Why_BMS_not_PLC

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Industry and Technology

Why BMS not PLCs forBuilding Automation?

Modern building automation solutions are required to

fulfill many often diverse and demanding objectives.

Foremost among these are the need to provide comfort,

security and energy efficiency. Depending on the purpose

of the building, facility, campus or enterprise, other

common requirements include: fire detection and life

safety assurance, enterprise system integration,

regulatory compliance support, critical parameter and

cleanliness level maintenance, data logging and alarm

management.


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INTRODUCTION

In recent years some debate has arisen as to whether Programmable Logic Controllers (PLCs) are suitable as an

alternative to Building Management Systems (BMSs) to meet these needs. This paper is intended to provide a

concise historical review of the technologies and an objective comparison between the two solution platforms,

their relative merits and application suitabilities.

SOME EARLY HISTORY

The PLC was invented in 1968 by Richard Morley and his colleagues within Bedford Associates, a small,

New Hampshire based engineering consulting firm. The Modicon (Modular Digital Controller) Company

was incorporated in 1969, building a substantial business around these new devices. The initial sales

success of PLCs was in the area of transfer lines in automotive plants. Over the years the capabilities

and application scope of PLCs have dramatically increased but automotive manufacturing remains their

most significant market. Nearly four decades later, the PLC is arguably the most widely used product

type in the industrial automation business, with a worldwide market of several billions of dollars per year

and available from hundreds of different sources, in many different form-factors (including embedded)

and prices ranging from tens of thousands of dollars (for triple redundant, failure-proof systems) to

commodity, catalog products at less than a hundred dollars.

In 1975, Mr. Morley was also a founder of Andover Controls. This companys first product, the

Sunkeeper, was targeted at the solar energy management market and introduced the worlds first Direct

Digital Control (DDC) system. The companys products quickly evolved encompassing much wider

building management responsibilities. The product lines included the highly successful AC256 introduced

in 1981, the Infinity system which debuted in 1989 and the currently available Continuum system which

followed in 1997.

One may speculate as to why PLCs were not simply applied to

building management applications from the very beginning. Were

business and technical needs left unmet, giving rise to building

automation companies such as Andover Controls? The simple answer

is yes; the deficiencies of PLCs for building management applications

effectively spawned the modern digital Building Management

Systems market. Indeed, in a recent interview with Mr. Morley, he

explained anyone trying to apply PLCs to building automation

applications is investing in excessive, divergent performance.

Today, both of the product heritages of Modicon and Andover

Controls are owned by Schneider Electric, the worlds power and

control specialists. This uniquely privileged position allows us to

present a fair and balanced comparison between these two

categories of powerful automation solutions.

Richard Morley with the worlds

first PLC, the Modicon 084.


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KEY REQUIREMENTS DIFFERENCES

It should be kept in mind that a fundamental difference in design intent exists between BMS and PLC

systems. BMS systems are primarily intended to control and protect the environment around people,

physical assets and data. PLC systems are primarily intended to control and protect the production

capacity of machines and manufacturing lines. This basic distinction underlies many of the following

comparison points not just in terms of the enabling technologies but also the types of companies andpeople involved in the successful completion of building automation projects and the ongoing service

and maintenance of these systems.

SYSTEM TOPOLOGIESBuildings are big. Space control requirements can range from a few hundred square feet well into the

millions. This space may be distributed across several buildings of a campus or even many multi-

building facilities across a country or continent. BMS systems have evolved to meet this type of control

challenge. PLC systems are usually physically limited to a single machine or production line within a

single building.

NUMBER OF USERS

BMS systems are designed to have many users with diverse needs interacting with the system on

several levels through multiple devices. This can range from an office worker changing the setpoint on

their local thermostat through to a Facility Director viewing plantwide energy efficiency statistics through

a web browser. Configurations supporting various classes of users with a wide range of privilege sets

are common. From an all-powerful administration account through to read-only access to a few points

on a single screen, modern BMS systems can easily be configured to provide vastly different users

experiences to a common system depending on the location, function and authority level of the various

user types.

