The control system points are usually provided by the design institute. The DCS system points are the sum of the instrument I/O points, electrical I/O points, and communication points between the DCS system and other systems. Accurate statistics of electrical and instrument I/O points can avoid coordination inconsistencies between electrical and instrumentation. Accurate DCS points can provide a basis for users to decide on the DCS system brand and DCS system cost.
The DCS system usually involves automation control content such as thermal detection, analog control, sequential control, and logic control. The DCS system points are obtained from the statistics of AI analog input points, AO analog output points, DI switch input points, DO switch output points, and communication points between DCS and other systems. The following introduces the calculation method of DCS system points for instrument and electrical majors:
How are AI input points calculated?
The analog input signals of the PLC can be directly input into the DCS system from the field. The AI input signals include thermocouples (J, K, T, N, E, R, S and B thermocouples), thermal resistor signals (Cu50, Cu100, Pt100 and Pt50), standard current signals (4-20mA, 0-20mA), standard voltage signals (1-5V, 0-5V and 0-10V) and pulse signals. If other forms of signals need to be sent to the DCS system, signal isolators, current transmitters, voltage transmitters, and other signal conversion devices must be used to convert the signal into 4-20mA or 1-5V before sending it to the DCS system.
How to calculate the number of AO output points?
AO refers to the analog output signal of the DCS system or PLC that controls the field execution equipment. There are five types of AO outputs: 4-20mA, 0-20mA, 0-5V, 1- 5V, and 0-10V. 4-20mA is the most commonly used DCS system AO output. AO output is usually connected to electric actuators, pneumatic actuators, frequency converters, power regulators, and industrial control modules. Usually, each controlled object corresponds to one AO output, and the number of AO output points is the same as the number of controlled devices.
How to calculate the number of DI input points?
DI refers to the switch input signal entering the DCS system or PLC. The DI input must be a passive contact, TTL, or CMOS level signal. After entering the DCS system or PLC, the DC24V or DC48V query voltage is often connected.
The instrument professional DI input usually comes from the alarm contacts of instruments such as on-site electric contact pressure gauges, electric contact bimetallic thermometers, electric contact water level gauges, liquid level switches, flow switches, flame detection, and electric contact water level gauges. Each alarm contact is calculated as a point DI input when connected to the DCS system.
How to calculate the number of DO output points?
DO refers to the switch output signal of the DCS system or PLC that controls the field equipment, which is usually connected to other electrical equipment of different voltage levels through an intermediate relay. Instrument professional DO output is often used to control external indicator lights, solenoid valves, sound and light alarms, electrical controls and multi-turn electric actuators, contactors, and other equipment.