WO2016082840A1

WO2016082840A1 - Low-power hart transmitter

Info

Publication number: WO2016082840A1
Application number: PCT/DK2015/050363
Authority: WO
Inventors: Stig Alnøe LINDEMANN; Mads Kolding Nielsen
Original assignee: Pr Electronics A/S
Priority date: 2014-11-28
Filing date: 2015-11-24
Publication date: 2016-06-02

Abstract

The pending patent application concerns a system adapted to perform communication with at least a first device, which communication is performed by a first communication performed by an analogue current signal generator, which analogue current signal is communicating over a communication line, which system further performs a second communication performed by a voltage injector at the communication line. It is an object of the pending patent application to achieve a system and a method for efficient communication by a first and a second communication system at a common communication line. The object of the patent application can be achieved by the second communication being performed by injection of a frequency-modulated voltage signal onto the first analogue current signal. Hereby it can be achieved that an analogue signal communication line at the same time can carry a frequency-modulated signal so that both a first analogue communication and a second frequency-modulated communication are effected over the same line. In this way a highly energy-efficient way of communication can be performed.

Description

Low-power HART transmitter

Field of the Invention

The pending patent application concerns a system adapted to perform communication with at least a first device, which communication is performed by a first communica- tion performed by an analogue current signal generator , which analogue current signal is communicating over a communication line, which system further performs a second communication performed by a voltage injector at the communication line which second communication is performed by injection of a frequency- modulated voltage signal onto the first analogue current signal at the analogue communication line.

Background of the Invention

The present invention relates to HART communication used for digital communication. An example of using HART communication could be data transfer between two HART capable units, e. g. a temperature transmitter (field device) and a handheld HART communicator (field device).

In order to establish two way communication both field devices must have a HART transmitter and a receiver (communications encoder and decoder). HART communica- tion is a type of binary frequency keying where a specific carrier frequency represent a "1" and another carrier frequency represents a "0" at a certain baud rate.

Object of the Invention

It is an object of the pending patent application to achieve a system and a method for efficient communication by a first and a second communication system at a common communication line.

Description of the Invention

The object of the patent application can be achieved by a system as disclosed in the preamble of claim 1 and further modified in that the frequency-modulated voltage signal can be generated by at least one voltage source, which voltage source is adapted to generate the amplitude of the voltage of the frequency-modulated signal, which system comprises a number of switches, which system further comprises at least one capacitor, which voltage source and capacitor is connected by a number of switches to the communication lines injecting the frequency-modulated voltage signal without interfering or changing the low frequency current signal from the analogue current signal generator when said low frequency current passing through the field device. Hereby it can be achieved that an analogue signal communication line at the same time can carry a frequency-modulated signal so that both a first analogue communica- tion and a second frequency- modulated communication are effected over the same line. In this way a highly energy-efficient way of communication can be performed. Because the frequency-modulated voltage signal over time will be totally neutral, this frequency-modulated voltage signal has no influence on the analogue current signal that also flows in the signal lines. In this way even a high-precise analogue communi- cation line can carry a frequency-modulated voltage signal without any influence on the analogue signal. In this way it is also possible to let the first and the second communication go in different directions without them having any influence on each other. Further it can be achieved that the system comprises one or more processors, one of which can process data to a display. Further it is possible that the frequency- modulated voltage signal also can be repeated by the system and transmitted to other devices. Further can be achieved that the frequency-modulated voltage signal is generated in a highly effective way based on a voltage source. By a number of switches it is possible to connect and disconnect the voltage source and in this way select between a low and a high voltage level. By changing the pulse width it is possible to achieve different frequencies for the communication. Further, by use of switches it is possible to charge a capacitor each time the voltage source is connected and thereby charging the capacitor. By connecting the switches in the opposite way, it is also possible to connect the capacitor in its opposite direction to the line and in this way led the charge of the capacitor reduce the actual voltage in the communication line be- cause a change from active voltage to low voltage is necessary in order to generate the correct frequency. Of course, the switches can be electronic switches and these switches can be controlled by processor means which are connected to the switches. By a further preferred embodiment of the invention can the system be adapted to perform analogue current communication with at least two devices, which system comprises a separation filter to reduce the frequency communication between the connected devices. Hereby it is be achieved that the system as such is communicating in more than one direction so that the system can communicate with two or more devices where communication is performed by a common current loop so that the first communication system based on the current is common between the system and devices, but where a second communication based on frequency communication, this second communication is blocked by a low-pass filtration.

