How to prevent overloading a loop

Need to add another loop controller to one of your systems but fear it would push you past your loop maximum? TheAnswer offers inexpensive ways to get around this type of problem.

The Problem:  We’re Overloading a Loop

"We need to add another loop controller to one of our systems. However, this would push us past our 600-ohm loop maximum. We see fairly expensive solutions to remedy this, but wonder if someone has some field experience that found an inexpensive way to get around this type of problem."

From September 2004 Control Design

 

The Answer(s) to Your Problem:

Resistance Isn’t Futile
We really should have a little more information on this one. "600-ohm loop maximum" says we are talking about a single loop here. But "Another loop controller" either implies multiple loops (typically one loop controller per one loop) or multiple controllers in one loop. In either case, if the need is to have more instruments in a loop than the driving force of the loop can handle, i.e., the 600-ohm max rating for this particular loop, there are a number of options. One is to install a 250-ohm resistor and operate some of the instruments in the loop (say recorders or indicators) on 1-5 VDC.

Another is to install loop isolator(s) (loop or separately powered) to create "sub-loops". In this case, the primary loop might be contain the transmitter, controller and loop isolator. Isolators are available with various loop resistances from 50–250+ ohms. The secondary instruments in the loop can then be operated on the output of the loop isolator. A third way, if a computer of some type is involved, it to set up a computer output that "repeats" the input from the transmitter and connect that output to the other loop instruments. Which specific method you use depends on process safety considerations (desired action should each loop component should fail) and the roles and impedances of the specific components.

Jim Federlein, PE, Federlein & Associates Inc., McMurray, Pa.


 

Pump Up the Signal

The challenge of driving all of the receiver instruments with a single transmitter has been around for quite some time. A 3-15 psig pneumatic instrument loop had volume boosters and repeaters long before the 4-20 mA (or 10-50 mA for the old Foxboro folks) devices came onto the scene. The problem is the same, but the technology has changed. From what I read of the problem, the OEM has too many receivers on the loop.

Since it probably isn't a good idea to add another transmitter, we should look at boosting the signal. This doesn't mean boosting the signal amperage, but amplifying the overall signal load capability. A simple two-wire loop-powered, or four-wire main-powered booster installed somewhere on the transmitted signal line, will do this very easily and cheaply. These boosters/amplifiers typically have a low input impedance of around 50 ohms and have and output capable of driving 600 ohms for a two-wire loop powered booster and 1,000 ohms for a 24 VDC/120 VAC 4-wire booster. These boosters can be DIN-rail mounted in the panel for easy access and calibration.

Chuck Savageau, Jacobs Engineering, Pasadena, Calif.


 

Monitor the Drop First

There are three classical solutions to solve the loading problem. First, check to see if the second controller supports a voltage input. If so, perhaps you could monitor the voltage drop across a device that already exists on the loop, e.g., the 1-5 VDC input on a DCS or PLC analog input card. With this approach you may be able to solve the application without any additional costs or loading on the primary 4-20 mA loop. However, you’ll want to “parallel sense” the last load in the loop to keep everything referenced to a single grounding location. Otherwise, you may run into grounding issues. And, you’ll want to keep your voltage wiring runs down to 50 ft. or less to avoid voltage drops and noise pickup.

Another classical solution would be to use a 4-20 mA isolator with low input impedance (75 ohms or less) to retransmit the signal to the second controller. We offer economically priced loop-powered or four-Wire (AC/DC powered) isolators with input impedances as low as 5 ohms. These products will give you another 600-1,800 ohms of retransmitted load capability on the output side. We also offer a splitter isolator instrument that will monitor a primary 4-20 mA signal (sourced or sinking) and deliver two isolated 4-20 mA outputs with load driving capability of 1,000 ohms on each output. Each independent output could drive its own controller (see diagram for a splitter illustration).

Donald Lupo, director of marketing & sales, process products, Acromag Inc., Wixom, Mich.


 

One of These Should Help

There could be several solutions for your overloaded loop problem. One would be to change out the source transmitter to one that has a higher loop drive capability, say 1,000 ohms. A second solution, assuming a two-wire transmitter is being used for the loop current source is to increase to power supply that powers the loop to 42 V. A third solution is to add a four-wire signal isolator that has a low input impedance in series with the original loop and use the output to drive the second controller. A fourth solution would be to use a signal spitting transmitter and divide up the loop load (including the new controller) between the two outputs. A fifth possibility, assuming the first controller has a 250-ohm resistor for an input shunt and the second controller has a 1-5 V input, would be to parallel the two controllers inputs. This would provide an input to the second controller and not add an additional burden to the original loop.

Carl Barnett, product support manager, Moore Industries-Intl., North Hills, Calif.


May’s Problem:

Must We Abandon Magnetic Encoders?
"We’ve used 12-bit magnetic, absolute rotary encoders with pretty good success. We like their durability. We have the opportunity to sell our equipment to customers in more difficult operating environments, and where there’s a lot more electrical noise than we’ve been used to. We’re told we’re asking for trouble by expecting the magnetic encoders to perform dependably and need to switch to optical encoders and fiberoptics. We’d like to hear the pros and cons from some experienced users."

Send us your comments, suggestions, or solutions for these problems. We’ll present them in the May 2005 issue and on ControlDesign.com. Send visuals, too—a sketch is fine. E-mail us at CDTheAnswer@putman.net or mail to The Answer to Your Problems, CONTROL DESIGN, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. You can also fax to 630/467-1124. Please include your company, location, and title in the response.

Have a problem you’d like to pose to the readers? Send it along, too.

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