Basics | Design World Motion Handbook

How can a universal motor operate on either DC or AC supply?

May 15, 2018 by Danielle Collins Leave a Comment

A universal motor is a series-wound motor — meaning that the field and armature windings are connected in series — and is mechanically commutated with brushes and a commutator. Although its construction is very similar to a series-wound DC motor, a universal motor incorporates several modifications that allow it operate properly on either DC or AC power supply.

For proper operation with AC supply, a universal motor incorporates a compensation winding, in series with the armature and field winding.
Image credit: circuitglobe.com

When the motor runs on AC voltage, the alternating flux causes a reactance voltage, which limits the current to a much lower level than would be produced with DC voltage. To limit the effects of this armature reaction, the universal motor uses a compensation winding, which is installed in the slots of the stator, 90 degrees electrical from the main field winding and connected in series with the armature and field winding. (This arrangement is referred to as “conductively compensated.”) The current flowing in each coil of the compensating winding is in a direction opposite to the current in the corresponding armature loop near it. The winding of equal coils in opposite directions produces a cancelling effect that reduces inductance, and therefore, reactance.

AC supply also induces more significant eddy currents than are produced when the motor operates on DC supply. To curtail eddy current losses, universal motors use laminated cores (rather than solid iron), which increases their resistance and reduces eddy currents.

Armature reaction is the interference of the main field flux caused by the armature flux. This distortion and weakening of the main field flux changes the location of the magnetic neutral plane, which is the axis along which the brushes should be placed.

Armature reaction changes the location of the magnetic neutral plane, which is the axis along which the brushes should be mounted.
Image credit: Precision Microdrives Limited

It is because the armature and field windings are connected in series that a universal motor can operate with either DC or AC supply. Being connected in series means that both windings are supplied by the same source, so if the voltage source changes polarity, as it does with AC supply, both the armature and field currents also change polarity and the direction of torque does not change. (The direction of rotation is reversed by reversing the current in the field circuit.)

To reduce inductance in the armature, the current flowing in each coil of the compensating winding is in a direction opposite to the current in the corresponding armature loop near it.
Image credit: Festo Didactic

One of the primary benefits of a universal motor is its ability to achieve (theoretically) unlimited speed, with speeds up to 20,000 rpm in real-world applications. The speed-torque curve is essentially a straight line between stall torque (zero speed) and no-load speed (zero torque), meaning that as the load (torque) increases, the speed decreases. In fact, like a series-wound DC motor, running a universal motor with no load (zero torque) could lead to a runaway condition, where the speed increases until the motor begins to break apart. The converse of this phenomenon is that universal motors produce very good starting torque (high torque at low speeds).

A universal motor can produce extremely high speed at zero torque, but speed drops rapidly as torque increases.
Image credit: Groschopp, Inc.

This ability to run at very high speeds means that universal motors are good for applications that involve rapidly spinning components, such as fans, blow dryers, and vacuum cleaners. And their ability to produce high torque at low speeds makes them suitable for applications such as blenders and portable drills. Because they’re mechanically commutated, universal motors generally aren’t suitable for continuous-duty applications, due to brush maintenance and wear. The brushes also make them relatively noisy, so their primary areas of application are small consumer appliances.

Feature image credit: Groschopp, Inc.

What are stand-alone motion cards?

May 11, 2018 by Miles Budimir Leave a Comment

Stand-alone motion controllers can take the place of PCs in some types of control systems. And there are a few different configurations possible. One is that the motion card can be housed in box with I/O and networking connections and mounted directly to the machine or process it controls. Control programs can be uploaded to the cards via USB link or Flash drives. This type of setup works well with single axis control and where the motion is fairly simple and doesn’t require interfacing with other controllers.

Another configuration is either rack mounted or DIN-rail mounted. The well-established PCI bus is the most common form factor for motion control cards in rack systems and so is readily available.

The PPCIe8443 controller board from Nippon Pulse can generate high-speed pulses to control input-type servo and stepper motors. The PCI Express board uses an updated PCI-express bus interface that can control up to 4 axes of coordinated motion.

