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Ratio controller for application configuration

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article image Ratio control -- a typical application.

THE Eurotherm 2604 triple loop controller is now available with a Ratio control function block for simple configuring of applications requiring ratio control, such as required in chemical dosing, water treatment, chlorination, mixing vessels, waste incinerators and gas furnace control.

Ratio control is a technique used to control a process variable at a setpoint which is calculated as a proportion of an uncontrolled (lead) input. It is one of the advanced control functions provided by the 2604 controller.

The ratio setpoint determines the proportion of the lead value to be used as the actual control setpoint. The ratio setpoint can be applied as either a greater or a fraction to the (lead) input.

A typical application is in a mixing vessel where in order to achieve the correct mix of product, inputs 1 and 2 need to be maintained at a constant ratio.

When the control loop is placed in manual or ratio has been deselected, setpoint tracking is used to recalculate the setpoint from the actual achieved ratio, thereby ensuring bumpless transfer between modes.

If the input from the lead process value becomes invalid (open circuit probe etc), then the working setpoint of the loop will remain at its current value.

The 2604 ratio function block maintains the relationship between two variables, provides for ratio setpoint tracking and ratio setpoint trim.

Another useful feature is the automatic calculation of the actual measured ratio which is then available to be displayed on the controller front panel.

The 2604 is part of the Eurotherm series 2000 family of process and temperature controllers. It is a powerful multi-function controller/programmer available in a single, dual or three-loop format.

It is a high-accuracy, high-stability instrument designed for demanding, high-accuracy multi-loop control applications, such as furnace and kiln control, environmental chambers, and a wide range of critical batch production processes in the chemical, brewing, pharmaceutical and plastics industries.

In addition to advanced control strategies such as Ratio control, function blocks are also provided for cascade and override applications as standard.

The 2604 features a dual 5-digit LED display for high accuracy measurement, with a dual line 20-character LCD panel for display of alarm messages, programmer and loop status information. User defined messages in the LCD panel simplify operation.

The 1/4 DIN panel mounting 2604 has a simple modular construction allowing for the addition of five plug-in I/O modules to be added to increase the standard I/O of two temperature and process inputs, eight selectable I/O, and one relay out.

In addition a 40 digital I/O expander is available for logic alarming and control functions. The plug-in I/O blocks can be retro-fitted at any time, allowing the function of the 2604 to increase with application requirements.

Two serial communication ports are available. Plug-in cards allow for ModBus (RS232, RS422 & RS485), DeviceNet and ProfiBus network connections.

As a slave device this allows the 2604 to be monitored by SCADA and PLC systems while as a master the 2604 can monitor and control other control devices on a ModBus network. The 2604 can be both a master and slave simultaneously.

The 2604 has numerous advanced control parameters, including a 500 segment, 50 program setpoint profile capability, real time clock, timers, totalisers, Maths advanced alarming and combinational logic function blocks, plus advanced PID control parameters including gain scheduling and digital event sequencing.

Automatic power supply feed-forward control provides further stability to normal process control.

The instrument incorporates a self-correcting input circuit to maintain calibration accuracy. Configuration is achieved either via a front-panel interface or by using Eurotherm's "iTools" configuration package running under the Windows 95 or NT operating systems.

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