Tech Tips September-October 2005

By Control Engineering Staff March 22, 2007

October 25, 2005


Avoiding ground loops

Ground loops (also referred to as ‘noise’) in an electrical system result from unwanted current flow in a conductor connecting two points that should be at the same electrical potential, but aren’t.

Minimizing electrical noise effects on control system performance and reliability requires:

  • Using isolated ac power sources;

  • Establishing a single, common system ground point;

  • Providing isolation for low-voltage signals (for example, thermocouples);

  • Minimizing influence of stray magnetic fields; and

  • Selecting appropriate cables and pathways, including adequate cable separation.

Improper grounding, such as grounding cable shield wires at both ends or at the wrong end, is well-documented as a source of introducing electrical noise. Cable shield wires should be grounded at one end, preferably at the power source end. The other end should be taped and protected. Most faulty grounding system designs result from mixing power and grounding subsystems (such as ac, dc, shields, cabinets, etc.) and/or failing to establish a single, common separate (isolated) ground point on the plant’s ground grid system.

Power and grounding subsystems should be separated up to the last possible and/or practical connection point. For example, in a grouping of four control and instrumentation cabinets—ac-, dc-, shield-, and cabinet—the grounds should remain separated throughout the cabinets and be connected to an isolated cabinet grouping ground bar. The insulated ground lead from the cabinet grouping ground bar should be routed to a master ground bar, and then joined by other cabinet groupings. The insulated ground lead from the master ground bar should be routed to an unshared (isolated) point on the plant ground grid.

In large plants, connecting all master grounding points to the same point on the ground grid may not be practical. Assuming the plant ground grid is properly designed, the difference in electrical potential between grid connection points should be negligible, making it permissible to use different ground grid connection points.

Source: Control Engineering, January 2005, Back to Basics, ‘ Avoid electrical ground loops .’

Also, ‘Control System Power and Grounding Better Practices,’ ISBN 0-7506-7826-7.

October 18, 2005


Ways to control step motors in closed-loop

Most step-motor-based motion systems operate in open loop for low cost and provide the only motion technology inherently capable of position control without feedback. However, for certain applications that require added reliability, safety, or product quality assurance, closed-loop control offers an alternative

Here are some methods to obtain closed-loop control of step motors:

  • Step verification , the simplest position control, uses a low-count optical encoder to ‘count’ number of steps moved. A simple circuit compares commanded versus measured steps, verifying that the step motor has moved to the desired position.

  • Back emf , a sensorless detection method, uses the step motor’s back electromotive force (emf) signals to measure and control velocity. When ‘back emf’ voltage drops below detection levels, the ‘closed-loop’ control shifts to standard open loop for the final positioning move.

  • Full servo control refers to full-time use of a feedback device on the step motor—encoder, resolver, or other feedback sensor—to more precisely control step motor position and torque.

  • Other methods include variants of back emf control based on motor parameter measurement and software techniques, used by some manufacturers. Here the stepper drive monitors and measures the motor windings and uses voltage and current information to improve step-motor control. Active damping uses this information to damp oscillations at speed, allowing more usable torque output and reducing torque lost to mechanical vibrations. Encoderless stall detect uses the information to detect loss of synchronous speed.

Source: Control Engineering, February 2005, ‘ Closed-Loop Stepper Motion Alternative .’

October 11, 2005

Ways to improve engineering communications

Anecdotal stories abound about engineers being poor communicators. However, there is a larger consideration here. Communication is not a soft skill reserved for human-resource folks and marketing types. Instead, it’s the skill of connection and understanding. It is what often makes the difference between a project’s success or failure.

Five communication tips can guide you and help lead your next project to success.

Create ‘living’ project agreements

Various changes can make the original goal of a project obsolete. When creating a project agreement, make it a “living document,” written knowing there will be changes.

