Process control instrumentation: Advanced applications for isolators and signal converters

Isolators can be used for many applications in process control beyond just eliminating ground loops and conditioning signals. In Part 2 of this exclusive two-part series, Greg Feliks of Moore Industries presents a selection of advanced applications for isolators and signal converters.

12/17/2007


In Part one of this exclusive two-part series, Greg Feliks of Moore Industries explained proper basic application of isolators, their characteristics and how the isolator gets its power. However, isolators can be used for many applications in process control beyond just eliminating ground loops and conditioning signals. In Part 2 of this exclusive two-part series, Feliks presents a selection of advanced applications for isolators and signal converters.

HART or HARTless
Many instruments have a HART signal. What do you do with the HART signal if the loop requires an isolator? By choosing the proper isolator, you can allow the HART signal to “pass” to the output side of the isolator, or you can block the HART signal from going beyond the primary loop (Fig. 1).


Fig. 1. The right isolator gives end users the flexibility to either pass HART signals to, or block HART signals from going beyond the primary loop.


There may be several reasons for not passing the HART signal to the isolator output. You may have an older receiving device with insufficient noise rejection on its inputs, and the HART signal causes interference with the analog measurement. Another reason might be that you have a flow signal that must go to your DCS and also to a PLC located in another area, such as a waste treatment facility, and you do not want the waste treatment techs reconfiguring your flow transmitter. An isolator with good common-mode rejection is generally all you need to block the HART signal from the output loop.
In other applications, you may want the waste treatment techs to work on the transmitter, so you need to send the HART signal to the output side of the isolator so the operator’s Rosemount 375 handheld can communicate to the flow transmitter. In this case, you need an isolator specifically designed to pass the HART signal. The isolator has a “hole” in its filtering to allow the 1,200 and 2,200-Hz HART signal through.
Converters for digital signals
A new emerging method of converting signals ignores all the previous rules laid down by analog isolators and converters. This new “digital signal conversion” is becoming especially popular in locations where power is sparse and wires are few. A common application deals with digitally converting or “mapping” HART digital signals to the Modbus RTU protocol.
Since almost all PLCs and DCSs %%MDASSML%% new and old %%MDASSML%% accept Modbus RTU, digital conversion becomes a quick and efficient way to get HART data into control systems that don’t natively accept HART. Since HART can be multi-dropped over one twisted pair and contains multiple pieces of process data per field instrument, it becomes a common interface in RTU locations where wire and power are limited (Fig. 2).


Fig. 2. Digital conversion and multi-dropping the signal from field instruments over a twisted pair is a good way to get HART data into non-native DCSs and PLCs, most of which accept Modbus RTU.


On the data acquisition side, you usually find an RTU or SCADA system that supports Modbus RTU. The HCS from Moore Industries gathers all of the HART data from the HART transmitters and converts it to Modbus, and it powers the HART bus using its 9-24 Vdc power input. Now an RTU or SCADA system can monitor any HART variable from any of the HART field devices.
Signal conversion
Signal conversion is often performed in isolators to get mostly old legacy signal types converted to 4-20 mA or some other convenient signal type. There are three approaches to performing signal conversion:
1. Use fixed-ranged products designed and built specifically for the conversion. The advantage is simplicity for the
2. Use an isolator that has switches or jumpers to re-range the input and/or output. The user has to do a little more work to make the product suitable to the application, but the fact that the product is configurable makes it more flexible in addressing multiple applications or changing conversion needs.
3. Using an isolator that is PC-configurable provides similar application flexibility plus some enhancements. Usually the rangeability has more resolution and there are no pots, jumpers or switches that can be easily changed without permission.
VFD noise %%MDASSML%% a new problem
One problem that has cropped up in the last 10 years or so is noise from variable frequency drives. After discovering the enhanced efficiencies of VFDs as compared to fixed speed ac and dc motors and mechanical devices for varying the output, many plants began replacing traditional motors and transmission devices.
Shortly thereafter, engineers and operators at these plants starting seeing a new set of problems: noise on the output cards going to the VFDs. VFDs by their very nature are a wonderful (or notorious) source of common-mode noise. A VFD usually receives an ac input source of 240/480 Vac. To control the output and torque of the motors, the frequency of this input is varied over a large range. This changing of motor load and input frequency creates and conducts a lot of extraneous noise that can be coupled directly onto surrounding electronics and even onto its own input signal coming from the control system.
How do you solve this problem? Simple: install a line-powered isolator to filter out the unwanted common-mode noise (Fig. 3).


