Tech Tips March 2005


MARCH 29, 2005


Motion control terminology

Motion control, as other technologies, has its fair share of special terms. Not all of them are rigorously defined; some require blending to suit the specific audience. Here's a sampling of some well-known and not-so-well-known terms.

A motion control system typically consists of a controller to process motion algorithms and signals; an amplifier to boost signals to a level needed to power an actuator that provides the motion output; and feedback (sensors/transducers) to allow adjustments for process changes, based on comparing measured output with the input.

An operator interface or host terminal front-end completes the system. Feedback implies that most motion control systems operate in closed-loop ; however, some run in open-loop , notably a step-motor-based system. Actuators come in various forms—motors, cylinders, solenoids, etc.—and can be electric, hydraulic, pneumatic, or other type.

Axis —Any movable part of a machine or system that requires controlled motion. Several axes of motion can be combined in a coordinated multiaxis system.

Circular interpolation —Coordination of two independent motion axes to produce an apparent circular motion. It's done through a series of straight line approximations via software algorithms.

Commutation —Sequential excitation of motor windings to maintain the relative phase angle between the rotor and stator magnetic fields, within specified limits, to control motor output. In brush dc motors, this function is accomplished by a mechanical commutator and carbon brushes; in brushless motors, it's done electronically using rotor position feedback.

Electronic gearing —A method that simulates mechanical gears by electrically 'slaving' one closed-loop axis to a second axis (open- or closed-loop) through a variable ratio.

Encoder —A feedback device that translates mechanical motion into electrical signals indicative of actuator position. Incremental and absolute encoders are common varieties; as the names imply, their output indicates incremental or absolute changes of position.

Feedforward —A method that 'precompensates' a control loop for known errors due to motor, drive, or load characteristics to improve response. It depends only on the command, not the measured error.

Indexer —An electronic unit that converts high-level commands from a host computer, PLC, or operator panel into step and direction pulses needed by a stepping motor driver.

Loop bandwidth —Maximum rate at which a control loop can respond to a change in a control parameter. It's indicative of loop performance and is expressed in Hertz (Hz).

Motion profile —The velocity versus time (or position) relationship of the move made by a motion axis.

Overshoot —A system response where the output or result exceeds the desired value.

Pulse-width modulation —A switch-mode control method used in amplifiers and drivers to control motor voltage and current to obtain higher efficiency than linear control. PWM refers to variable on/off times (or width) of the voltage pulses applied to the transistors.

Quadrature —A technique that separates signal channels by 90° (electrical) in feedback devices. It is used with encoders and resolvers to detect direction of motion.

Resolver —A position transducer that uses magnetic coupling to measure absolute shaft position during one revolution.

Servo mechanism —An automatic, closed-loop motion control system that uses feedback to control a desired output such as position, velocity, or acceleration.

Tachometer —An electromagnetic feedback transducer providing an analog voltage signal proportional to rotational speed.

This sampling of terms draws on various sources, notably, Programmable Motion Control Handbook (1992), National Electrical Manufacturers Association (Washington).

Frank Bartos, executive editor

Source: Frank Bartos, 'Terminology in Motion,' Back to Basics, Control Engineering, Jan.'98, p. 108.

MARCH 22, 2005


Developing mathematical models.

A mathematical model is algorithm or set of equations that represents the interesting behavior of a system. Experts with thorough knowledge of the system and its interaction with the environment typically perform model creation tasks in development projects for complex dynamic systems. The development of a model for a complex, dynamic system is an iterative process that involves significant effort to verify the correctness and accuracy of the resulting implementation.

The process of model development begins with a specification of the requirements the model must meet. The following are issues that must be addressed in development a model of a complex system.

What effects should be included in the model? A system may exhibit many different kinds of behavior (for example, the motion of motors, vibration, wear of moving parts, etc.), but not all of these behaviors need to be modeled to produce an effective simulation. Limiting the effects model to only those that are truly necessary will make the model less complex and easier to build, test, and maintain-as well as requiring less computational resources to execute.

How detailed must the model be? In many cases a simple model is all that is needed, but if precise determination of system behavior is required, the model may need to be very elaborate.

What interactions between the system and the outside environment must be modeled? For example, a communications satellite's motion model must operate in conjunction with a model of the earth's gravitational field, as well as with models of other relevant phenomena, such as solar pressure.

What techniques will be used to develop the model? A fundamental choice is whether to use physics-based equations or measured data as the basis for the model. The answer to this question is often obvious to those with expert knowledge of the system.

