Control system design is a fundamental skill that every industrial control engineer must master, and IO inventory, which is what we often call point counting, is the first task to be done.
The number of system I/O points is usually provided by the design institute. This number includes the instrumentation I/O points, electrical I/O points, and communication points between the control system and other systems. Accurately counting the electrical and instrumentation I/O points avoids inconsistencies between the electrical and instrumentation disciplines, and accurate system I/O points provide a basis for the user to decide on the control system brand and cost.
Control systems typically involve automation control functions such as thermal detection, analog control, sequential control, and logic control. The number of points is derived from statistics on five aspects: AI analog input points, AO analog output points, DI digital input points, DO digital output points, and communication points between the control system and other systems.
1. How are the AI input points calculated in the automatic control system?
AI refers to the analog input signal entering the control system. AI input signals that can be directly input into the system from the field include thermocouples (J, K, T, N, E, R, S, and B type thermocouples), resistance temperature detector (RTD) signals (Cu50, Cu100, Pt100, and Pt50 type), standard current signals (4-20mA, 0-20mA), standard voltage signals (1-5V, 0-5V, and 0-10V), and pulse signals. Other types of signals, if they need to be sent into the system, must be converted to 4-20mA or 1-5V using signal conversion equipment such as signal isolators, current transmitters, and voltage transmitters before being sent to the DCS system.
(1) Statistics of thermocouple AI input points
A single assembled thermocouple or a single armored thermocouple is counted as 1 AI point; a double assembled thermocouple or a double armored thermocouple requires the system to display the temperatures of two sensors at the same measuring point, which is counted as 2 AI points; if only one temperature at the measuring point is displayed, it is counted as 1 AI point; a single multi-point thermocouple or a multi-point thermocouple is often used to monitor the temperature of different parts of the same measuring point. The number of AI inputs for each thermocouple is calculated based on the number of measuring points.
(2) Statistics of AI input points for thermal resistors
The method for counting the number of AI input points for resistance temperature detectors (RTDs) is the same as that for counting the number of AI input points for thermocouples.
(3) Statistics of standard current and voltage AI input points
For each 4-20mA, 0-2mA, 0-5V, 1-5V, or 0-10V signal input to the system, calculate one AI point and simultaneously determine the corresponding measurement range. For two-wire transmitters (including temperature transmitters, pressure transmitters, level transmitters, flow transmitters, etc.), which involve DC 24V power supply, it is best to calculate the AI points separately to facilitate system integration and wiring.
Special note: Field instruments such as pressure gauges, bimetallic thermometers, and glass rotor flow meters displayed on-site are not included in the system's point count calculation.
2. How to calculate the number of AO output points?
AO refers to the analog output signal issued by the control system to control the field actuators. AO outputs generally come in five types: 4-20mA, 0-20mA, 0-5V, 1-5V, and 0-10V. 4-20mA is the most commonly used AO output in DCS systems. AO outputs are typically connected to devices such as 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.
3. How to calculate the number of DI input points?
DI refers to the digital input signal that enters the control system. The DI input must be a passive contact, TTL or CMOS level signal. After the DI enters the DCS system or PLC, it is usually connected to a DC24V or DC48V query voltage.
Instrumentation-specific DI inputs typically come from the alarm contacts of instruments such as field-mounted electric contact pressure gauges, electric contact bimetallic thermometers, electric contact level gauges, level switches, flow switches, flame detectors, and electric contact level gauges. Each alarm contact connected to the system is counted as one point DI input.
4. How to calculate the number of DO output points?
DO refers to the switching output signal issued by the control system to control field devices. It is usually connected to other electrical equipment with different voltage levels through intermediate relays. In instrumentation, DO outputs are commonly used to control external indicator lights, solenoid valves, audible and visual alarms, electrical controls, multi-turn electric actuators, contactors, and other equipment.
The number of I/O points required to control different devices varies. The following are the common I/O point counts for controlled objects:
(1) Switch type electric actuator: Each actuator has 1 AI input for valve position feedback 4-20mA calculation, 2 DO outputs for valve forward/reverse control calculation, 2 DI inputs for valve open/close signal calculation, and 2 DI inputs for valve over-torque/over-torque fault signal calculation.
(2) Switch type multi-turn electric actuator (AC380V power supply): Each actuator valve position feedback 4-20mA calculates 1 AI input point (if there is no feedback signal, the number of AI points will not be calculated), valve forward/reverse control calculates 2 DO output points, valve open to the position/valve close to the position (limit switch) calculates 2 DI input points, actuator over-torque/over-torque fault signal calculates 2 DI input points.
(3) Regulating electric actuator: Each actuator calculates 1 AI input for valve position feedback, 1 AO output for valve control signal, and 1 AI input for actuator fault alarm signal (fault alarms are common in intelligent electric actuators; if there is no fault alarm signal, the AI points are not calculated).