Fig. 3. A line-powered isolator prevents unwanted VFD noise from disrupting the output signal from the control system.

Rather than the input to the isolator originating in the field, the control system now supplies the input to the isolator and it, in turn, drives the output to the VFD. The isolator simultaneously isolates and shields the control system from the unwanted noise.
AC current/voltage conversion and isolation
Normally, when people think of isolators, they think of solving some type of problem at the instrument/control level %%MDASSML%% typically dealing with dc signals. However, a more common application is using an isolator to monitor, trend or alarm on ac signals.
With preventive maintenance budgets shrinking, more proactive monitoring of expensive and critical equipment is being implemented. Pumps, motors and fans quickly fall into this category. Since much of this equipment is powered with ac voltage at high current levels, CTs are installed. The need for CTs is twofold. First, other than expensive Watt transducers, there isn’t much instrumentation available that will directly accept an input of 480 V and 100 A. Therefore, CTs are used to step down the current to a level that can easily be monitored. Second, safety is always a large concern. No one, especially a plant safety manager, wants technicians used to working with 24 Vdc grabbing hold of 100 Aac.


Fig. 4. By using isolators, a 480 V, 100 Aac signal can be stepped down, all the way to 0-5 mAac.


Isolators with ac current input are used in these situations to convert and isolate this “high level” ac signal to a lower level 4-20 mA dc signal (Fig. 4). The secondary of the CT, which is almost always 0-5 A, can be directly wired into the input of the signal converter. As an added measure of protection, Moore Industries offers an externally mounted CT option that uses a “mini-CT” to step the 0-5 Aac down to 0-5 mAac. This much lower ac signal is now very safe to wire and deal with.
Hazardous area isolation
Isolators and signal converters cannot always be installed in “back of panel” service or safe areas. Equipment location, lack of wiring and process flammables may be reasons that require isolators and signal converters to be installed in hazardous areas within a plant. When such needs arise, technicians must determine whether the area is classified as NEC Class I, Div 1 (IEC Zone 0/1) or Div 2 (IEC Zone 2).
Today, isolators come in all flavors to accommodate such installations. Intrinsically safe, non-incendive and explosion proof are options from which to choose. One requirement that often arises is an explosion-proof signal converter/isolator with a local indication for remote operators. The SDY from Moore Industries, for example, is an output loop powered isolator that provides isolation, signal conversion and local indication in an explosion proof housing (Fig. 5).


Fig. 5. An isolator provides isolation, signal conversion, linearization and local indication. Proper certifications allow it to be installed in intrinsically safe, explosion proof and non-incendive areas.


More than ground loops
Signal isolators, splitters and converters have come a long way since their days of simply protecting instruments from ground loops. These simple and inexpensive devices can often solve complex and expensive problems.
Greg Feliks is a senior application engineer at Moore Industries. He has been in the industrial and process industries for more than 20 years involved with field instrumentation, recorders, controllers, control systems and various other types of industrial controls. Feliks owned a manufacturer’s representative firm in the Atlanta area, and worked for Dynisco Instruments and Eurotherm before joining Moore Industries. He has held positions in sales, marketing and product management. For the past three years, he has specialized in signal conditioning products including isolators, signal converters, alarm trips, remote I/O and related devices.





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