What data must be gathered to perform the modeling? For example, an aerodynamic model of an aircraft may require extensive wind tunnel testing.

How much time and how many people are available to develop and text the model? As model complexity increases the development and test hours will increase as well.

What computing resources are available for the model? A large model may consume significant amounts of memory, disk space, and CPU time. However, given the capabilities of current computers, this may not be a critical issue.

Will the model eventually be used in a hardware-in-the-loop (HIL) simulation? This may place severe constraints on the execution time allow for the model. Alternatively, a complex model may require high-performance computing hardware for use in an HIL simulation, perhaps involving the use of multiple processors.

How can verification and validation be performed for the model implementation? There must be reasonable ways of conforming that the model has been implemented correctly, and that its behavior matches the system being modeled to an acceptable degree.

These issues should be addressed as part of planning for the simulation effort. The questions listed can be applied at the highest level of the entire system being simulated initially, and again as the system is broken down into subsystems and individual components to be modeled. These questions are also useful ion the development of additional models needed for a complete simulation, such as the gravitational field and solar pressure models in the communication satellite example above.

Source: Jim, Ledin, Control Valve Handbook, CMP Books, CMP Media LLC, Lawrence, KS, 2001, p. 28-29.

MARCH 15, 2005


Network security.

Industrial network security is increasingly focusing on networks as they continue to evolve beyond their historical isolation. Networks are making themselves and their data accessible via Ethernet TCP/IP, IT-related systems, and/or the Internet, and are facing the inherent vulnerabilities of these technologies. The question is how can users access their networks remotely without exposing themselves to unauthorized intrusions?

To begin improving security, managers, control engineers and system administrators must first think of their network as a whole, and become aware of their company-wide infrastructures. It's useful to literally sketch out the entire network; take an inventory of everything connected to the network; and then ask 'Is this network linked to a company intranet or to the Internet?' and 'Is the network completely hardwired or are there wireless components?' Next, managers should check what security measures are presently available, and make sure they're enabled and operating.

Routers rule
Undoubtedly the most important tool for increasing security is having a router/firewall between local networks and larger systems, especially those tied to the Internet. While switches operate at the data link layer (layer 2), routers generally operate at the network layer (layer 3) with most routers handling TCP/IP messages. A router/firewall matches private Internet addresses with data requests, allowing through only specified messages. Very few unauthorized messages get through routers.

Another security question is: 'Will the highly repeatable communications on the plant floor be able to handle corporate-level data transfer sizes and bandwidth? To manage these communications, many users employ switches with broadcast storm control capabilities, which block broadcasts from overly noisy ports.

Also, these switches assign slightly different bandwidths for accessing each port on a network. This ensures that each device gets only the messages it's supposed to receive.

VLANs vital
Beyond basic routing, some users implement virtual local area networks (VLANs) between their plant-floor networks and office systems. Located in the switches' hardware, VLANs block unauthorized messages between network ports.

In fact, two VLANs overlapping to a specific degree can share data if, for example, a device on the factory floor also sits on the corporate VLAN. This exposes only one device to potential vulnerabilities, and leaves other devices protected.

Yet another option is to simply install an additional router between two locations, which can be dedicated solely to sending data between them. This security strategy doesn't mask addresses, but it too allows only specific traffic between plant-floor addresses and office addresses. This method is similar to a VLAN, but instead uses the added router to do its job.

Check connected PCs
Back on the infrastructure side, network managers must also be cautious about what devices might be using up available bandwidth on the plant-floor. Ethernet switches are designed to be very inviting, and someone plugging into an available RJ-45 port can potentially hinder or damage manufacturing processes with unauthorized or untested network traffic.

So, besides checking the security of one's own network, managers also must be careful about the protocols used on PCs and laptops that may connect to switches on their plant-floor network. Managers can test new software or devices by running a plant-floor network in safe mode or by setting up small test networks.

Bennet Levine, R&D manager Contemporary Controls,

Source: Bennet Levine, “Securing network security,” Back to Basics, Control Engineering, Oct. ’04, p. 68.

Locking in security to-do list

  • Think of network as a whole, and sketch it out-literally

  • Inventory what network is connected to-Internet? Wireless?

  • Check for existing security features, and make sure they're enabled

  • Install router switch/firewall between plant-floor network and other networks

  • Enable router's broadcast storm control capability

  • Use virtual local area networks (VLANs) to block unauthorized messages between ports

  • Overlap two VLANs to allow specific data sharing

  • Use additional dedicated router to allow only authorized traffic between two networks

  • Test PCs and laptops plugging into plant-floor network

MARCH 8, 2005


Common valve plug guiding methods.