By a further preferred embodiment of the invention can the second communications be performed by injection of a HART voltage signal onto the analogue current signal, which HART voltage signal is performed by a Frequency Shift Keying voltage, which signal injection is performed without compromising the precision and accuracy of the analogue current signal. By using the HART protocol this HART protocol is switching between two frequencies. If these frequencies are oscillating primarily symmetrical in relation to the current of the first communication, there will be no influence from the HART voltage signal at the analogue current values. The mean value of the frequency can be selected to be as close to zero as possible but the current is probably generated by a current generator having a relatively low impedance where the HART voltage signal is generated by relatively high impedance. Therefore the influence by the high impedance on the low impedance will be relatively small and probably so small that it can never be measured. By a further preferred embodiment of the invention can the voltage source be adapted to generate the amplitude of the HART voltage signal, which HART voltage signal by the switches is added to the voltage level at the load impedance, when a first switch is in position 11, and a second switch is in position 22, and a third switch is closed, and when the first switch is in position 12, and the second switch is in position 21, and the third switch is open the HART voltage signal is subtracted from the voltage level at the load impedance. Hereby, by a processor means the switches can be controlled so that they are following the HART protocol and by using the three switches it is possible to generate the HART voltage signal with very low power consumption in the system. By a further preferred embodiment of the invention can the system is adapted to perform HART voltage injection whereby the HART signal injection can be disabled by continuously keeping the first switch in position 1 1, the second switch in position 21, and let the third switch be open. Hereby it can be achieved that the HART voltage signal is stopped and therefore there is no influence on the analogue current communication.

By a further preferred embodiment of the invention can HART voltage injection system can be used as a master end HART voltage transmitter. In systems where the HART voltage injection system is communicating with several different devices or further systems, it is rather important when using the HART communication protocol that there is established a master and slave relationship between the connected devic- es.

By a further preferred embodiment of the invention can the system be adapted for injection of a HART voltage signal onto an analogue current signal by a simple and very low power consumption circuit comprising at least the first, the second and the third switches, and at least one capacitor and at least one voltage sourcewhich voltage source is adapted to generate the amplitude of the HART voltage signal, which by the switches is added to the voltage level at the load impedance when the first switch is in position 11, the second switch in position 22, and the third switch is closed and subtracted from the voltage level at the load impedance (ref), when the first switch 1 is in position 12, the second switch in position 21, and the third switch 3 is open, which HART signal is performed by a Frequency Shift Keying voltage modulation, which signal injection is performed without compromising the precision and accuracy of the analogue current signal. Hereby it is possible to achieve a highly effective HART communication system in combination with an analogue current communication where the HART frequency-modulated voltage signal is generated simply by activating and deactivating the switches. This activation can effectively be by electronic switches and where these switches are connected to a processor that controls the HART communication. By a further preferred embodiment of the invention can HART voltage injection system be used in an intermediate device between a HART enabled measuring device and the master end as e.g. a field-mounted display. Hereby it can be achieved that the field-mounted display is being controlled by the HART voltage communication.

The pending patent application further concerns a method for communication between a system and a device as disclosed in the following sequence of steps, a. the system perform a first communication by an analogue current lines connected between the system and the device, b. the device transmit analogue measured values by the first communication to the system, c. the system performs a second communication towards the device, d. the system performs the second communication by a voltage injector (108) at the communication line, e. the second communication is performed by injection of a frequency- modulated voltage signal onto the first analogue current signal.

Hereby it can be achieved that the highly effective communication by two different communication systems is performed over the same communication lines. In some situations, communication can be in both directions without any interference. The injected voltage into the current communication will have no influence if the mean value of the injected voltage is as close to zero as possible. Even a change in frequency or pulse/pause ratio will only have very limited influence on the analogue current communication.

The main focus of this patent is a HART transmitter capable of generating a HART communication signal with a very low power consummation. The transmitter can be part of a more complicated HART network setup. HART communication is used for data transfer of process data to a display which also functions as a HART repeater to a secondary HART network as shown below.

In this instance two separate HART networks exists, where it is important to notice the HART repeater (fig 2)) has two HART transmitters and two HART receivers. The transmitter to be patented is the HART repeaters transmitter used in HART network 1 in figure below.

The main focus of this patent is a HART transmitter capable of generating a HART communication signal with a very low power consummation.

The transmitter can be part of a more complicated HART network setup. HART communication is used for data transfer of process data to a display which also func- tions as a HART repeater to a secondary HART network as shown below.

In this instance two separate HART networks exist, where it is important to notice the HART repeater fig 2 has two HART transmitters and two HART receivers. The transmitter to be patented is the HART repeaters transmitter used in HART network 1 in figure below.

