Which type of modulation is used in hart communication?

FSK Frequency Shift keying in which a digital signal is superimposed on analogue signal

The primary form of communication used in analog transmitters is a current loop with a normal range of 4 mA to 20 mA, employing a transmitter, a receiver, and a power supply. It enables features such as remote calibration, fault interrogation, and transmission of process variable data. Low-power transmitters and receivers must operate on the minimum current, 4 mA or less, depending upon the “headroom” required for error indication. These current loops are reliable, robust, and highly immune to environmental interference over long communication distances. A significant disadvantage, however, is that a single loop allows only one-way communication from a sensor, or to an actuator, and can only transmit one process variable.

The introduction of the HART standard provided a means to create “smart” transmitters, by adding digital communication capability, sharing the same twisted-pair line used for traditional 4-mA-to-20-mA instrumentation. A 4-mA-to-20-mA analog current is modulated by a 1-mA peak-to-peak FSK signal—without interrupting the original primary variable transmission—while still leaving headroom for operating the loop. The HART protocol has become the global standard for sending and receiving digital information over analog wires between smart devices and a control or monitoring system

operates using the

it is working by frequency shift keying(FSK) principle. The digital signal is made up of two frequencies1,200 Hz and 2,200 Hz representing bits 1 and 0, respectively. Sine waves of these two frequencies are superimposed on the direct current (dc) analog signal cables to provide simultaneous analog and digital communications Because the average value of the FSK signal is always zero, the 4–20 mA analog signal is not affected. The digital communication signal has a response time of approximately 2–3 data updates per second without interrupting the analog signal. A minimum loop impedance of 230 Ω isrequired for communication.

Yokogawa and Invensys Foxboro and Emerson Rosemount RS3 system and Siemens, S-5, S-7 control system.

Frequancy shift key (FSK) signal to establish digital communication over the 4-20mA analog signal. 

HART uses FSK modulation techniques frequancy shift key

This enables two-way field communication to take place and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument. The HART Protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a smart field device. As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal.

The HART Devices ( Highway Addressable Remote Transmitter ) uses a frequency Shift keying digital communication which is superimposed over an analog signal. The signal uses two frequencies which corresponds to a  '0' and '1' digital data bits and the data is transmitted at a speed of 1200 bps. 

Frequency Shift Keying (FSK)

HART  is an acronym for Highway Addressable Remote Transducer. The HART Protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA.

The HART Communication Protocol (Highway Addressable Remote Transducer) is a hybrid analog+digital industrial automation protocol. Its most notable advantage is that it can communicate over legacy 4-20 mA analog instrumentation current loops, sharing the pair of wires used by the analog only host systems.

used in analog transmitters is a current loop with a normal range of 4 mA to 20 mA, employing a transmitter, a receiver, and a power supply. It enables features such as remote calibration .FSK Frequency Shift keying in which a digital signal is superimposed on analogue signal