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WirelessHART’s Silver Bullet

22 June 2009

Tens of millions of analogue instruments installed around the world have been waiting for this—even if they’re not HART-based instruments. The “Bullet” adapter will upgrade them to become wireless instruments reporting their data via radio signals to a central control system, with no wiring needed.

MACTek president Thomas Holmes
MACTek president Thomas Holmes

The Bullet uses WirelessHART technology. It may be installed permanently by screwing it into an available spare port of the field device, or by using a short, right angle conduit. If installation on the field device is difficult, the engineer has a convenient alternative: it can be located at any point on the 4-20 mA current loop.

A wired HART field device retrofitted with a Bullet will do two things simultaneously. It will continue to communicate its process variable over the existing 4-20 mA line to the distributed control system and at the same time communicate diagnostic and alarm data over the WirelessHART network to process management, asset management, and engineering applications.

THREE EXAMPLES OF USE

MACTek’s president Thomas Holmes says he sees three “use cases” for his Bullet.

FIRST, and the most obvious, is to retrofit the legacy analogue HART instruments that are using only the 4-20 mA line to send the PV to the control system. WirelessHART will allow the instruments to send diagnostic data to a maintenance system. HART people

SECONDLY, it might be used on a newly installed instrument that might be difficult to connect into the control system. If there is local power, either from the mains or from a solar power system, no 4-20 mA line needs to be installed. The Bullet enables the instrument to send all of its data wirelessly to the control system.

THIRD, it is possible to consider installation of the Bullet on a standard, non-HART-based analogue instrument. The Bullet can “read” the 4-20 mA signal and send its value by radio to the control system.

The Bullet can support up to six devices in multidrop configuration. The target list price for the device is US $650.

WHERE DOES IT GET ITS POWER TO OPERATE?

Where does it get its power? For existing wired HART instruments, it can draw enough power from the loop to operate. This is true in North America, in Europe, or any other part of the word.

A second option is to use direct power when local DC power is available (e.g. system power, local power supply or solar/battery). In direct power mode, the Bullet can switch the field device on/off providing battery power management by cycling the field device power to take periodic readings.

However, in some applications that involve intrinsic safety in Europe, the Bullet would have to be battery powered due to current limitations in hazardous areas, explains Mr. Holmes.

The potential problem arises when using a Wireless HART adapter in a plant where intrinsically safe barriers are used. This results from the fact that IS barriers reduce the power available on the unsafe side of the 24 VDC loop.

Old IS barriers reduce available power by more than newer ones. Use of IS barriers in Europe is, of course, quite common.

“We had to design a wireless adapter that did not further reduce the power available to power the transmitter by too much. Adding the wireless adapter into the loop causes an additional voltage drop, which we call the ‘insertion voltage drop.’ The reduction of loop power available to the transmitter caused by inserting the wireless adapter in the loop is independent of the advanced power management technology we designed to scavenge the power from the loop that is used to power the wireless adapter.

“When I was doing market research with customers to identify wireless adapter features the consensus among the customers I spoke to was that the insertion voltage drop for the wireless adapter should be 1 volt.

“For the sake of illustration, assume you plan to add a wireless adapter to a transmitter that needs 12.5 volts to operate, and the IS loop voltage available to this transmitter is just 14 volts due to cable losses and the IS barrier.

“Connecting the wireless adapter with 1 volt insertion drop to the transmitter in this example would reduce the power available to the transmitter to 13 volts, which in this example would be sufficient power for the transmitter to keep operating. However, if the wireless adapter insertion voltage drop were 1.5 volts or greater adding this wireless adapter in the loop would cause available power to drop below 12.5 volts and the transmitter could not operate.

“One of the early adopters of WirelessHART technology planned to offer a wireless adapter that had a 2.5 insertion voltage drop. A wireless adapter with a 2.5 insertion voltage drop would not work on many loops in plants using IS barriers.

“An important feature of The Bullet is our patent pending StepVolt™ technology that allows the user to set the insertion voltage in steps from 1 to 2.5 volts. If power is limited, say because of IS barriers, the user would probably leave the insertion voltage at its default value of 1 volt. There is a trade off between insertion voltage and bandwidth and the update rate. Higher insertion voltage means a higher update rate. If a particular transmitter fitted with The Bullet has power to spare the user can set the insertion voltage drop higher to increase bandwidth and the increase its update rate.

“To summarixe, the bullet can operate on loop power in zone 0 and 1 areas. The Bullet meets the customer requirement of being able to operate without limitation using a 1 volt insertion voltage drop. We offer the additional capability to the user of increasing the default insertion voltage drop from 1 to 2.5 volts in steps to increase bandwidth and update rates when the loop power at the transmitter is larger than is required to power the transmitter.”

To visit the MACTek website www.mactekcorp.com CLICK HERE









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