Tech Tips April 2006


April 25, 2006


Using diagnostic features of digital valve positioners

Digital valve controllers/positioners (DVCs/DVPs) have added significant intelligence to their features over recent years. End-users can tap into this intelligence to ease their valve maintenance and diagnostic chores.

Diagnostics available from most DVC manufacturers include:

  • Final control device tracking parameters for monitoring total (accumulated) travel and the number of reversals (cycles);

  • DVC health parameter tests, providing alerts about memory, microprocessor, and/or sensor problems. These permit operator alarms or, for more severe problems, unit or process shutdown actions;

  • Final control device alerts. These communicate expected travel deviations, travel to high/low limits, and/or when preset accumulated travel and cycle targets are exceeded;

  • Dynamic error band tests to check hysteresis and deadband-plus-slewing condition in final control devices;

  • Drive and output signal tests that vary the transducer setpoint at a controlled rate and plot final control device operation to determine dynamic performance; and

  • Predefined final control device maintenance and signature tests.

Some digital fieldbus protocols include predefined step maintenance tests for checking and/or establishing a final control device performance 'signature.' Some DVC manufacturers include a fourth dynamic step test designed to determine the current performance signature of the control device. If the device is new, performance signature becomes the benchmark for future comparisons. If the device has been in service for a while, comparing present and benchmark signatures can determine when repairs and/or calibration will be required.

Source: Control Engineering, Jan. 2003, ' Making Valve Controllers/Positioners Smarter is Smart Business .'

April 18, 2006


How to implement proximity sensors—Part 2

Part 1 of 'How to implement proximity sensors' (previous week's 'Tip') mentioned the two common forms of these sensors: shielded units than can be embedded flush in their mounting without affecting the detection field; and unshielded inductive proximity sensors that offer longer sensing distances, but aren't suitable for flush mounting.

Inductive proximity sensors are designed to have a type of hysteresis in their circuitry that's used to eliminate output chattering. As a target approaches the sensor's detection face, it eventually triggers a sensor output. When the target moves away from the sensor's detection face, the triggered output holds until a certain distance has been passed. This distance—called the reset distance or distance differential , can be as high as 10% of the sensor's total sensing distance. This holds proportionally true for detection objects that cause reduced sensing distances.
Make sure that the target object is completely removed beyond the sensor's reset distance to avoid all potential chattering from detectable object vibrations or other environmental factors.

When using numerous inductive proximity sensors in an application, be aware of a mutual interference effect. This occurs when one proximity sensor's magnetic field affects that of another sensor, causing it to trigger an output. This false triggering can be erratic and difficult to detect.

Look for inductive proximity sensors that feature alternate frequency models. These sensors oscillate their magnetic fields at different frequencies and do not interfere with one another as much as two inductive proximity sensors using the same frequency. Simply take one frequency proximity sensor and mount it next to one of a different frequency.

Inductive proximity sensors are simple and effective sensing tools. Avoid problems by well-planned implementation in your application.

Source: Control Engineering February 2001, Back to Basics, ' Proximity sensor implementation .'

April 11, 2006


How to implement proximity sensors—Part 1

These sensors come in two forms: shielded and unshielded. Shielded inductive proximity sensors can be embedded flush in their mounting material without affecting the field of detection. Unshielded inductive proximity sensors have the advantage of longer sensing distances, but the disadvantage of not being embeddable.

Sensing distance

Rating of inductive proximity sensors for sensing distance refers to their ability to detect the 'standard detectable object' at its specified sensing distance. Standard detectable object is a 1-mm (0.04 in.) thick, square piece of ferrous metal, with its side dimension equal to the proximity sensor's face diameter.

If the target material is nonferrous, effective sensing distance will change. For example, effective sensing distance due to target material is reduced by 30% for copper, 40% for aluminum, 50% for brass, and 80% for stainless steel.

Other factors accounting for reduced sensing distances include the target material's capacity for conductance. Conductive materials disperse eddy currents needed for inductive sensing. Highly conductive materials make poor detectable objects and create reduced sensing distances. However, thin materials—for example, aluminum foil—hold eddy currents and make excellent targets for inductive sensing.

Proximity sensor users also must ensure that specified power requirements are met. Reduced line voltages can lead to weak magnetic fields eminating from the sensor's detection face and result in smaller sensing distances.

Further guidance for implementing inductive proximity sensors will be offered in Part 2, in the next Tip of the Week.

Source: Control Engineering February 2001, Back to Basics, ' Proximity sensor implementation .'

April 4, 2006


Pros and cons of resistance temperature detectors (RTDs)

Among many ways to measure temperature, resistance temperature detectors (RTDs) are considered one of the most accurate. In an RTD, the device's resistance is proportional to temperature. The most common resistive material is platinum, but some RTDs are made from nickel or copper. RTDs can measure over wide temperatures in the range of -270 to 850 °C (-454 to 1,562 °F), depending on their construction.

Advantages of RTDs over other temperature-measuring devices include:

  • Very stable device;

  • Most accurate of all temperature measurement devices; and

  • More linearity than thermocouples.

There are a few drawbacks, however:

  • RTDs are more costly than thermistors and thermocouples;

  • They require a current source; and

  • RTDs have a small delta R, which means there is a low resistance to temperature change. (For example, to change one °C, an RTD might change its reading by 0.1 ohm. However, low absolute resistance could lead to measurement errors if using the two-wire sensing method.)

Other common conditions must be accounted for when using RTDs—for example, self-heating . Measurement inaccuracy could result if the RTD self heats under the test current. When measuring low temperatures (such as below 0 °C), heat from the RTD could raise the expected temperature. Also, if uncompensated test leads are used, even more error (noise and uncertainty) could be introduced into the measurement. Using a four-wire method helps eliminate this type of error.

Another effect to watch for is selecting the proper RTD temperature range. Trying to measure outside an RTD's temperature range can result in more errors or even sensor damage. Always select the appropriate RTD for the expected measurement. See the reference below for further guidance on 2-, 3- and 4-wire RTD measurement methods.

Source: Control Engineering November 2005, ' Resistance Temperature Detectors .'

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