PLC

BMS

Bits & Bytes

Object Oriented

Human Comfort & SafetyMachine Efficiency


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In contrast, PLC systems often have only very localized user interfaces. This makes sense because of the

need for close proximity of the operator to the machine to verify its correct operation and to perform

mechanical maintenance. Where connection to enterprise systems is provided it is often through

standardized interfaces such as OPC. 3rd party SCADA vendors provide many of the system level views

of PLC-based configurations.

DEGREE OF DETERMINISM

Generally speaking, BMS systems operate around human time. Air conditioning control sequences are

triggered by occupancy conditions and the comings and goings of facility area occupants are captured in

familiar time/date formats.

In contrast, PLCs are usually designed to operate in the 1-50 millisecond scan-time range and are

generally concerned with machine time (i.e. their operation is based upon mechanical and electro-

mechanical system dynamics.) As a consequence, all other functions are subjugated to logic and I/O

processing in order to ensure highly deterministic system operation. BMS systems are typically more

adaptive in that they give communications functions a much higher priority. Part of the reason for this is

that the individual controllers have been designed from the very outset to be part of a larger system andhence awareness of their availability and ability to interact with the rest of the system is fundamental to

the overall system operation.

An example of an important difference in communications approaches between the two types of systems

is that BMSs, along with regular I/O scanning mechanisms, have long featured (over 20 years) built-in

report-by-exception capabilities.

Three types of information can be reported by exception:

1. Alarms

2. Change of value

3. Object attributes referenced by other controllers

This mixed scanning/exception reporting approach makes highly efficient use of communications

bandwidth, thereby delivering optimal system performance, while only minimally impacting overall

system determinacy.

Although both BMS and PLC field controllers can function in a standalone manner, it is much more

common for PLCs to act as localized islands of automation. While there is a very significant trend in

networking PLCs over various fieldbusses, they remain far more functionally isolated than controllers in

a typical BMS installation.

ALARM DETECTION AND MANAGEMENTAlarms within BMS architectures are generally detected at the field controller level. The various alarm

limits and notification destinations (where the alarms should be sent) are basic attributes of the

monitored values and are distributed and processed as close to the signal source as possible. In PLC-

based architectures alarm management is almost always performed only at the SCADA level. While

functional, this is a far less robust approach, as the workstations are much more likely to become

unavailable than the PLCs. Another disadvantage to this approach is that transient alarm conditions are


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far more likely to be missed because of the communications overheads and asynchronous task

processing of the two system levels.

DATA LOGGING

BMS systems are often used to provide both short and long-term records of environmental parameters.

Great quantities (1,000s) of these logs can be stored near their signal sources within distributed fieldcontrollers as well as being aggregated in more centralized database resources. BMS systems are

designed to automatically provide this capability with simple configuration options. Of course, PLCs have

local storage capabilities but are generally oblivious to human real time (time of day etc.) and a great

deal of custom application programming is required to even approximate the logging functionality

natively available within a BMS field controller.

NATURE OF THE MEDIUM

One often overlooked attribute of the air handling aspect of building automation is that the medium

under control (the air) affords the control system a degree of natural fault tolerance. BMS

architectures are designed to allow subsystems to be temporarily taken out of service for maintenance

or repair without dramatically impacting the quality of the overall environment. This is due to the fact

that other operational subsystems will generally compensate for the non-functional unit. The overall

system may be temporarily unbalanced but a comfortable environment is normally maintained. BMS

systems have evolved to anticipate such circumstances. In contrast, systems controlled by PLCs are

usually rendered entirely inoperable should the PLC fail.

PURPOSE BUILT = LOWER INSTALLED COST

When contrasting the I/O types of PLC and BMS systems it soon becomes evident that the BMS I/O is,

not surprisingly, more adapted to control the types of devices commonly found in air handling and

security management applications. For example, BMS field controllers are available with built-in airflow

sensors and damper actuator motors. These controllers are purpose-built to be mounted directly onto

ductwork. No equivalent to this exists in the PLC world.

