PID and Honeywell Modutrol IV Motor

[Pulsar2003 0]

Hello. I have a 1746-NT4 thermocouple input card. I think I understand well how to set it up and get the PID running. My other component are a Honeywell burner controller RM7800 and a Modutrol IV motor M9484F4001. Which analog output card is best to drive this motor? NIO4I or NIO4V? This motor drive a gas burner. There is a start up cycle for the Honeywell burner controller, it shorts two of the three wires to force the motor to it's full 90 degrees; this is part of the purge cycle. There are two resistors placed between the blue, red and white wires (237 and 66.5 ohms); the output card will be connected to two of these wires. I wonder if during this cycle I should not isolate the analog output with a small relay with gold plated contacts to avoid damaging the card? Currently there are two Omron PID and there are two relays that the PLC turn on, one is to select which of the PID's temperature will be used and the other is to send the control signal to the modutrol. I wonder if I need to reproduce this situation except that the two Omron PID will be removed and I will use my NT4 card and a ladder logic to set the desired temperature value. Thank you.

[rcshete 0]

Hi I have worked with the Honeywell Modutrol motor 5-6 years back and recollecting, I am sure I used the 4-20 mA signal to drive it since the resistors you mentioned across terminals will basically convert it to voltage signal anyways. The Modutrol electronic circuit design is basically based on accepting potentiometric inputs, either from Hand signal of a Potemtiometer or the "Pressuretrol" used in earlier boiler control systems. you can refer to the manual available which shows a basic circuit diagram. As far as damaging the card is considered, it wont get damaged as the output of card is current controlled and hence shorting wont affect it, but if you have the freedom to add a relay, it might prove helpful. Your reference to 2 PID controllers being used cant be understood as there is generally a single loop for this control. But if you can provide details, we can look into it further. Hope this helps.

[paulengr 44]

I've seen similar configurations before. The first relay is selecting two different thermocouples. It completely violates basic principles of thermocouple wiring but some plants do this, think that everything is working correctly, and never realize that their thermocouple readings are now not reading correctly. The two PID controllers are either in there to allow for two different sets of tuning parameters (and presumably two different inputs), or for pure and simple online spares (redundancy). With roughly 10 thermocouples per kiln and 6 kilns where I work at, for whatever reason, we get about one dead Honeywell UDC controller every 3-6 months. I think the failure rate (60 controllers, failing at a rate of 2-4 per year, MTBF=15-30 years) still seems high but I'm sort of sidestepping it and replacing them with PLC control as time and money allows.

[Pulsar2003 0]

I want to give some more information about this oven. It has two buggies or chariot, left and right. Each has three timers and one PID. Depending on which side is in the oven it will select the left or right PID. This means that if the left buggy is in the cooking cycle it will be the left PID that will control the oven's temperature according to the set point. Usually we cook at 300C while there is only one product that will be at 315C. Each PID has it's own thermocouple. It is the output of either one that will be selected to control the temperature. These relays have been troublesome as the engineers have use Omron's MK2P-S with what I believe to be silver contacts, these end up oxidizing with time and the signal is not passing through. I managed to solve this by replacing it with a relay for lower currents, possibly gold plated. Hopefully that was helpful.....

[Pulsar2003 0]

By the way, any body has some examples of PID? I keep wondering if I've done things right. At this point I'm getting ready to make some test. I will also need to show the oven's temperature on the HMI.

[paulengr 44]

Three likely reasons for those failures. First if it's wet/hot/humid in the area, you accelerate oxidation. Second, and more likely, it's the number of cycles. Relays have a finite lifespan that can be measured in terms of the number of cycles, even if they are hermetically sealed and in an environmentally controlled room. As you said, the contacts do oxidize, and thus during a closing operation, they must arc slightly to blow the oxide layer off. This causes them to wear out over time, usually about 1 million cycles or so. Doesn't matter if it's copper, silver, tungsten, or gold. Regardless of the metallurgy, in the end, same problem. The only contactors I know of that last almost "forever" in this type of operation are vaccuum bottles (or a couple decades ago, SF6) the vaporized metal condenses and cycles over time. Of course they are very expensive so they are not used in practice except at high current levels where the oxidation-type of operation fails prematurely (usually NEMA size 4 contactors for 480 VAC or larger). Instead of the lifespan of the contacts themselves, they fail as air slowly leaks into the bottle over time (or SF6 leaks out in older designs). At low current levels, a second option is to use an SCR to block current flow. An SCR is simply a diode with a "reversed" diode right in the middle of it. In normal operation, it blocks current flow in BOTH directions. However, applying current to the gate input causes it to open in one direction only (the forward conduction direction for the overall diode). Two SCR's wired back-to-back forms a triac, and this is a solid state switch with NO moving parts and no contacts to wear out. SCR's can be constructed to almost arbitrarily large sizes and have reverse bias voltage limits of well over 1 kV are available. Lifespans are usually measured in years, and are independent of the number of switching cycles. An additional advantage is that an SCR can only turn off at a transition from on-to-off, which eliminates ringing (and the resulting surges and spikes) common with relays. Perhaps the only disadvantage is that the reverse-biased resistance is very high, but measureable, unlike relays. So an open circuit on an SCR will still act like it has voltage on it but there is almost no available current. Fluke sells a "stray voltage eliminator" plug-in for their meters which is just a prepackaged 3K resistor across the meter input if this anomaly annoys you. My first suggestion would be to look very closely at your application. Can you add some hysteresis (aka deadband) to reduce the number of switching cycles? Relays die very quickly if they "chatter" because as I said, they have finite switching cycle limits. My second suggestion would be to look at your circuit and buy either a solid state relay (SSR), which you can find from several electronics houses (Newark, Allied, Digikey, Mouser), or go with a vaccuum contactor that you can buy from say Joslyn Clark or Toshiba, depending on current levels. Both will totally eliminate the problem by eliminating the troublesome physical contacts in the first place.

[Pulsar2003 0]

I had no idea I could use a solid state relay in this type of application. Thank you.