Accurate guiding of the value plug is necessary for proper alignment with the seat ring and efficient control of the process fluid. The names of the common methods are generally self descriptive, and they include:

  • Cage guiding - The outside of the diameter of the value plug is close to the inside wall surface of the cylindrical cage throughout the travel range. Since bonnet, cage, and seat ring are self-aligning on assembly, correct valve plug/seat ring alignment is assured when the valve closes.

  • Top guiding - Value plug is aligned by a single guide bushing in the bonnet or valve body, or by packing arrangement.

  • Stem guiding - Valve plug is aligned with the seat ring by a guide bushing in the bonnet that acts on the valve plug stem.

  • Top-and-bottom guiding -Valve plug is aligned by guide bushings in the bonnet and bottom flange.

  • Port guiding - Valve plug is aligned by the valve body port. This construction is typical for control valves using small-diameter valve plugs with fluted shirt projections to control low flow rates.

Source: Control Valve Handbook, Third Edition, Fisher Controls International and Emerson Process Management, Marshalltown, IA, 2003, p. 59-60. 52.

MARCH 1, 2005


Radio frequency identification (RFID) basics.

Radio frequency identification (RFID) has been in use for almost 20 years. Industrial automation applications include automobile manufacturing, electronic component or appliance assembly, and machine tool tracking. 'RF' is a method of contactless identification without requiring direct line of sight. The 'ID' comes in the form of a portable database on a coded tag that can store manufacturing process data or identification numbers. Barcode technology, for example, requires line of sight as well as some direction orientation.

The 'tag' in an RFID system has a memory that can be read-only, write-once-read-many (WORM), read-or-write, or a combination of all three. The tag can store information about the item to which it is attached. It may store information about processes that must be performed. With read/write capability, process data can also be written to the tag.

Two tag types
Tags are available in a variety of memory types and data capacities. Permanent, read-only memory generally has a serial number programmed during the manufacture of the tag. It has unlimited life. Random access memory (RAM) requires a battery to maintain its data. Data capacity of RAM varies from 2K bytes up to 32K bytes or more. Many electronically erasable programmable read only memory (EEPROM) tags have specifications for limited write cycles of about 100,000, although the can often be written reliably to more than 500,000 cycles. Read cycles are unlimited. Data capacity of EEPROM memory varies from a few hundred bytes to several hundred, and the data itself has a life of 10 years or more.

Recently developed Ferroelectrical Random Access Memory (FRAM) is gaining popularity among factory automation applications. Without a battery, FRAM has the virtually unlimited write cycles of RAM.

RFID has two basic tag types. Active tags use a battery for power to transfer data to and from the reader. These tags are generally RAM memories. Active tag advantages include longer transmission distances between the tag and the reader. Disadvantages include battery maintenance.

Passive tags use the RF field of the reader for power. These tags can have any of the four memory types. Advantages of passive tags include longer service life since no battery is required for data transmission. In the case of RAM memory, batteries are used only for data storage. Batteries in passive tags, depending on the amount of data transferred and number of times, can last for many years. One disadvantage is slower overall tag read or write times, as there is a startup time when the tag enters the RF field of the reader.

Position reader head
Reader heads-the other end of the RFID equation-come in a wide variety of shapes and sizes. Distances between tags and readers vary from a few millimeters to several meters. The frequency of the data transmission and the size of the reader and tag determine the distance. Frequencies used in most RFID applications vary from 125 KHz to 2.45 GHz. Higher frequencies, in the MHz and GHz range, enable longer transmission distances. Readers and tags are normally sized proportionally to one another. In general, a smaller tag will require a proportionally smaller reader and will have a shorter read distance between the two. Reader sizes vary from a 10-mm diameter cylindrical barrel type to units built like metal detectors that you could walk through.

RFID use is growing. It is a reliable and proven technology and will continue grow as manufacturers look for more methods to reduce manufacturing costs.

Ed Rogin, product manager, intelligent sensing,
Siemens Energy & Automation,

Source: Ed Rogin, 'RFID gets the message,' Back to Basics , Control Engineering, Dec.'01, p. 52.

Main RFID selection criteria :

  • Distance between tag and reader

  • Tag speed at read

  • Amount of data

  • Type of machine control

  • Size and shape of reader head and tag

  • Type of data

  • Whether read only or need to write to tag

  • Data life span on tag

  • Number of anticipated read/writes

Source: Control Engineering with input from Siemens Energy & Automation

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