Teaching Technical Practices (Labwork)

Chapter 38 - Educational Concepts and Models for the Field of Instrumentation - Advice for Teachers

Labwork is an essential part of any science-based curriculum. Here, much improvement may be made over the “standard” educational model to improve student learning. In my students’ Instrumentation courses, I forbid the use of pre-built “trainer” systems and lab exercises characterized by step-by-step instructions. Instead, I have my students construct real working instrumentation systems. The heart of this approach is a “multiple-loop” system spanning as large a geographic area as practically possible, with instruments of all kinds connecting to a centralized control room area. None of the instruments need perform any practical purpose, since the goal of the multiple-loop system is for students to learn about the instruments themselves.

[multiple-loop system]

A model for a multiple-loop system might look something like this:

Instruments may or may not be grouped together to form complete control systems, since process control is not necessarily the purpose of this system. The primary purpose of a multiple-loop instrument system is to provide an infrastructure for students to investigate instrumentation apart from the dynamics of a functioning process. The separation of controls from process may seem counter-productive at first, but it actually provides a rich and flexible learning experience. Students are able to measure instrument signals and correlate them with actual physical measurements, take instruments in and out of service, check instrument calibration, see the effects of calibration on measurement accuracy and resolution, practice lock-out and tag-out (LOTO) procedures, diagnose instrument problems introduced by the instructor, practice installing and removing instruments, remove old wire and pull new wire into place, practice sketching and editing loop diagrams, and many other practical tasks without having to balance the needs of a working process. The system may be altered at any time as needed, since there are no process operating constraints to restrict maintenance operations. The fundamental advantage of a process-less instrument system is there are no process limitations restricting educational objectives. In this sense it is as flexible as a computer simulation, but with the advantage of using real-world components.

The first academic year I attempted to build such a system with my students was 2002-2003. Our system cost almost nothing1116, with a control panel fabricated from a discarded fiberglass electrical enclosure and 4-20 mA loop wiring salvaged from discarded spools of category-5 data communications cable (four twisted pairs per cable). We stapled the cable runs to the lab room wall, and used cheap terminal block assemblies to provide connection points between the cat-5 trunk cables and individual instrument cables. Our first loops built with this system included the following:

• Air compressor receiver tank pressure measurement – measurement only
• Air compressor temperature measurement – measurement only
• Regulated (service) air pressure measurement – measurement only
• Wash basin water level measurement – measurement only
• Water column level and temperature control – measurement and control
• Air reservoir pressure control – measurement and control

The first four of these instrument loops were “permanent” in that they were never disconnected once installed. The water level and temperature control system was a later addition made toward the end of the academic year. It began as a pneumatic system, then was upgraded to electronic (single-loop digital controller), then as a PLC-controlled process, then finally as a DCS-controlled process. The air pressure control system was much the same. All the time we left the process vessels and field instruments in place, used the same signal tubing and wiring, but merely changed the control instruments at the other end of that tubing and wiring.

In addition to these six permanent and semi-permanent loops, students used the system throughout the year to connect individual instruments for loop calibration. Usually there was no control involved, as they were simply studying individual instruments and were not ready for a complete control system yet. Every time they had a transducer to calibrate, a control valve to test, or a transmitter to configure, I required them to tie it into the loop system and document the loop using ISA standard loop diagrams. Then, I would fault their loops (usually electrically by creating opens or shorts in signal wiring, or pneumatically by creating leaks or by plugging tubes with foam earplugs) and have them troubleshoot the loops using real test equipment, documenting their diagnostic steps for grading purposes. After successful commissioning, calibration, and troubleshooting, students disassembled the loop so the instruments could be used again in a different loop.

Our multiple-loop instrument system – despite its crude appearance and low cost – was extremely successful as an educational tool. My students gained a tremendous amount of practical knowledge and skill in addition to the basic theory. Abstract principles of measurement and instrument application “came alive” for them as they saw the pieces fit together to make a working system. The intentionally distributed nature of the system – with the control panel located in one far corner of the room and field instruments scattered around the rest of the room – forced students to think and work in a manner much more similar to the real work environment. There were days they were so excited about working on this system that I had to coax them out of class when the school day was over!

In the summer of 2006 I upgraded the loop system to include a 12 foot by 8 foot metal control room panel (donated by a local paper mill), a set of computer workstations for DCS and SCADA system consoles, industry-standard terminal block assemblies located in electrical enclosures, with plenty of electrical conduit runs between different locations in the lab facility to allow pulling of new wires and cables. Students still must connect each instrument they learn about into the system, configuring either a panel-mounted or computer-based display to register the measured variable in proper units (or to receive a control signal if the instrument in question is a final control element). Construction of working control systems (transmitter, controller, valve or motor) is quite easy with this infrastructure in place. The geographically distributed nature of the system lends itself well to realistic troubleshooting, with students working in teams (communicating via hand-held radios) to diagnose problems intentionally placed into the system.

A new feature of the 2006 multi-loop system is that it included digital communication as well as analog (4-20 mA) signaling. Multiple Ethernet hubs were installed throughout the lab, interconnected to form a single 10 Mbps network linking personal computers with loop controllers and PLCs. Non-dedicated category 5 cabling was also used for RS-232 and RS-485 communication between serial devices (e.g. data acquisition modules) as needed. FOUNDATION Fieldbus wiring was also installed (twin-lead shielded cable with 100 $$\Omega$$ characteristic impedance) allowing the interconnection of fully digital field instruments such as transmitters and digital valve positioners.

The following photographs show the appearance of the new (2006) multiple loop system, beginning with the control panel and computer workstation cluster. These two elements comprise the “control room area” of the lab:

In another area of the lab room is a pneumatic control panel and a cabinet housing the distributed control system (DCS) I/O rack:

The rest of the lab room is dedicated as a “field area” where field instruments are mounted and wires (or tubes) run to connect those instruments to remote indication and/or control devices:

Note the use of metal strut hardware to form a frame which instruments may be mounted to, and the use of flexible liquid-tight conduit to connect field instruments to rigid conduit pieces so loop wiring is never exposed.

A less expensive alternative1117 to metal strut is standard industrial pallet racking, examples shown here with 2 inch pipe attached for instrument mounting, and enclosures attached for instrument cable routing and termination:

The multiple-loop system is designed to be assembled, disassembled, and reassembled repeatedly as each student team works on a new instrument. As such, it is in a constant state of flux. It is not really a system so much as it is an infrastructure for students to build working loops and control systems within.

In addition to the multiple-loop system, my students’ lab contains working processes (also student-built!) which we improve upon every year. One such process is a water flow/level/temperature control system, shown here:

Another is a turbocompressor system, built around a diesel engine turbocharger (propelled by the discharge of a 2 horsepower air blower) and equipped with a pressurized oil lubrication system and temperature/vibration monitor:

Yet another permanent process is this electrical power monitoring unit, where protective (overcurrent) relay operation may be demonstrated:

Measurements of voltage and current in this particular system may be integrated into the rest of the multi-loop system by using voltage and current transducers with 4-20 mA output signals. Digital protective relays may be connected to the multi-loop system using serial data communication (RS-232, RS-485) signals.

The process piping and equipment on these permanent systems are altered only when necessary, but the control systems on these processes may undergo major revisions each year when a new group of students takes the coursework relevant to those systems. Having a set of functioning process systems present in the lab at all times also gives students examples of working instrument systems to study as they plan construction of their temporary loops in the multiple-loop system.

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