Regardless of configuration, stand-alone motion cards can be networked together into larger, more complex multi-axis control systems. Typical options include USB, Ethernet, or other Ethernet-based connections.

However, stand-alone motion cards can also reside on PCs in some cases. The way it commonly works is that the motion control card, with a dedicated processor to handle motion control tasks requiring fast processing speeds, handles motion-critical tasks while offloading less demanding tasks to the PC processor.

How an IDE reduces development time in PACs

May 10, 2018 by Danielle Collins Leave a Comment

Applications that require complex process or motion control also require elaborate and complicated programming — often across multiple components, functions, and parameters. But thanks to the integrated develop environment found in programmable automation controllers (PACs), the programming task for these complex applications is faster, less burdensome, and more reliable.

The Parker Automation Manager (PAM) software is an integrated development environment that allows users to program robotics applications in a user-friendly, graphical environment.
Image credit: Parker Hannifin Corporation

When ARC Advisory Group introduced the term “programmable automation controller,” they specified five characteristics that signify a PAC, including that it:

Employs a single development platform using common tagging and a single database for development tasks across a range of disciplines. The same software is used for all development; this IDE (integrated development environment) reduces development time.

Note that ARC’s definition specifically states that the purpose of the integrated development environment (IDE) in a PAC is to reduce development time. One of ways it does this is by managing all the tasks related to programming, including editing, compiling, and debugging. And because all programming is done in one software package — rather than requiring multiple software packages from different vendors — the IDE makes programming less tedious and easier to scale.

Also key to a PAC’s ability to simplify and reduce programming time are the two specific features of the IDE mentioned in ARC’s characteristics — common tagging and a single database. Common tagging means that each variable, or tag, the programmer defines (name and definition) can be used in every software application, whether it’s for control, human-machine interface, vision, or plant-wide ERP systems. And if a tag is changed at a later time, it’s easy to propagate the change throughout the system.

This common tagging is enabled by the use of a single database. In other words, the tag definitions are kept in one database that is available to all the applications in the IDE. This means that programmers don’t have to define and synchronize tags across different applications.

The integrated development environment that underpins PAC software simplifies programming for all the applications related to the control system.
Image credit: Opto 22

Integrated development environments aren’t exclusive to programmable automation controllers. While the idea of an IDE goes back to TurboPascal in the early 1980’s, Microsoft’s Visual Basic, launched in the early 1990’s, is commonly recognized among programmers as the first IDE. (Note that Wikipedia identifies Maestro I, from Softlab Munich, as the first IDE, but does not specify the characteristics that made it recognizable as such.)

Probably the most recognized benefit of IDEs during their initial development and adoption was their ability to write, modify, compile, and debug code within a single program, simplifying the iterative process of writing, testing, debugging, and re-testing that programmers were accustomed to.

How does a servo drive connect to a motion controller?

May 4, 2018 by Miles Budimir Leave a Comment

Drives and controllers are key components in any motion system. A drive accepts commands from a motion controller and converts these signals into high current signals to drive a motor. So, how to connect a drive to a controller?

Manufacturers offer a range of options for drive and controller connectivity. These typically break down into stand-alone components that can be connected together to meet specific design requirements as well as integrated products, with the drive and controller integrated into one package.

A diagram of a motion control system shows a controller and drives connected via EtherCAT with individual drives powering separate motors. (Image via ACS Motion Control)

The fairly common integrated drive/controller (or drive/controller and servo motor) has internal connections between all the components. There is usually one external connection, often to a network bus, that ties individual units together. Common examples of this are such integrated units controlling individual axes on a machine.

The SmartMotor from Moog Animatics is a programmable, integrated servo motor system. The servo motor is integrated together with an encoder, an amplifier, a controller, and RS-232/RS-485 communication and I/O. (Image via Moog Animatics)

The most common way to connect stand-alone drives to controllers is via I/O connections or common networks for motion control applications such as EtherCAT, SERCOS, PROFINET or others. Which network is used depends on factors such as the speed of the application, system complexity, or the need for highly synchronized motion across multiple axes.