Improve team dynamics

Lack of commitment, lack of interaction, and lack of interest in constructively resolving conflict may arise. Many projects also lose and gain people during project execution. When this happens, it’s important that the team spend time together developing new team guidelines and meeting protocols. Redeveloping guidelines and protocols is done for the same reason it is done initially—to facilitate working relationships, to create a way to positively interact, and to prevent destructive conflict.

Create institutional memory

Industry standard project management practices require a critical project closeout phase that collects lessons learned and gives your organization powerful historical knowledge from across the enterprise. A company that can learn and grow, rather than continually repeat mistakes, will move faster.

Create contagious commitment

People need to see, hear, smell, and taste success; even small victories have a big impact. It’s important to communicate and show success with early adopters, so people will understand what you’re doing and how they can be a part of the ongoing success.

Create a safe ‘blue sky’ environment

For people to communicate openly, they have to be in an environment that’s safe and allows for some “blue-sky” and off-beat thinking. If you foster an environment that shoots down ideas, then people will stop sharing ideas, and instead just take the path with least resistance.

Think of communication as a tool that clarifies, illuminates, and unifies, and ultimately brings you closer to the goal ahead.

Source: Control Engineering, September 2005, “ 5 Ways to Improve Engineering Communications .”

October 4, 2005

Using predictive leak-testing methods

Leak testing can be complicated, particularly for high-speed processes as applied in the pharmaceutical industry. A key challenge is the time taken for the part under test to stabilize under pressure (or in vacuum); and as cycle time decreases, the difficulty in performing reliable leak testing increases. To avoid excessive equipment costs of multiple leak test stations, predictive testing is increasingly used to reduce leak test cycle time. Predictive leak testing takes a leak-rate measurement before the part has completely stabilized and scales that value to a final leak rate from a fully stabilized test.
Here are typical leak-testing methods:

Linear scaling of the final leak-rate value: One scaling factor is used to scale the measured leak rate so that at the end of the abbreviated test, the value is the same as a full-length test. Only the final scaled value is compared to the leak limits. The method’s limitation is that it ignores the shape of the leak rate and pressure curves until the end of the test. Part or process anomalies may cause the test to end up at a valid leak rate, while in fact there was a problem.

Non-linear scaling: Over a given range of leak rates, no linear correlation may exist between abbreviated leak-rate values and fully stabilized values. Thus, using linear scaling to predict the final leak rate can be inaccurate, especially for parts that take the longest to fill and stabilize. This gets worse as the test gets shorter. Advantage of this method is that non-linear scaling considers multiple correlation points to more accurately predict the final leak rate, versus linear scaling, which works with just two points.

Continuous verification against a trained curve: It is important to detect anomalies in the part or process causing irregularly shaped pressure and leak-rate curves. To achieve this, signals must be monitored continuously throughout the test and checked against preset limits. It’s important to note out that a test system can be set up to reliably pass or fail a part based purely on these limits. When an actual leak rate value is required, one of the above two predictive methods should also be employed.

Source: Control Engineering, July 2005, Inside Process, “ 7 Steps to Predictive Leak Testing .”

September 27, 2005

Tip of the Week
I/O system design suggestions

Input/output systems add intelligence, communication capability, and flexibility to centralized or distributed control architectures. Here are some suggestions to design “the best I/O system:”

  • Consider I/O and control system structures before selecting products to meet application needs.

  • Evaluate system requirements, then define overall control attributes (response times, network communications, diagnostics, redundancy, and scalability).

  • Ensure network compatibility; for example, avoid inadvertent increase of number of local area networks needed by selecting incompatible devices.

  • Identify the application’s physical structure/layout to help determine if I/O modules should be local, remote, distributed, or a combination.

  • Figure the mix of rack-mounted and distributed I/O systems in the application: Rack-mounted I/O is used for points requiring fast response and for local devices where wiring is minimal. Distributed I/O modules connect to some remote devices.

  • Determine processor configuration, that is, whether a single processor (PC or PLC) or split control across several processors, or multiple processors for specialized tasks are needed.

