Chapter-3

Direct Digital Control System

Reference:

•Direct Digital Control of Building Systems- Theory and Practice by H. Micheal Newman

•Direct Digital Control System by J.B.Knowles

•Direct Digital Control for HVAC systemby Thomas B. Hartman

•Objectives

• Upon Completion of this chapter, the reader will be able to:

  Explain the details of Direct Digital Controller’s hardware

  Describe the DDC I/O points and operational features

  Understand the DDC communication and application software

IT & Networking Bldg Services

EEE

BAS

What is BAS?

DDC block diagram

Basic features of DDC hardware components:

• microcontrollers that can carry out programmed control of a physical process such as temperature, pressure, humidity and flow rate, etc

• consist of hardware, software and firmware

• Stand alone/ networking capability

•

DC power Supply • usually powered by an internal 24 V power supply

derived from the 240 V ac supply. • The 24V is also used for driving sensor and output

control circuitry. • The DDC’s microelectronic circuits only operate on

low-voltage dc, typically up to 5V• In case of mains failure, or when the mains supply is

switched off for maintenance, the DDC memory is powered by a back-up battery.

• The back-up battery must be reliable, of adequate life expectancy, and of sufficient capacity to retain the DDC memory programs for sufficient time until mains power is restored.

Power Supply• powered by an internal 24 Vac power

supply derived from the 240 V ac supply.

• LV(ac) is used for driving sensor and output control circuitry

• DDC’s microelectronic circuits only operate on low-voltage dc, typically up to 5V, also derived from the ac supply.

• Why shall it be LV?

Backup Power Supply

• DDC RAM memory is powered by a back-up battery during power break down.

• Without which most of the user's control programs resident in the DDC RAM would be lost.

• must be reliable, of adequate life expectancy, and of sufficient capacity to retain the DDC memory programs for sufficient time until mains power is restored.

• Usually 72 hrs back up capacity shall be expected.• Most powerful one is lithium battery with compact

size.

Communication interface

• communication interfaces to:

- a modem (modem port)

- a local area network port (LAN) (network terminals)

- other operator interface devices, e.g., serial communications port (USB) for connecting it to a portable PC (PDA) for local programming and re-configuration.

The microprocessor (CPU)

• contained in a dual-in-line (DIL) package with between 16 and 64 pins.

• (i) Control unit: • (ii) CPU memory:• (iii)Arithmetic and logic unit (ALU):• (iv)CPU clock unit: (Timer interval)• (v) DDC Clock• (vi)Address and data buses • (vii)DDC RAM and ROM

(i) Control Unit

• controls the operations of the microprocessor ALU and other associated chips, such as memory chips

• coordinates all the microcomputer's functions, and interprets the instructions in the program to perform the control functions necessary to run that program

(ii) CPU Memory

• small memory composed of a number of registers which will hold information – such as sensor data, and the immediately

required program instructions for the ALU and control unit to work on.

(iii) Arithmetic and logic unit (ALU):

• performs calculations, such as addition and multiplication, as well as logical decision-making processes of selecting, sorting and comparing of data.

• the ALU processes the data from input devices (sensors and transducers) to produce a control output command based on computation.

(iv) CPU clock unit: (Timer interval)

• - sends out timing pulses from a quartz crystal to enable the control unit to time and synchronize the operations of the microprocessor

• - the clock unit speed mainly determines the length of the processor cycle time (or machine cycle time), which is the basic speed of executing an average program instruction.

(v) DDC Clock

• The DDC clock unit is different from the CPU clock unit.

• is a real time clock, driven by the CPU clock, that keeps time of day and the date.

• It enables the DDC to control items of building plant (fans, pumps, contactors etc.) on a time program, or schedule.

• Example: is used for timing the automatic switching of plant, e.g., to switch off a set of lights at night and during weekends and holidays.

(vi) Address and data buses• provide connection between the microprocessor and

the other chips in the DDC. • Every piece of data or program instruction stored in

the DDC memory must have an address. • Example: each sensor will have its own address and

storage area in the memory set aside for its readings.• (i) Data bus - used for the transfer of data between

DDC chips• (ii) Address bus - used for locating where in the

memory the required data is, or where a program instruction is located.

(vii) DDC RAM and ROM• (i) RAM

• - used for temporary data storage, e.g., DDC application programs, configuration information, computational results, etc.

• - information in RAM is called software, as they can be changed by the programmer.

(vii) DDC RAM and ROM

• (ii) ROM

- used for storing standard control programmes for the DDC, such as a time schedule, on/off control, PID routines

- the routines stored in ROM are used together with configuration information and application software stored in RAM to run application programs

- the control routines or programs, written in ROM chips are considered as firmware, and cannot normally be changed by the programmer.

Memory size

• Design/ selection criteria for memory size– Time duration for storage– How many bit/byte will be taken up for one

data packet?– How fast is the communication?– How accurate shall it be?– FIFO (first in - first out)

Operation of a DDC

DDC input/output unit interface

• consists of a set of electronic chips ie. op amps, A/D and a number of other circuits mounted on a PCB.