What is an inching drive?

May 2, 2018 by Danielle Collins Leave a Comment

When you hear the term “inching drive,” you might think of a small actuator, possibly driven by a piezo or voice coil, suitable for short, precise movements in applications such as optical inspection. (At least, that’s what I envisioned when I first heard of an inching drive!) But inching drives are so named because of their ability to move very large, high-inertia loads at very slow speeds.

The two most commonly used terms are “inching drive” and “auxiliary drive,” but other terms for these low-power, high-torque drives include “barring drive,” “creeping drive,” “jack drive,” “turning gear,” and — my favorite — “Sunday drive.”

Shown here is a gear drive with an integrated inching drive (front left).
Image credit: Rexnord Corporation

An inching drive is used as an auxiliary system to the main drive for a large machine such as a ball mill, industrial kiln, conveyor, or elevator. Its purpose is to turn the equipment at a speed slower than the normal operating speed — typically 1 to 2 rpm, although fractional rpms are also common — and to do so at high torque — typically 120 percent of the normal drive torque.

Three main components make up the inching drive — an electric motor or internal combustion engine, a gear reducer, and a connecting element to automatically or manually engage with the equipment being driven. The use of either an electric motor or an internal combustion engine depends both on the type of equipment being driven and the conditions under which the inching drive will be used. For example, a kiln needs to be rotated constantly. If the main power supply fails, an inching drive with an internal combustion engine can rotate the kiln in the absence of power. But for most other types of equipment, the inching drive is used primarily during maintenance and repair operations. In these cases, electric motors are the more common choice because they can provide positioning accuracy and good holding capabilities.

One of the uses for an inching drive is to slowly rotate a ball mill, as shown here, for inspection or replacement of the inner lining.
Image credit: Mauricio Neves, Arnaldo Andrade, and D.N. Silva

The mounting configuration of an inching drive can be parallel shaft, concentric shaft, or right angle. The mounting configuration is important for space considerations, but it also determines the type of gear reducer used in the inching drive. Parallel shaft configurations primarily use a helical gear or double-worm gear, although worm gears have low efficiency and are typically chosen only when self-locking is desirable. Concentric shaft designs also use helical gears, while right-angle designs most often use a combination of spiral bevel and helical gears.

In addition to the motor, gearbox, and coupling, most inching drive systems incorporate safety features, such as a brake or backstop, to prevent the drive from rotating when power is removed and to keep the equipment from rotating if it’s stopped in an unbalanced position.

Inching drives can use either electric motors or internal combustion engines. For driving a kiln, as shown here, an internal combustion engines is used for its ability to provide continuous rotation, even if power is lost. Kiln applications also incorporate a backstop to prevent rotation in the reverse direction.
Image credit: IEEE Standards Association

A variable speed drive can generally provide inching or creeping capabilities — which eliminates the need for a separate inching drive — as long as the motor is capable of operating within the speed range required for inching.

What type of graphical user interface do PC-based controllers use?

April 27, 2018 by Miles Budimir Leave a Comment

Controllers that are PC-based are typically programmed via a Windows computer and used to communicate with, monitor, and control industrial machines and equipment. As the name implies, being PC-based means that the PC monitor is the main graphical user interface (GUI).

One advantage of PC-based controllers is their GUI. PC-based controllers are programmed via a PC that, using a monitor, is a better GUI than what a typical PLC or standalone controller offers. Usually these latter controllers have either an LCD display combined with a small keypad or maybe a touchscreen display, which can make viewing data as well as programming the controller difficult.

PC-based controllers, such as those pictured here from Beckhoff, come in a range of forms with different GUI options. For instance, controllers can be mounted inside control cabinets or embedded within machines, or are available as panel PCs with large monitors for easy access and programmability. (Image via Beckhoff)

Another advantage of PC-based controllers is that the PC platform is familiar and so there is less of a learning curve for commissioning and programming and setup. A big reason why it’s so familiar is the PC GUI which makes it much easier to program and work with the system than other types of controllers that may have GUIs that are not as intuitive or user friendly. PC-based controllers are also easily scalable for expanding and changing motion control requirements.