The transmitter to be patented is part of a HART network which transmits data from a HART repeater to a temperature transmitter. HART signals can be communicated by excitation in either voltage or current. The HART signal piggybacks onto a high preci- sion analogue process value, in this instance a high precision current.

A HART separation filter is needed since communication on HART network 1 and HART network 2 both piggybacks on the same high precision analogue process signal. This can be achieved by utilization of a passive low pass filter which lets the high precision analogue process value passes unaltered through the HART repeater, while suppressing the mutual HART communication interference from HART network 1 and 2.

It is only the HART communication which is being modified by the repeater, not the analogue process value.

HART transmission could be implemented as either a current or voltage excitation. In HART network 1 both types are utilized - the HART repeater transmit a HART signal by voltage carrier excitation to the temperature transmitter and receive a HART cur- rent signal. Correspondingly the temperature transmitter transmits a HART signal by current carrier excitation to the HART repeater and receive a HART voltage signal. This is depicted in the figure below.

HART network 2 communicates solely by voltage signals.

An example is given In the figures 1-11 of how the HART transmitter and receiver could be implemented in HART network 1. The HART current receiver is implemented by a differential measurement of a voltage over a series resistance. The HART voltage transmitter is implemented by injecting a voltage signal by use of a switch capacitive network.

Besides the HART separation filter the two HART networks are also isolated from each other by use of a series of voltage regulator which are high impedance towards HART network 2 and low impedance towards HART network 1. The example above illustrates a loop powered repeater (by PSU_EXT) and transmitter setup with an external resistive load R_EXT- The analogue filter block in the HART repeater offers a means for analogue signal conditioning of the generated HART signals e.g. removal of higher harmonics etc. An adaptive feature could also be added to the voltage regulator in order to provide a frequency depended impedance characteristic.

A voltage signal is injected onto HART network 1 - this is described in the following section. The focus of this patent application is the voltage transmitter where the remaining system can be simplified to a high impedance load and a low impedance load as depicted in the figures 1-11.

A HART voltage signal is injected by stacking a fixed or controlled voltage reference onto a regulated DC voltage (V_regou_T) by use of switches in the configuration

V_HA_{RT T}x : HIGH, in this configuration the storage capacitor is charged to the potential VjjARTampl. ·

The voltage potential of the storage capacitor is subtracted from the regulated DC voltage (Vregoui) by use of switches in the configuration V_HA_{RT TX} : LOW.

While communicating the transmitter toggles between the two states HIGH and LOW, which effectively allows a DC current to flow during communication. When no HART communication is to be send the transmitter returns to V_HA_{RT TX} : IDLE

The idle state can also be realized by having the switches in the opposite position.

This implementation of the HART transmitter is very power efficient due to the utilization of a switch capacitive energy transfer while maintaining the capability of trans- ferring a high precision analogue process value during communication.

Note that opposite to HART network 2 the HART transmitter in HART network 1 rely on a low impedance in order to function.

Further information related to HART communication protocol can be found in FSK Physical Layer Specification HCF SPEC-054, rev. 8.1

can be achieved: Very low power consummation • HART communication o Combined with high precision analogue process value o Simple implementation

• Combination with specific HART system setup suggested o HART to HART o HART repeated to HART

Detailed Description of the Invention

Figure 1 shows a communication system for HART communication between a field device and a field device communicator 102. The figure shows the field device 104 communicated by HART communication 106 with the field device communicator 102. The field device communicator 102 comprises a HART communication transmitter receiver 108 and the field device communicator 104, such as a temperature transmitter, comprises a HART communication transmitter receiver 110.

Figure 2 also shows the field device 104 such as a temperature transmitter comprising a HART communication transmitter receiver 110 communicated by the HART communication 106 towards the field device communicator 102. As previously disclosed, this field device communicator 102 comprises a HART communication transmitter receiver 108, and the field device communicator 102 further comprises a second HART communication transmitter receiver 112 which over a HART communication line 118 communicates with one further field device communicator 114 comprising a HART communication transmitter receiver 116. Hereby it can achieved be that the field device communicator 102 operates as a HART relay device so that the field device communicator 102 receives the HART communication and performs additional communication of the HART communication towards a field device 114. The field device 104 and the field device 114 can perform a first communication by an analogue current loop (126,128 fig 4) transmitting e.g. temperature measurements. The HART communication is a frequency-modulated voltage added to the current loop (126,128 fig. 4) of the first communication. In this way the HART communication can be performed over existing current loop (126,128 fig. 4) communication lines without hav- ing any influence on the analogue values that are communicated via the first current loop (126,128 fig.4) communication line.

Figure 3 discloses many of the same features as those found in figure 2. As already described above, the field device 102 comprises two HART communication transmitter receivers 108 and 112.