  • Consider response time, which is related to bus communications, diagnostics, redundancy, scalability, and modularity. It includes input signal filtering time, I/O scan time, time for the processor to act on the input, time to send the signal to the output device, and time for the output device to generate the output signal.

Further guidance is given in the reference below.

Source: Control Engineering, September 2005, Back to Basics, “Input/output systems.”

September 20, 2005

Tip of the Week
Mapping a thermography route for plant-equipment inspection

Consistency is key to effective and efficient periodic thermography inspections. The latest thermal imaging systems provide on-board intelligence and advanced user interfaces (HMIs) to help supervisors ensure that technicians acquiring images visit each piece of equipment along a carefully planned route.

To ensure effectiveness of the inspection route, the thermal imager’s HMI should incorporate a reminder system showing the planned route, location of each stop, images needed at each stop, and any required reminder notes. Maintenance supervisors must keep three considerations in mind when developing routes:

  • Travel time—Time needed to acquire images at each stop is typically less than the time required to move from point to point, so plan routes to minimize travel time and time used at each stop.

  • Logical inspection sequence—Whenever possible, the route should follow a logical sequence. For example, the sequence might follow the plant’s production process.

  • Safety—Written route descriptions should help ensure the safety of technicians as they travel from one inspection point to another, as well as when they acquire data.

Source: Control Engineering, August 2005, “Thermography Improves Predictive Maintenance,” an article in the Inside Machines section.

September 13, 2005

Tip of the Week
Protecting control systems from the Internet

While plant control systems are inherently protected from cyber attacks, they can become exposed whenever access is provided to those seeking plant-floor information via the Internet.

Three general options exist to mitigate potential intrusions into process control systems, depending on the risk tolerance and benefit you’re seeking:
Isolate the network. The safest, most restrictive approach is to keep the control network locked down, allowing only physical access by authorized persons (inside or outside the plant) to operator stations and connected machines.

Go ahead and connect. Simply connecting the control network to plant and business networks is the fast and easy approach, but entails just hoping for the best.
Make connections in an intelligent, controlled fashion.

  • Use firewalls and routers to segment the network. Properly established firewalls block specific messages or message types, enabling network administrators to control what traffic flows into/out of a control network. If ports, such as HTTP and RPC must be open, risk of control network penetration increases. Unfortunately, these are ports that many applications require to be open.

  • Establish policies and procedures for maintaining firewalls and ensure they are properly configured. Rules should identify who can change the firewall, define permitted changes, and provide for oversight. System security is chiefly a process issue—not a technology issue.

  • Enhance firewall protection by using intrusion detection systems, which monitor network traffic to identify inappropriate activity. These systems can help identify when firewalls are ineffective or when an attack is underway through open ports.

  • All the firewalls in the world won’t protect a system with weak passwords. Automatically generated passwords are best, but tools are often required to help generate and manage them, such as Password Minder and Password Safe. Finally, keep all non-essential software off computers directly connected to the control network. It helps reduce the risk of a virus.

Source: Jon Westbrock, senior technologist at Emerson Process Management, in Technology Update, Control Engineering, August 2005 (p. 14).

September 6, 2005

Tip of the Week
Application guidance for step-motor-based motion control

A step-motor-based system provides a simple, low-cost method of motion control. However, successful use stepper motion control is highly application dependent. Here are some general points of application guidance:

  • Stepper motors are physically limited to under 1 kW output power

  • Useful torque production drops off rapidly with motor speed. Limits speeds to 3,000 rpm or less as a rule of thumb.

  • Stepper motors offer excellent position holding, without “dithering” as in servo motors

  • When properly sized for the driven load, step motors produce little velocity ripple while executing a motion profile

  • If properly sized, steppers supply repeatable motion and positioning

  • Stepper motors typically run without a feedback device, hence add reliability to a system

  • Stepper motors are less costly than servo motors

  • In a multi-axis machine application, some of the motion axes may be better suited to stepper control and some to servo motor control. Newer drives and controllers can accommodate both motor types.

Source: Various Control Engineering articles on stepper (and servo) motor control.