West Instruments

West Controller Overview

Principle of Operation
West offers a wide variety of digital temperature/process controller models and accessories designed for temperature, process and environmental control applications. West temperature and process controllers are available in the following varieties and control mode technologies:

Single Loop versus Multi-Loop Controllers
West temperature controllers come in many different styles with a vast array of features and capabilities. There are also plenty of ways to categorise controllers according to their functional capabilities. In general, temperature controllers are either single-loop or multi-loop in nature.

Single-loop controllers have one input and one or more outputs to control a thermal system. On the other hand, multi-loop controllers (e.g. the West MLC9000+) have multiple inputs and outputs, and are capable of controlling several loops in a process. More control loops permit the control of more process system functions. Reliable West single-loop controllers range from basic devices that require single manual set-point changes to sophisticated profilers that can automatically execute up to eight set-point changes over a given time period.

Beside single-loop controllers which can control only one process loop, multi-loop controllers can control more than one loop, meaning they can accept more than one input variable.

Generally speaking, a multi-loop controller can be thought of as a device with many individual temperature controllers inside one single chassis. These are typically mounted behind the panel as opposed to in the panel or front-to-panel. Programming any one of the loops is very similar to programming a panel mounted temperature controller, however multi-loop systems tend not to have the traditional, physical user interface (no display or switches), instead a dedicated communication link is available.

Multi-loop controllers need to be configured by a specialised software programme on a PC that can 'download' the configuration to the multi-loop temperature controller using the dedicated communication interface.

Multi-loop controllers provide a compact modular system that can operate either within a stand-alone system or in a PLC environment. As a replacement for temperature controls in PLCs, they provide fast PID control and off-load much of the maths intensive work from the PLC processor, allowing for faster PLC scan rates. As a replacement for multiple DIN controllers, they provide a single point of software access to all control loops. The cost of installation is reduced by eliminating much wiring, panel cutouts, and saving panel space.

Information is retrieved via a communications interface. Common communications interfaces that are supported include DeviceNet, Profibus, MODBUS/RTU, CanOPEN, Ethernet/IP, and MODBUS/TCP.

Multi-loop controllers provide some additional features not available on traditional panel mounted controllers. For instance, multi-loop controllers have higher loop density for a given space. The West MLC9000+ multi-loop temperature control system can have up to 32 loops of control in a DIN rail mounted package not much longer than 8". They also reduce wiring by having a common connection point for power supply and communications interfaces.

Multi-loop temperature controllers also have enhanced security features, such as not having buttons for a person to use and change critical settings. By having complete control over the information being read from or written to the multi-loop controller, the machine builder can limit the information that any given operator can read or change, preventing undesirable conditions from occurring, such as setting a setpoint too high to a range that may damage product or the machine.

Some additional considerations for a multi-loop controller is the lack of need for panel cutouts. In addition, controllers can be hot-swapped. This lets a controller be changed out without having to power down the system. Modules also auto-configure after a hot swap.

Single-loop controllers are also referred to inter-changeably by Danaher Industrial Controls as front-of-panel (FTP) controllers whereby the West MLC9000+ multi-loop controller is a form of behind-the-panel (BTP)/DIN Rail mount controller. The advantages and limitations to both front-of-panel (FTP) controllers versus behind-the-panel (BTP)/DIN Rail mount controllers are further outlined below.

Front to Panel (FTP) versus Behind-the-Panel (BTP) or DIN Rail Mount Controllers

Front to Panel Controllers
The most common type of controllers are the front-of-panel (FTP) type controller. These controllers provide an easy-to-read display that is shown directly on the front of the controller. This display allows the operator or engineer to make changes and programme the controller while standing directly in front of it.

Advantages FTP

• Designed for snap-in machine panel cut-out installation on OEM machinery or wall mount (if recorder based)
• DIN Size specified by OEM manufacturer
• Danaher Industrial Controls FTP model controllers conform to all industry DIN size standards

Limitations FTP

• For applications requiring multiple controllers, each controller must be individually wired and programmed which can be time consuming
• Network protocol options tend to be more limited than BTP (e.g. Modbus, DeviceNet)

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Behind-the-Panel (BTP) or DIN Rail Mount Controllers
Behind-the-Panel (BTP) controllers, like their name implies are typically found encased within a power supply cabinet on a manufacturing floor (like a fuse box). These types of controllers typically do not have displays, and a laptop computer is used to view controller parameters, so it is not necessary for the controller to be assessable on the front of the panel.

Advantages of BTP

• Moderate to high accuracy
• Compact, space-saving DIN rail mount design (can operate with PLC or Scada HMI environments
• Available in a variety of Bus Control Modules (BCM) and Loop Control Modules (LCM)
• Detachable terminals optimised for easy maintenance and wiring (with features such as hot-swap and auto-configuration)
• Allow control of multiple loops from a single controller
• Overcomes weaknesses of PLC temperature control or multiple front to panel discrete controllers.
• PC configurable and programmable in minutes versus hours for discrete controllers; can save recipes for backup or reprogramming
• BTP controllers offer more network protocol options than discrete controllers typically (e.g. Profibus, DeviceNet, CANopen, EtherNet and Modbus)

Limitations of BTP

• No local display compared to FTP
• No local configuration; configuration software running on a PC is usually required. Although sometimes the controller can be set-up via DIP switches or jumpers
• No VMD, limit device or profiling capability on BTP controllers - currently only available on FTP models
• If you only need 1 loop of control, BTP may not be the best choice as you are forced to buy multiple loops which may not be necessary for your application

 

Controller Modes/Technologies
The common categories of  West closed-loop temperature and process controllers are Limit, Valve Motor Drive (VMD) and Profile controllers.

