The article describes the standardized analog pneumatic signals (20 to 100 kPa) and electrical signals (4 to 20 mA), as well as the innovative analog and digital hybrid signals HART (Highway Addressable Remote Transducer) and the state of the art of current digital communication protocols commonly called BUS.
TYPES OF CONTROL SIGNALS: ANALOG, HYBRID, DIGITAL
Analog Control Signals
The traditional most commonly used transmission signal of the type:
- Direct current signals (Table 1): for connection between instruments on long distances (i.e. in the field area)
- Direct voltage signals (Table 2): for connection between instruments on short distances (i.e. in the control room)
|LOWER LIMIT (mA)||UPPER LIMIT (mA)|
|(1) Preferential signal|
Table 1- Standardized signals in direct current (IEC 60381-1)
(1) Voltage signals that can be derived directly from normalized current signals
(2) Voltage signals that can represent physical quantities of a bipolar nature
Table 2- Standardized signals in direct voltage (IEC 60381-2)
The signal different from 0 (live zero) for the variable at the beginning of the measuring range (true zero), is used for electrical instruments to power the instrument and in general to highlight connection losses (as in the pneumatic instruments).
Moreover, given their characteristics, the current signals are used in the field instrumentation, while the voltage signals are used in the technical and control room instrumentation.
Finally, the current signal with respect to the voltage signal has the advantage of not being affected by the length and hence the impedance of the connection line at least up to certain resistance values, as it is illustrated in Figure 1.
Figure 1 – Example of the limit of the operating region for the field instrumentation in terms of its connection resistance Omega to the supply voltage V
- Vdc = Actual supply voltage in volt
- Vmax= Maximum supply voltage, 30 V in this example
- Vmin= Minimum supply voltage, 10 V in this example
- RL= Max. load resistance in ohm at the actual supply voltage:
- RL <= (Vdc – 10) / 0,02 (in the Example reported in Figure 1)
Hybrid Control Signals
Hybrid signals, that is of the analogical-digital protocol type, were standardized “de facto” by a Consortium of Manufacturers as:
HART (Highway Addressable Remote Transducer)
which precisely superimposes to the analog normalized signal (4 ¸ 20 mA) a digital signal modulated in frequency according to the Standard Bell 202, with amplitude of +/- 0.5 mA and with frequency found in Table 3, which given the high frequency of the superimposed signal, the added energy is virtually zero, so this modulation does not cause any disturbance on the analog signal.
NOTE: Remember that to operate the HART protocol requires a resistance of 250 ohms in the output circuit!
Table 3 – HART protocol with signals standardized BELL 202
Digital Control Signals
Digital signals were normalized towards the end of the 1990s by the International Standard IEC 61158 on Fieldbus Protocol, but still not much applied since it standardizes as many as 8 communication protocols, and as each digital protocol is essentially characterized by following features (see Table 4):
- Transmission encoding: Preamble, frame start, transmission of the frame, end of the frame, transmission parity, etc.
- Access to the network: Probabilistic, deterministic, etc.
- Network management: Master-Slave, Producer-Consumer, etc.
|(1) Protocol initially designed as unique standard protocol IEC|
Table 4 – Standardized protocols provided for by the International Standard IEC 61158
Finally, Figure 2 shows the geographic path of the measurement signals from the “field” to the “control room” through the “technical room”, where the sorting (also called “marshalling”) takes place and the transformation of the current signal in the voltage signal for the controller (DCS: Distributed Control System) and then through digital signals flow in “control room” for the operator station and video (HMI: Human Machine Interface).
Figure 2 – the Typical path of a measurement chain from the field to the control room
INSTRUMENTATION POWER SUPPLY
- For pneumatic instrumentation: 140 ± 10 kPa (1.4 ± 0.1 bar) for the pneumatic instrumentation (sometimes the normalized pneumatic power supply in English units is still used: 20 psi, corresponding to ≈ 1.4 bar)
- For electrical instrumentation: Continuous voltage: 24 V dc for field instrumentation, Alternating voltage: 220 V ac for control and technical room instrumentation
The connection and transmission signals between the various instruments in the measuring and regulating chains are standardized by the IEC (International Electrotechnical Commission):
- Pneumatic signals (IEC 60382): 20 to 100 kPa (0.2 to 1.0 bar) (sometimes the standardized signal is still in English units: 3 to 15 psi, ≈ 0.21 to 1.03 bar)
- Electrical signals (IEC 60382):