Microprocessor-based control retrofits: Old and new technology for contractors

July 12, 2000
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Beginning in the early 1980s, microprocessor-based controls have been used to replace electrical-mechanical control systems on compressor and chiller packages. Traditional electromechanical components (pressure switches, rotary drum timers, temperature switches, freezestats, wiring, relays, etc.) have given way to microprocessors, EEPROMs, transducers, RTDs, analog circuits, and digital I/O modules.

The trend toward microprocessor-based systems is prevalent in both the hvac and industrial refrigeration industries.

The basic control philosophy has not changed and operates by maintaining control of a pressure or temperature relative to a setpoint; however, that is where the similarities end. Even early microprocessor-based systems had significant advantages over their electromechanical counterparts, such as:

  • Operator keypad interface;

  • Display of operating data; and

  • Display of shutdown messages.

    While earlier microprocessor controllers were operationally superior to electromechanical control systems, they still had their limitations. Early microprocessor designs were proprietary and strictly limited to the oem’s equipment design.

    In most cases, this meant your microprocessor control system would be very limited in:

  • Data editing/entry functions;

  • Compressor type (screw, centrifugal, reciprocating, etc.);

  • Functionality (controlling other devices);

  • Analog interface;

  • Communications;

  • Diagnostic capability; and

  • Custom applications (sequencing, condenser control).

These limitations and more have been addressed with the development of the FES “Micro III” compressor control panel, Figure 1, and Micro III systems panel.

Data entry/editing functions

The Micro III control panel keypad and LCD graphic display are large, clear, and easy to use, Figure 2. The “change display” and cursor (arrow) keys allow the operator to efficiently move between display groups and setpoints. The displays are logically grouped to assist in locating parameters based on a particular topic.

The display groups include: operating data, analog data, operating parameters (control setpoints) and alarm parameters (user diagnostic setpoints), historical data, compressor data, alarm messages, and shutdown messages.

Alarm and shutdown conditions are immediately and clearly annunciated by flashing LEDs on the respective “shutdown” or “alarm” push-button whenever a problem exists. Depressing the shutdown or alarm key will immediately transfer the display to a description of the event and a time stamped of when it occurred.

Historical data displays also provide both short-term and long-term analog data at user-selectable intervals.

Historical data displays allow the operator to view past analog data and compressor operating status. Additional snapshots of historical data are taken each time an alarm or shutdown condition occurs.

The combination of historical data and alarm/shutdown information provides the contractor with a powerful set of tools for troubleshooting almost any type of problem.

Compressor types

The FES Micro III compressor control panel is designed to replace electromechanical panels or outdated microprocessor controls on most brands of equipment, including dual and two-stage packages.

The Micro III panel can be readily applied to the following compressor types:

  • Rotary screw;

  • Centrifugal;

  • Rotary vane; and

  • Reciprocating.

The Micro III compressor control panel can also be custom configured to control and monitor additional devices on compressor or chiller packages. FES utilizes “LOGIC BLOCKS” programming, which replicates PLC functions within the Micro III processor.

The use of logic blocks provides a greater level of integration by providing single panel control of the entire refrigeration system, eliminating the need for many individual control devices. A few examples of what can be configured using FES logic blocks are:

  • PID control of electronic valves;

  • Modulating condenser water regulating valve;

  • Variable-frequency drives;

  • Refrigerant monitors; and

  • Power usage (kWh).


Important details

The standard Micro III compressor control panel, provides capability for up to 40 optically isolated I/O modules and 16 analog input channels.

Each analog input is jumper selectable for 1-5 vdc, 4-20 mA, or ICTD input and can be software configured to accept almost any transducer-scaling requirement. The Micro III processor also supports four analog output channels that can be configured as either isolated (4-20-mA loop externally powered) or non-isolated (4-20-mA loop powered from the Micro III).

The combination of these powerful design features allow the Micro III panel to be adapted to almost any type of analog system easily.

The Micro III control panel supports high-speed FES “ComMENT” network interface, which makes external data acquisition a breeze. All control functions and operating parameters that can be accessed at the panel can be remotely accessed via the FES ComMENT network.

A ComMENT gateway provides a sophisticated yet simple interface with supervisory systems by emulating industry-leading PLCs such as Allen Bradley, Modicon, and GE.

The emulation provides easy connection to your DCS system or to a FES “MicroLINK” or FES “LiteLINK” supervisory system. The FES LiteLINK Windows 95/98/NT PC-based supervisory software provides an affordable way to dial-up (via modem /ComMENT gateway interface) and access real-time information.

The MicroLINK provides a completely custom graphical MMI solution with historical data, custom reporting, and advanced security features.

Built-in diagnostics software can quickly test the operation of all hardware components of the microprocessor panel. The diagnostics include functional tests for the display, keypad, transducers, I/O and communication interface using simple keyboard commands.

Individual analog outputs can also be manually controlled to assist in troubleshooting most interface problems that may arise.

Micro III control panels can be configured to include a multitude of additional standard options such as compressor sequencing via FES’ ComMENT network communications, condenser control, chiller control, refrigerant detector monitoring, vessel level control, etc. Unique or custom control functions can also be provided if needed.

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