 

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Limit Device Controllers
Limit control is the simplest form of control, used by almost all domestic appliance thermostats and furnaces. Used for processes which require an extra device for safety reasons to prevent damage to the appliance and possibly any 'product' that may be damaged by excessive temperatures. These controllers provide safety limit control over process temperature. Put simply, they are an independent safety device to be used alongside an existing control loop and are capable of accepting thermocouple, RTD, or process inputs with limits set for high or low temperature just like a regular controller. Limit control is latching and part of a redundant control circuitry to positively shut a thermal system down in case of an over-limit condition. Power to control the element is switched through the contacts on the limit device, which "drop out" if the temperature goes above or below a specified range or preset limit. Many limit controllers are factory mutual (FM) approved and have latching relays which require a front panel reset to re-engage the contacts.

Advantages

• Simplest form of control
• Involves only one parameter that needs to be tuned
• Provides fast latch output reaction
• Low cost controller
• Available in all popular DIN sizes

Limitations

 

• Low Accuracy
• Has NO ability to control temperature on its own
• The latching limit output must be reset by an operator, it will not reset by itself once the limit condition does not exist.

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Valve Motor Drive (VMD) Controllers
Valve Motor Drive controllers are specifically designed to control valve motors used in manufacturing applications such as gas burner control on a production line. Special tuning algorithms give accurate control and fast output reaction without the need for slide ware feedback or excessive knowledge of three-term PID tuning algorithms. Valve motor drive controller are used to control the position of the valve, somewhere between 0% open to 100% open, depending on the energy needs of the process at any given time. They use the ON/OFF Duplex function which is a very simple algorithm and like its counterpart, ON/OFF control, is another low cost controller with fast output reaction but low accuracy.

Advantages

• Provide accurate control of valve motor drives with little PID knowledge 
• Simplest form of control
• Involve only two parameters that need to be tuned (input and output)
• Minimises control element cycling
• Provides fast output reaction via special tuning algorithms
• Low cost controller 
• Available in popular DIN sizes

Limitations

• Low accuracy
• Can only be used with valve motor drives

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Profile Controllers
Profiling controllers, also called Ramp-Soak controllers are controllers that will allow the operator to programme a number of setpoints and the time to sit at each set point. The changing of the setpoints are called "Ramp" and the time to sit at each set point is called "Soak" or "Dwell". One ramp and one soak is considered to be one segment. A profiler offers the ability to enter a number of segments to allow complex temperature "profiles". There are many applications for a profiling controller. The profiles are often referred to as "recipes" by the operator and are often kept on a computer and downloaded via a communications channel directly into the controller as needed. Most profilers allow storage of multiple recipes for later use. Smaller profilers may allow for 4 recipes with 16 segments each while more advanced profilers would allow for more. Profile controllers are able to execute ramp-and-soak profiles such as temperature changes over time, along with hold and soak/cycle duration, all the while being unattened by an operator, allowing the operator to perform other tasks. Typical applications for profile controllers include heat treating, annealing, environmental chambers, and in complex process furnaces.

Advantages

• Available in all popular DIN sizes
• Moderate to high accuracy (model dependent)
• Allow operator to programme a number of setpoints and the time to sit at each set point.
• Ability to enter a number of segments to allow complex temperature "profiles" for enhanced control
• Once programmed profilers can operate unattended by the operator, allowing the operator to perform other important tasks 

Limitations

• Available in basic and high end models
• Vary in the number of segments and programmable profiles.
• Moderate to high cost

 

West Digital Controller Products Comparison Table... At A Glance

	West™				PMA™
	Temperature and Process Controllers				Multifunction Controllers
	ELECTRONIC				ELECTRONIC
	DIGITAL				DIGITAL
Types	General-Purpose Limit	General-Purpose VMD	High-End Profile	High-End PID	High-End Electronic
Sizes	1/4, 1/8 and 1/16 DIN	1/4, 1/8 and 1/16 DIN	1/4, 1/8 and 1/16 DIN	1/4, 1/8 and 1/16 DIN	1/4, 1/8 DIN
Mounting Base Front to Panel (FTP); Behind the Panel (BTP)	FTP Only	FTP Only	FTP Only	BTP and FTP	BTP and FTP
Features / Functionality	Moderate	Moderate	Moderate - High	Moderate - High	Moderate - High
Programmable	Yes	Yes	Yes	Yes	Yes
Ease of Use	Very Simple	Very Simple	Moderate - Complex	Moderate - Complex	Moderate - Complex
Tuning Options	Moderate	Moderate	Multiple	Multiple	Moderate - Complex
Delivery	3 Days	3 Days	3 Days	3 Days	2-4 Weeks

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