A common type of PC-based controller is the industrial PC. They run on processors like other PCs but are built with more rugged components designed to withstand the harsh conditions of industrial environments. These conditions can include shock and vibration, noise and dust, as well as exposure to harsh chemicals and extreme variations in temperature and humidity.

Selecting a servo drive: 9 things you need to know

April 25, 2018 by Danielle Collins Leave a Comment

Sizing a motor for a servo application requires evaluating the move profile and torque requirements to determine the mechanical demands of the system, such as maximum velocity and acceleration, RMS and peak torque values, and load-to-motor inertia match. Once the motor is chosen, the next step is to select a drive.

On the surface, selecting a servo drive (also referred to as a “servo amplifier”) may seem to be a matter of simply matching the drive’s voltage and current output to the motor’s requirements. But there are many factors that need to be considered to ensure the drive operates satisfactorily within the complete servo system. While some applications may require other, more specialized functions from the drive, here are nine factors that guide the selection of a servo drive for most applications.

1. Motor type

A servo drive can be used with any motor that operates in a closed-loop system — including stepper, induction, and asynchronous — but the two most common types of motors that are paired with servo drives are brushless DC motors and synchronous AC motors. Of these, synchronous AC motors are more common in motion control applications.

2. Commutation

The type of commutation required by the drive depends on the type of motor being driven and the application’s sensitivity to torque ripple. Brushless DC motors can operate with either trapezoidal or sinusoidal commutation, while AC synchronous motors use sinusoidal commutation. Trapezoidal (also referred to as “six-step”) commutation is the simpler method, using three Hall sensors to determine the commutation sequence. But it produces high torque ripple. Sinusoidal commutation virtually eliminates torque ripple and provides more precise control by continuously varying the current supplied to the motor windings. But it requires a high resolution feedback device to determine the rotor position.

Sinusoidal (left) and trapezoidal (right) current waveforms for synchronous AC and BLDC motors, respectively.
Image credit: STMicroelectronics

3. Feedback

Servo systems require feedback in order to provide closed-loop operation and detect and correct any errors in the motor’s actual position, torque, or velocity. This feedback can be provided by Hall sensors, resolvers, or encoders, although most high-end systems use a resolver or an encoder. Regardless of the feedback mechanism, the drive must be compatible with its signal to process it and communicate it to the controller.

4. Voltage and current

The most basic requirement of the motor-drive relationship is that the power from the drive — maximum voltage, continuous current, and peak current — must be sufficient to produce the mechanical output required by the motor — torque, speed, and position. Because the operation of the motor and the drive are co-dependent, manufacturers provide torque-speed curves that define the performance of specific motor-drive combinations.

5. Operating mode

Servo control loops within the drive are used for controlling torque, velocity, or position (or a combination of the three). While all servo drives incorporate a torque control loop and a velocity control loop, only digital drives can provide position control.

Servo drives often have a multi-loop structure, with the current loop nested inside the velocity loop, which is nested inside the position loop. Analog drives can provide current and velocity control, but only digital drives incorporate position loops.
Image credit: nctu.edu

6. Analog or digital

Traditional servo drives were analog and converted ± 10 volt signals from the controller to current commands for the motor to control torque or velocity. In order to tune an analog drive, gain values and other parameters are set via potentiometers. With newer digital drives, the command can be executed by either digital or analog inputs, and tuning is done via software. Along with torque, velocity, and position loops, digital drives can also manage higher-level functionality, such as path generation. Digital drives are also capable of monitoring internal functions of the drive (such as following error) and providing more detailed fault diagnostics.

7. Communication

In order for the drive and controller to “talk” to each other, they must have a common language. To accomplish this, servo drives are offered with a variety of communication protocols, from basic serial links, such as RS485, to more advanced protocols, such as traditional fieldbus networks (DeviceNet, for example) or Ethernet protocols. The choice of communication protocol will largely depend on the required communication speed, whether there is a master-slave arrangement between multiple drives, and in some cases, the type of controller being used.