(4) Adjustable multi-turn electric actuator: Each actuator calculates 1 AI input point for valve position feedback, 1 AO output point for actuator 4-20mA control signal, 1 DO output point for ESD emergency control signal (ESD emergency control signal is common in intelligent multi-turn electric actuators; if this function is not available, the number of DO points is not calculated), and 2 DI input points for over-torque/over-torque alarm signal.
(5) Frequency converter: Each frequency converter has 1 AI input point for frequency feedback calculation, 1 AO output point for frequency set signal calculation, 1 DO output point for run/stop set command calculation, 1 DI input point for frequency converter fault alarm calculation, 1 DO output point for fault reset calculation, and 1 DI input point for frequency converter running status calculation.
If the frequency converter and communication method are connected to the DCS system, only one communication point needs to be calculated, and the number of other points does not need to be calculated.
(6) If the system is connected to external devices such as solenoid valves, indicator lights, contactors, etc., calculate 1 DO output for each device (if multiple devices share a control signal, it is usually accomplished by adding intermediate relay contacts, and only 1 DO output needs to be calculated).
5. How does an electrical engineer calculate the number of control system points?
(1) Number of system points for conventional electrical control
The simplest motor control circuit requires two DI inputs and one DO output. The operating status of each circuit (from the contactor auxiliary contacts) is calculated at one DI input, the start/stop control signal (connected to the contactor coil) is calculated at one DO output, and the fault signal (from the thermal relay or motor protector overload signal) is calculated at one DI input.
If the motor circuit requires current display and local/remote control, in addition to calculating 2 DI and 1 DO, the number of AI input points for the current signal (from the current transmitter) should be calculated as 0-3 points (small power motors usually do not need to monitor current, so the number of AI input points is not calculated; for high power three-phase motors, the number of AI input points should be calculated according to the number of phase currents that need to be sent to the DCS display. Each 0-5A current signal must be converted into a 4-20mA signal by the current transmitter and sent to the control system, with a maximum of 3 points); if the motor needs to be controlled from multiple locations, the control location selection switch should calculate 1 DI input.
To facilitate understanding, the following diagram illustrates the electrical secondary control principle of the GGD electrical cabinet, field control box, and control system, which are controlled from three locations.
Motor control secondary circuit function description: The stop button on the electrical cabinet and field control box can stop the motor in any state; the control location selection switch can select "cabinet control", "field control" and "automatic control", and the start button at the corresponding position of the selection switch can start the motor; when the selection switch is in "automatic control", the motor start/stop operation can be performed on the system.
Electrical Component Description: In the secondary schematic diagram, 1SS is the stop button on the electrical cabinet, and 1SS1 is the stop button on the field control box; 1SB is the start button on the electrical cabinet, and 1SB1 is the start button on the field control box; DO is the system start/stop control output contact; 1HR5 is the power indicator light; 1HR is the running indicator light on the electrical cabinet, and 1HR1 is the running indicator light on the field control box; 1HG is the stop indicator light on the electrical cabinet, and 1HR1 is the stop indicator light on the field control box; 1KK is the operating ground changeover switch; 1KH is the thermal relay; 1KM is the contactor; 1KA is the intermediate relay; and 1FU is the secondary circuit fuse.
(2) Number of system points for reduced-voltage starting electrical control
For each reduced-voltage starting circuit, the full-voltage operation status signal of the motor (from the auxiliary contact of the main contactor 1KM1) is calculated as 1 DI input point; the system start/stop control signal (connected to the contactor coil) is calculated as 1 DO output point; the electrical fault signal (from the thermal relay or motor protector overload signal) is calculated as 1 DI input point; and the motor current signal (from the three-phase current transmitter) is calculated as 3 AI input points (motor A, B, and C phase current transmitters). If the motor needs to be controlled from multiple locations, the control location selection switch status (when automatic control is selected) is used to calculate 1 DI input point.
To facilitate understanding, the following diagram illustrates the control principle of a stepped-down starting secondary circuit controlled from three locations: an electrical cabinet, a field control box, and a control system.
(3) Number of system points controlled by frequency converter
Each inverter operating status signal (from the intermediate relay contact) is calculated as DI input 1 point; the DCS system start/stop control signal (connected to the intermediate relay coil) is calculated as DO output 1 point; the inverter fault signal (from the inverter) is calculated as DI input 1 point; the fault reset is calculated as DO output 1 point; the inverter frequency feedback signal is calculated as AI input 1 point; and the inverter frequency setpoint signal is calculated as AO output 1 point.
(4) Number of system points for motor forward and reverse rotation control
For motor forward/reverse operation (from contactor auxiliary contacts), calculate 2 input points D1; for forward/reverse fault signals (from thermal relay), calculate 2 input points DI; for forward/reverse control (connected to contactor coil), calculate 2 output points DO; for motor current feedback signals, calculate up to 3 input points AI (this point is not calculated if there is no current feedback).
The above method for calculating the number of system I/O points can quickly determine the actual number of system hardware points required. The actual system configuration also needs to consider system redundancy, which is usually increased by 20% based on the actual number of system I/O points required by the user.
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