Figure 4 shows a system overview of the pending application where a field device, such as a temperature transmitter, comprises a first and a second communication system where the first system is an analogue current loop 126, 128 communication, and the second one is the HART communication. The HART communication is sent by line 106 to the HART communication transmitter receiver 108. From the HART communication transmitter receiver 108 the communication is transmitted further to a communication handler relay 120 which is connected to a field display 122. The communication handler relay 120 is further connected to the second HART communication transmitter 112 which by line 118 transmits HART communication to the second field device communicator 114. Further there is indicated that the analogue process value 126 delivered/supplied from the field device 104 is sent to the HART separation filter 124 into line 128 and further into field device communicator 114.

From this figure 4 it is possible to see that the analogue processor value is communicated to the HART separation filter 124 directly between the field device 104 and the field device 114.

Figure 5 shows the same features as those found in figure 4 which is described above. In figure 5 it is possible that HART transmission could be implemented as either a current or voltage excitation. In HART network one both types are utilized. The HART repeater transmits a HART signal by voltage carrier to the excitation to the temperature transmitter and receives a HART current signal. Correspondingly, the temperature transmitter transmits a HART signal by current carrier excitation to the HART repeater and receives a HART voltage signal.

HART network 2 communicates solely by voltage signals. Figure 6 shows a possible embodiment of the invention where the field device 104 comprises a voltage V HART RX generator 142 and a current IT HARTX and IMESH DC generator 140, both connected between communication line 126 and ground. The communication line 126 passes through an analogue filter 138 before the signal 126 reaches the HART communication handler 108, which comprises a current generator 134 IHARTRX and voltage generator VHART TX 136. By line 130 the IHART is communicated to a communication handler relay 120, and further a line 132 is communicating from the communication handler relay 120 to the VHART TX 136. The line 126 carrying the analogue current signal passes through a HART separation filter 124 and further into a voltage regulator 144 and further into the HART communication handler 112. This HART communication is receiving a voltage signal 154 from the communication relay handler 120 delivered to a current IHART TX generator 146. Further, a voltage detector 148 is communicating via line 152 to the communication handler relay 120. The analogue current communication line 128 continues to a HART communication device 114, and in the meantime it is connected to an external power supply 152 and to a resistor 150 indicating the resistance in the communication line.

Figure 7 shows a more detailed embodiment of the HART generator 108. The remaining part of figure 7 is more or less identical to figure 6.

The HART communication handler 108 now comprises a first switch 162, a second switch 166, and a third switch 164. Further, there is indicated a voltage generator 168 defined as VHART AMPL such as a variable voltage generator. Further there is indicated a capacitor 170. By the switches 162, 164, and 166 it is possible to add the voltage from the power supply 168 to the analogue communication line in one situation, and at the same time charge the capacitor 170. By another position of the switches 162, 164, 166, it is possible instead to use the off-set polarity of the capacitor 170 and in that way reduce the voltage of the line. In this way it is possible from the communication handler relay 120 to generate an oscillating HART signal in the communication line.

Figure 8 shows many of the same features as those disclosed in figure 7, but in figure 8 the switches 162, 164, and 166 are in a position where the voltage from the power supply 168 is connected through the switch 164, the switch 162 towards the equivalent circuit for the field device 104. In this situation, the capacitor 170 will be charged by the voltage from the voltage generator 168. Thus, by the system in figure 8 there is generated a high voltage in the communication line. This can be seen in figure 11.

Figure 9 shows the same embodiment of the invention as the one found in figure 8, but now in a situation where a low voltage is generated in the communication line towards the equivalent circuit for the field device 104. The switch 164 is open and the switch 162 is in the opposite position as is the switch 166 which is now in the 2-1 position. The capacitor 170 is now connected with its opposite polarity to the line towards the equivalent circuit for the field device 104. Hereby a negative charge is added to the line, and in this way the voltage is rapidly reduced. This is further indicated as VHART TX LOW in figure 11 which refers to figure 9.

Figure 10 shows a third idle position of the switches because the switch 164 is open, and the switch 162 is in position 1-1, and the switch 166 is in position 2-1. In this idle position, the voltage in the communication line towards the equivalent circuit for the field device 104 is in an open situation where the actual voltage is defined by the equivalent circuit 114 where impedance set low and a voltage V REGULATION OUT defines the voltage.

Figure 11 shows different voltages depending on the position of the switches as described above.

By operating the switches from a communication handler 120 it is possible in this way to generate the HART voltage regulation in a very efficient manner at very low power consumption.