8. Integrated safety

Functional safety is now mandated on many types of machines and is governed by the EN/IEC 62061 and EN/ISO 13849-1 safety standards. To comply with these standards, safety functions, such as Safe Torque Off (STO) and Safe Stop 1 (SS1), are integrated into many higher-level drives.

When the STO function is activated, the drive stops supplying power to the motor, and the motor coasts to a stop.
Image credit: Siemens AG

9. Mounting

For industrial applications, servo drives are, for the most part, either panel-mounted or PCB-mounted. Panel-mounted drives are often installed in a control cabinet where they are protected from environmental hazards and can be externally cooled. PCB-mounted drives (also referred to as “plug-in” drives) provide a compact solution for direct integration onto a PCB and are commonly used in high-volume OEM applications.

Panel-mounted drives are typically mounted in a control cabinet.
Image credit: ABB

What is non-uniform memory access in industrial controls?

April 20, 2018 by Miles Budimir Leave a Comment

Non-uniform memory access (NUMA) is a kind of memory architecture that allows a processor faster access to contents of memory than other traditional techniques.

In other words, in a NUMA architecture, a processor can access local memory much faster than non-local memory. This is because in a NUMA setup, each processor is assigned a specific local memory exclusively for its own use. This eliminates sharing of non-local memory, reducing delays when multiple requests come in for access to the same memory location.

The illustration shows the differences between a NUMA and UMA architecture. The NUMA layout has local memory assigned to each processor, while in the UMA setup multiple processors share memory via a bus. (Image via Database Technology Group)

NUMA architecture is common in systems with multiple processors. For example, in industrial controls, particularly in systems with multiple processors, this is beneficial because it speeds up overall system performance times. NUMA allows for fast and easy access to memory for the processors, as opposed to shared memory architectures where access times may be longer, thus slowing down execution of key processor and system tasks.

NUMA architecture was developed largely due to the advent of modern microprocessors that are faster than memory speeds. As a result, local memory is placed closer to the processor, shortening signal path lengths and reducing latency times with fewer delays in accessing memory.

Stepper drives: What’s the difference between an L/R drive and a chopper drive?

April 19, 2018 by Danielle Collins Leave a Comment

There are two fundamental types of stepper drives: L/R drives and chopper drives.

L/R stepper drives are also referred to as “constant voltage” drives, because they supply constant voltage to the motor windings. The current produced in the windings depends on the motor’s time constant, which is the relationship between its inductance and resistance (L/R). The time constant causes current to increase slowly with each voltage pulse. So for very high pulse rates (i.e. high motor speeds), full current, and thus full rated torque, may not be reached. This limits the use of L/R drives to primarily low-speed applications.

Torque vs. pulse rate for a typical unipolar L/R drive with a 15 degree step angle.
Image credit: AMETEK Inc.

Current in the windings can be increased by using higher supply voltage, but motor temperature rise then becomes a problem. When the applied voltage is high, the “off” time (when no power is applied) must be long enough to prevent the motor from overheating. Another option is to add resistors in series to improve the time constant (L/2R or L/4R, for example) and increase the voltage. But resistors waste power through heat generation, so efficiency is reduced.

Resistance (R) affects the maximum current in the windings, according to Ohm’s law, I = V/R.

Inductance (L) affects the rate of change of current in the windings, according to the relationship, dI/dt = V/L.

Because of the torque and speed limitations of L/R stepper drives, an increasing number of stepper motor applications now use chopper drives. These drives are also referred to as “constant current” drives because they supply constant current to the motor windings, by “chopping” the output voltage (i.e. turning the output voltage on-and-off very rapidly).

With each motor step, a very high voltage (typically eight times the motor’s nominal voltage) is supplied to the motor windings. This high voltage gives a very short current rise time, according to the relationship for an inductor (dI/dt = V/L), and it causes higher current to be produced, according to Ohm’s law (I = V/R).

