Archive for category Loop controllers

How to pick a setpoint profile program on a Honeywell UDC3500 controller

Honeywell UDC3500 universal digital controllerYou’ve heard this phrase before: “It’s simple. But nobody said it would be easy.”

And this is exactly one of those cases.

The Honeywell UDC3500 digital controller can support up to four setpoint programs, the ramp/soak profiles used in batch control operations. But after configuring all four profiles, I was stuck on how to select the one I wanted the controller to use.

There’s no “Program Select” button on the keypad. So I was mystified on how I was going to select my setpoint profile program #3.

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How do I get an RTD signal to two different devices?

RTDs are great temperature sensors – accurate and easy to install.  But they are not friendly when it comes to trying to get a single RTD to go to two places, like when an RTD temperature measurement has to go to both a controller and a recorder.  People call and ask, “How do I split an RTD signal?”  The short answer is, “You can’t.”

An RTD cannot be wired in parallel or in series to a second device.  Any RTD input supplies a known, regulated ‘excitation’ current to the RTD.  Mixing RTD inputs would mix currents and that’s a Big No-No.

There’s also a lead wire compensation circuit for 3- or 4-wire RTDs that would create problems if a single RTD were connected to two different RTD inputs.  There’s just no feasible means of making two RTD analog inputs play nice together.

But all is not lost. There are several ways to achieve your goal.

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Using a Honeywell loop controller as a manual station

Industry statistics reveal that a fair percentage of control loops are controlled manually, and are not automated.   This fact was brought home last week, when a caller told us he needed “something to adjust the valve position so that the valve stays where it’s been set.  And it’s real important that it can’t accidentally go off its own”.   

What he was describing is what we call a manual station.   It’s a controller where the 4-20mA current output stays fixed where it is until someone pushes buttons on on the front of the controller: up to manually raise the output value, and down to manually lower it.  Typically, the output value is displayed for the operator as a digital number from 0 to 100 percent.  

So what’s available to fill the bill?
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Tested and proven: Honeywell PIE software reads and writes do not affect a UDC controller’s output

A customer has several UDC 3200 loop controllers with newly added Ethernet cards.   He needed to configure each of the controllers’ IP addresses using Honeywell Process Instrument Explorer (PIE) software.  Because the controllers are working in a 24×7 continuous process, he was concerned about how making those changes would affect each controller’s performance.

So he asked me:  Would a PIE action of uploading config files from or downloading them back to a controller affect the controller’s performance?

In the past, I’d only ever changed a controller’s IP address when it was on my workbench, not when it was actively controlling a process. So I’d never paid attention to whether PIE communications would affect the controller’s output or its PID action. Since I couldn’t answer the question, I told the customer I’d run a test to find out for sure.

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How can I use a controller to detect thermocouple drift?

If you use thermocouples in high-temperature applications, you’re aware of the issues thermocouple drift can cause. Thermocouples drift. It’s not a question of IF, it’s a question of WHEN. And thermocouple drift costs processors time and money in processing errors, waste, downtime, and lost production.

Thermocouple drift occurs due to metallurgical changes of the metal alloy elements over the extended use of the sensor. Thermocouples can drift by as much as several degrees per year.

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How do I tune a process control loop?

Industry surveys say that nearly half of all processes aren’t accurately tuned. If you read my post on accuracy, stability, and repeatability, you’ll know that a poorly tuned process can result in bad readings, downtime, and wasted materials.

If you use a Honeywell UDC2500, UDC3200, or UDC3500 1/4 DIN universal digital controller, there’s a great built-in function called Accutune that can help make sure your control process is properly tuned.

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How do you keep production going when a control thermocouple burns out?

There’s no such thing as a fail-proof thermocouple. Over time, thermocouples fail. To compensate for that, a temperature controller will normally go into upscale burnout mode, and drive the furnace burner to low fire or turn down the SCRs.  But then, you have to deal with the downtime, rework, or even the potential of losing product.

Not long ago, a plant operator called to see if there we had a way to work around this burnout mode, so he wasn’t wasting time and materials.

His heat treat load had almost finished its final soak when the control thermocouple broke open. The controller, as expected, drove the furnace burner to low fire. The operator then popped the controller into manual mode, so he could nurse the load through the remainder of its soak cycle. He used the temperature reading on a recorder, fed from a second, unbroken thermocouple in the protection tube as temperature indication for the load.

If the situation had happened in the middle of the night, it may not have been handled with the same attention the day-shift operator had provided.

So, he asked if there was any way to have the controller automatically “fail over” to a second thermocouple. 

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