• The sensor and actuator cables are connected via pairs of input/output terminals (line and common terminals)

IU

ADC

DDC

4.71 V- AI1

1.5 K

CPU

0001 11110000 0010

DDC input unit interface

• can be configured to carry out one or more of the following functions:– obtain input signals from sensors/transducers,

meters, etc., to determine the current status of a measured variable

– condition the input signals as necessary– convert analogue input signals to digital signals by

analogue to digital (A/D) conversion– receive on/ off status signals from volt-free

contacts (static switches) to determine the status of binary devices such as relays or alarms

DDC output unit interface

• carries out one of the following functions based on the results of computation and logical operations carried out by the CPU:– sends out a single digital signal to change the state

of the controlled device – sends out a number of pulses for a certain period to

move the output device incremental basis– convert the digital output command to an analogue

signal to move the output device to a definite position, using a digital to analogue (D/A) converter

DDC operating sequence:

1. The sensor, associated transmitter sends back a signal to the DDC input unit representing the physical variable measured such as T, RH, FR…

2. The IU samples the sensor reading and the input unit converts it to a suitable digital electrical signal for the CPU to process. (stored in IU memory/buffer)

Sampling Rate:

Sampling Method:

3. When sensor data is ready for processing, the CPU locates/scans the address of the sensor data in IU buffer.

4. The CPU sends a control signal on the control bus to fetch the sensor reading on the data bus, and transfer it to temporary storage in the CPU memory.

DDC operating sequence:

DDCIU CPU

5. A program counter determines the next application step - the CPU reads this step and loads it into the instruction register of the CPU memory.

6. The CPU fetches any data stored in RAM required for executing the next program step.

7. The control routine is then executed by a software program (modules.. PLC, PID, etc) in the ROM

DDC operating sequence:

DDC I/O point types• a) Input points:provide data from sensing devices

– Digital Inputs (DI): a two-state input signal used for status monitoring.

• Examples: fan on/off status, high/low temperature, alarm on/off, etc.

– Analogue Input (AI): a continuous signal derived from a sensor and its transmitter, representing the current value of a physical variable

• Examples: temperature, pressure, humidity, etc..

– Pulsed Input (PI): consists of accumulated pulses from the data environment used to represent physical variables in meaningful engineering units.

• Pulsed inputs are usually every 100 milliseconds with a duty cycle of 50% (10 Hz).

• Examples: KWH transducer

AI-Types• AI can be configured as either active or passive

type.– the active AI receives either voltage or current

signal input with specific ranges, such as:0 to10V (0 to 5V) 4 to 20mA (0 to 20mA)

Mainly used for T, P, F, RH, Speed..and many more.

– the passive AI receives resistance values from the resistance temperature detector or RTD.

Only used for T

• b) Output points: generate commands (in the form of control signals) that are the ultimate result of DDC computations or logical operations.– Digital Output (DO): performs two-state control functions,

usually by energizing or de-energizing the relay coil of a contactor or switch.

– Analogue Output (AO): a software command generated by the CPU, and converted into continuous, variable voltage or current control signals for controlling actuator and drives.

Example: damper and valve actuators, variable speed drives…

Specific ranges for different AO types are usually either 0~10 V, 0~5V or 0~20 mA, 4~ 20mA.

DDC I/O point types

DO Types

• In BAS industry, DO can be configured as any of the following types:– On/off (driven from a logic variable)– On/off (driven from a numeric variable)– Duration Adjusted Type (DAT) (driven from a

numeric variable)– Position Adjust Type (PAT, incremental control)– Start/Stop (S/S) (driven from a logic variable)– Pulse (driven from a logic variable)

DDC Data Point

• Notes: Apart from I/O points, many other internal data points are available in DDC database as a result of software modules’ computations. – These are called pseudo-points (in both analog or

digital format.) – They are stored in Static RAM.

Example: LRS, AO, AOF, DO, OUH, UNR, OVR. etc..

DDC software modules

• DDC software consists of a number of internal modules that exist in the DDC as memory (ROM) resident software.

• These software modules can be programmed/ configured by graphical CAD or text based methods to carry out the required control functions.

• Typical DDC software modules include:– control modules ( e.g., on/off, PID)

– arithmetic modules (e.g. averaging, summing, totalization)

– programmable logic modules (PLC)

– interlock modules , etc (e.g., EF with EF OUT)

DDC packaging • Very large scale integration (VLSI) technology has led

to the development of application-specific integrated circuits (ASICs), or microcontrollers, which combine processors, memory, timers, counters, and input/output capabilities in one package.

• DDCs use VLSI technology to provide sophisticated, cost‑effective computer control systems for building plant and services.

• Under mass production, we can’t choose to have various designs or types of DDC and generally classified into two main groups of packaging.

1) General‑purpose DDCs - field-programmable, multi-function DDCs-    usually modular in design, i.e., I/O capabilities can be added as required by the

application at hand in whatever types and numbers are needed, usually by simply adding another printed circuit card or expansion modules

2) Application-specific DDCs (Unitary Controllers)-     smaller DDCs that are pre-programmed for specific

control applications, e.g. control of VAV terminal boxes, pumps, fan coil units, and packaged air conditioners

- operation almost entirely determined by their firmware, which is specifically tailored by the manufacturer, for a particular application.

DDC packaging