Torque vs. pulse rate for a typical bipolar chopper drive with a 15 degree step angle.
Image credit: AMETEK Inc.

Voltage chopping also modulates the width of the output pulses (pulse width modulation) and is typically done at a frequency of 20 kHz or higher — above the audible range. The impedance in the windings varies with the motor speed, so it plays a role in determining the voltage on-time.

At slow motor speeds — and low impedance — the chopper drive provides a short on-time for the voltage, producing a small pulse width. Alternatively, at high speeds —and high winding impedance — the chopper drive provides a long on-time for the voltage, producing a large pulse width and allowing time for the current to rise sufficiently to produce the rated torque.

Relationship between voltage and current in a (chopper) drive.
Image credit: Oriental Motor USA Corp.

A current-sensing resistor placed in series with each winding regulates the current and ensures the shortest time possible for current to build up and decline. As the current increases, voltage develops across the resistor. This voltage is monitored by a comparator, and at a predetermined reference voltage, the output voltage from the drive is turned off (chopped) until the next pulse occurs. This allows current to build and decline as the voltage is switched on and off.

By controlling the duty cycle of the drive through chopping, the average voltage and current are kept equal to the motor’s nominal voltage and current ratings. The result is precise torque control, and more importantly, higher torque production at high motor speeds.

Microstepping drives are common versions of chopper drives.

Feature image credit: AMETEK Inc.

When to use a programmable automation controller (PAC)

April 12, 2018 by Danielle Collins Leave a Comment

In a broad sense, a programmable automation controller (PAC) is an industrial controller that combines the functionality of a PLC with the processing capability of a PC. The term “Programmable Automation Controller” is generally accepted as having been coined by the ARC Advisory Group, which specified five characteristics that define a PAC:

Multi-domain functionality
A single, multi-discipline development platform
Flexible software tools that maximize process flow across machines or processes
An open, modular architecture
Compatibility with enterprise networks

PACs can operate in multiple domains simultaneously – such as motion control, process control, sequential control, logic, data management, and communication – using a single platform.
Image credit: Opto 22

But with no industry-standard definition of a PAC, the distinction between PACs and PLCs is blurry. Higher-end PLCs now incorporate some of the characteristics described above and are encroaching on what was once considered PAC territory. In fact, many PLCs now include standard programming languages, the ability to expand functionality through add-on modules, and connectivity to various bus systems.

However, PACs still differentiate themselves from PLCs by employing a more open architecture and modular design. They’re also more capable than PLCs at monitoring and controlling a large number of I/O, such as in a large processing plant or a complex automation system. They do this because data can be exchanged between devices and applications in different domains, such as motion and process control. And, a programmable automation controller can send and receive data to and from other PACs, creating a distributed control system of PACs. With large memory capacity, the ability to handle complex or high-speed analog I/O, and high-speed communication capabilities, PACs are well-suited for vision applications, including vision-guided motion.

PACs are often used in Cartesian, SCARA, and 6-axis robot applications, which require coordinated motion across multiple axes as well as integration with other motion and data systems.
Image credit: NEXCOM International Co., Ltd.

The ability of PACs to gather, store, and track large amounts of data means they can handle predictive maintenance and operations monitoring. Data is often stored and accesses through an Ethernet network or a USB storage device.

PACs use the IEC 61131-3 programming languages (ladder diagram, function block diagram, sequential function chart, instruction list, or structured text), and some include standard PC programming languages such as C/C++, so familiarity is retained and the learning curve for programming is low, regardless of prior experience or expertise.

Programming is done in an integrated development environment (IDE) that uses a single, tag-name database. This means that all the defined variables (tags) are kept in one database, which is used by all the software applications — such as HMIs, ERP systems, and vision applications. This greatly simplifies and reduces programming work and makes it easier to scale to larger systems.

In a nutshell, PACs are best suited for applications that require complex controls — in automation, this often means multi-axis, coordinated motion or circular interpolation — while PLCs work well for simple applications such as single-axis motion control.