Another feature of BMS field controller I/O is that of built-in signal conditioning. This often overcomes

the need for separate and expensive signal conditioning blocks. A variety of signals from input devices

such as thermistors and airflow sensors are characterized directly within the firmware of the I/O module

to provide the software with linearized SI and Imperial standard signal representations. In most PLCs,

costly, specialist I/O modules are required to perform the equivalent task. It is also very common for

BMS field controller I/O to feature built-in manual override switches and potentiometers - so called

Hand/Off/Auto switches. These are extremely useful during system commissioning and servicing and

when a controller is temporarily offline from the rest of the system. It is very unusual to find this feature

in PLCs.


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Thermistors are a good example of fitness-for-purpose of sensors in building automation applications. As

PLCs became applied to an ever widening range of industrial control applications, the absolute and range

of temperatures they became expected to monitor and control were only able to be provided from

thermocouples. Thermistors, on the other hand, while less linear are far more cost-effective and

practical for the temperatures typically encountered in building automation applications. In fact, one of

the earliest and still very evident differentiators of PLCs and BMS field controllers is in their ability tonatively condition thermistor signals.

INTEGRATION NOT INTERFACING

PLCs are sometimes interfaced to a few complementary automation devices. This frequently requires

custom coding and significant engineering time. In comparison, BMSs usually support extensive, native

integration capabilities and are often tied to dozens of facility subsystems. As shown below, such

subsystems include: power, utilities, process, security and life safety. The BMS can also form a

consistent means to integrate with higher level enterprise business applications such as Manufacturing

Execution (MES) and Enterprise Resource Planning (ERP) systems. For example, to support security

integration; BMS systems have dedicated I/O modules to perform functions such as door controls. Along

with dedicated digital I/O (request to exit, door strike etc.), these modules incorporate specialized

communications protocols (such as Weigand) that allow easy integration with a huge variety of ID

verification devices including proximity card readers and biometric recognition devices.

+Airflow Sensor+

Actuator+

Room Sensor+

Signal Conditioners+

Lots of Engineering+

Enclosure (Plenum-mount)

=

PLC High Installed Cost


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SOPHISTICATED OBJECT MODEL

BMS are built around a sophisticated, distributed software object model. The system database holds a

complete topological map, data/program image and revision history for every physical and logical entity

within the system. This approach supports a high degree of automatic configuration management. The

object types themselves range from simple discrete I/O to complex compound objects such as doors.

While outwardly a door may appear to be a simple control object, dozens of attributes may apply to

each instance.

REGULATED INDUSTRY APPLICATIONSIn highly regulated industries such as pharmaceutical and medical device manufacturing, a common

requirement is to be able to provide proof of compliance with regulations through the use of automated

systems. A clear advantage of BMS systems in this area is that they are database centric. PLCs are local

memory centric. This means that a BMS has a far greater capacity to keep long term records of all

critical facility related events and parameters. Indeed, a fundamental requirement of the Electronic

Records and Signatures regulation (21 CFR Part 11) is that comprehensive audit trails be maintained.

The database centric nature of a BMS makes this a straightforward task to accomplish. In contrast, the

limited local memory of a PLC has little, if any, capacity to capture this critical information.

THE MYTH OF IEC 61131-3

The IEC standard 61131-3 has been available now for over 15 years. The intention of this standard is toprovide a high degree of basic functional uniformity from the control products of industrial automation

systems manufacturers. The idea was that system users would be able to create rich, vendor-

independent libraries of reusable control code.

While this is a commendable objective, it has met with limited success. From a building automation

standpoint a fundamental problem is that the standard is written around the use of traditional PLC

programming languages and software structuring tools - Ladder Diagram, Function Block Diagram,

Instruction List, Structured Text and Sequential Function Charts.

These languages are well suited to machine and production line applications but not to BMS applications.

They are not particularly object oriented and do not intuitively map to common BMS control

requirements.

Another major problem is that the IEC 61131-3 languages are extensible. This means that the different

manufacturers can and do create proprietary extensions to their implementations which severely limits

the degree of portability and reusability of any application code developed.


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NETWORKING DIFFERENCES

BMS and PLC networks have evolved separately and differently. Today almost all BMS manufacturers

offer systems based on either or both of the open standards, LONWorks and BACnet. These

communications standards are sophisticated, well-defined and their implementation is well controlled.