System for injection of a HART voltage signal onto an analogue current signal characterized in a simple and very low power consumption circuit comprising three switches 162, 164, 166 , one capacitor 170 and one voltage source 168 which voltage source 168 is adapted to generate the amplitude of the HART voltage signal, which by the switches 162, 164, 166 is added to the voltage level at the load impedance (ref) when switch 162 is in position 11, switch 166 in position 22, and switch 164 is closed and subtracted from the voltage level at the load impedance when switch 162 is in position 12, switch 166 in position 21, and switch 164 is open. HART signal injection is performed without compromising the precision and accuracy of the analogue current signal.

HART voltage signal injection system whereby the HART signal injection can be disabled by continuously keeping the switch 1 in position 11, switch 2 in position 21, and switch 3 is open. HART voltage injection system where the injection system can be used as a master end HART voltage transmitter.

HART voltage injection system where the injection system can be used in an intermediate device between a HART enabled measuring device and the master end as e.g. a field-mounted display.

Claims

1. System adapted to perform communication with at least a first field device (104), which communication is performed by a first communication performed by an ana- logue current signal generator (140) generating an analogue current signal which is communicated over an analogue communication line (126), which system (102) further is adapted to perform a second communication performed by a voltage injector (142) at an analogue communication line (126), which second communication is performed by injection of a frequency- modulated voltage signal (106) onto the first ana- logue current signal at the analogue communication line (126) characterized in that the frequency-modulated voltage signal is generated by at least one voltage source (168), which voltage source (168) is adapted to generate the amplitude of the voltage of the frequency-modulated signal, which system (108) comprises a number of switches (162, 164, 166), which system further comprises at least one capacitor, (170) which voltage source (168) and capacitor (170) are connected by the number of switches (162, 164, 166) to the communication lines (126) injecting the frequency- modulated voltage signal without interfering or changing the low frequency current signal from the analogue current signal generator (140) when said low frequency current passing through the field device (102).

2. System according to claims 1, characterized in that the system is adapted to perform analogue current communication with at least two devices (104, 114), which system comprises a separation filter (124) to reduce the second frequency communication between the connected devices (104, 114).

3. System according to one of the claims lor 2, characterized in that the second communication is performed by injection of a HART voltage signal onto the analogue current signal (126), which HART voltage signal injection is performed without compromising the precision and accuracy of the analogue current signal (126).

4. System according to the claims 3, characterized in that the voltage source is adapted to generate the amplitude of the HART voltage signal, which HART voltage signal by the switches (162,164,166) is added to the voltage level at the signal line (106) connected to a load impedance, when in a first mode of operation the first switch (162) is in position 11, and a second switch (166) is in position 22, and a third switch (164) is closed for generating a HART high level voltage, in a second mode of operation the first switch (162) is in position 12, and the second switch (166) is in position 21, and the third switch (164) is open, the HART voltage signal is subtracted from the voltage level at the signal line connected to the load impedance .

5. System according to one of the claims 3-4, characterized in that the system is adapted to perform HART voltage injection whereby the HART signal injection can be disabled by continuously keeping the first switch in position 11, the second switch in position 21, and by letting the third switch be open.

6. System according to one of the claims 3-5, characterized in that the HART voltage injection system is to be used as a master end HART voltage transmitter.

7. System according to the claim 4, characterized in that the system is adapted for injection of a HART voltage signal onto an analogue current signal (126) by a simple and very low power consumption circuit comprising at least the first, the second and the third switches (162, 164, 166), and at least one capacitor (170) and at least one voltage source (168) which voltage source is adapted to generate the amplitude of the HART voltage signal, which by the switches (162, 164, 166 ) is added to the voltage level at the load impedance (114) when the first switch (162) is in position 11, the second switch (166) in position 22, and the third switch (164) is closed and subtracted from the voltage level at the load impedance (114), when the first switch (162) is in position 12, the second switch (166) in position 21, and the third switch (164) is open, which HART signal is performed by a Frequency Shift Keying voltage modulation, which signal injection is performed without compromising the precision and accuracy of the analogue current signal.

8. System according to one of the claims 3-7, characterized in that HART voltage injection system is used in an intermediate device between a HART enabled measuring device and the master end as e.g. a field-mounted display (122).

9. Method for communication between a system (102) and a device (104) as disclosed in one of the claims 1-8, characterized in the following of steps, a. the system performs a first communication by an analogue current line connecting the system and the device, b. the device transmits analogue values by the first communication to the system, c. the system performs a second communication towards the device, d. the system performs the second communication by a voltage injector (108) to the analogue current line, and e. the second communication is performed by injection of a frequency- modulated voltage signal onto the first analogue current signal.