conditioner — include an oscillator to generate the drive signal, along with a demodulator, an amplifier, and a low-pass filter to convert the AC output voltage to a DC signal. Because they rely on the coupling of magnetic flux, LVDTs have almost infinite resolution, limited only by the noise in the signal conditioner. Similarly, repeatability is extremely high — typically less than 0.1 percent of the measurement range. Common measurement ranges are from ± 0.25 mm to ± 750 mm.

Signal conditioning electronics can be contained inside the LVDT housing. This configuration is typically referred to as a DC LVDT because the primary coil is supplied with DC voltage. Although internally-housed electronics reduce complexity, DC LVDTs have lower resistance to shocks, vibrations, and temperature extremes than traditional AC LVDTs.

LVDT devices are extremely robust, since there is no physical contact, and therefore no friction or wear, between the moving core and the transformer bore. The transformer is typically encapsulated with epoxy to protect against contamination and moisture, and the housing can be made from a wide variety of materials — from stainless steel to nickel alloys or titanium.

Typical applications include tensile test and other material testing devices, load cells, and weighing devices. LVDTs are also used to measure displacement in hydraulic cylinders and actuators.