This affords BMS users a high degree of integration capability. These networks natively support theobject types and services required by the building automation world. LONWorks and BACnet are also

natively supported by a wide range of complementary devices such as variable speed drives, power

monitoring and metering equipment and lighting controllers.

The network standards prevalent among PLCs are generally less sophisticated and rooted in the

proprietary technology of the larger vendors. Profibus and DeviceNet are good examples. While these

network technologies are highly functional for machine automation applications, they are cumbersome

to implement and not a good fit for building automation applications.

DON'T PAY FOR WHAT YOU DONT NEED

As PLCs were originally invented to replace cabinets full of electro-mechanical relays situated near (andsometimes even on) production equipment, it was imperative that they were able to withstand the

electrical and mechanical rigors of these harsh environments. The I/O boards of PLCs therefore generally

feature a high degree of channel to channel isolation, electrical noise suppression capability and

mechanical ruggedization. Some models even allow defective I/O modules to be replaced under power

or so-called hot-swapped. While these features are necessary for such industrial control applications,

they do add significant cost to the design of PLCs. BMS field controllers have been designed to operate

in the utilities environment of a facility which is typically far less severe. The electrical and mechanical

design of BMS field controllers has therefore evolved to be better suited to such conditions. A significant

commercial advantage of BMS field controllers over PLCs is that the user does not pay for a degree of

product ruggedization unnecessary for the application.

PLC Program BMS Configure


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EASE OF SOLUTION

Perhaps the greatest single advantage of BMS over PLCs falls in the area of ease-of-solution or

engineering costs. As the workhouse of the industrial automation world, modern PLCs are undoubtedly

highly flexible and reliable devices. Given the target physical application domain of facility management,

there exist no significant reliability difference between BMS and PLCs. However, a tremendous difference

does exist in application engineering time and costs. In part due to the very flexible nature of PLCs, theamount of work required to perform even rudimentary facility automation tasks with these devices can

be many times that required when employing purpose-build BMS solutions. For example, consider the

work involved in setting up a control sequence for a fan-coil unit application as shown in the figure

below. This would require a great deal of bespoke code development in a PLC while a BMS controller

would only require straightforward configuration of its built-in functionality.

CONCLUSION

PLCs and BMS have evolved into powerful automation solutions. PLCs are extremely versatile and withenough effort can be used to meet the needs of almost any automation application. BMS, on the other

hand, have a more restricted application domain but a far higher degree of fitness for their intended

application scope. As shown in the diagram below, for small standalone system installations PLC and

BMS costs may be comparable but for larger, more sophisticated installations, overall engineering,

maintenance and operating cost will become increasingly higher for PLC-based over BMS based systems.

Another important consideration is that PLC-based systems will reach practical application size,

complexity and manageability limits long before BMS-based solutions.


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Of equal, if not greater, importance than the control equipment used on a building automation

application is the expertise and experience of the organizations and individuals providing system

engineering and support services. It should be recognized that, generally speaking, PLC integratorstypically have no building automation expertise and their staff are traditionally from electrical

engineering backgrounds. In more recent years this type of integrator has been taking on more staff

with software development expertise. Equally, BMS integrators rarely have process or machine control

expertise and their staff are traditionally from mechanical automation backgrounds. In more recent

years this type of integrator has been taking on more staff with IT expertise.

As we have seen, while they have some historical and technical commonalities, PLC and BMS systems

have followed necessarily different evolutionary paths with BMS systems being more highly adapted to

the building management applications domain. Equally, PLCs perform a vitally important role in the

industrial manufacturing and process automation worlds.

PPLLCC

BBMMSS

Application

Complexity

Low

IntelligentBuilding

High

Installed Cost

Simple

EquipmentControl


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It is important to appreciate that these two automation solution platforms deliver complementary

functionality. As industrial processes and the surrounding environments of the machines and people that

control them are increasingly recognized to be interdependent, integrating PLC and BMS systems is

becoming ever more common. Fortunately, straightforward integration is facilitated by many network

protocol, software interface and database standards such as Modbus, TCP/IP, XML, OPC, SQL and ODBC.

Low

High

Applications

SCADAPLC

BMS

Fitness


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