Seeking advice on temperature control systems! ! ! Urgent~~~~~~~~~
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These past few days I encountered a strange situation: A Honeywell temperature controller is connected to two relays (normally open and normally closed), as well as an electric control valve of the on/off type. With this wiring method, the temperature is maintained within 1 degree of the set temperature! It’s stable… The electric control valve moves at most 1 or 2 millimeters up or down. With the same wiring and under the same operating conditions (with a distance of 1 meter between them), using a different temperature controller together with another electric control valve fails to maintain the desired temperature; the temperature fluctuates by up to 20 degrees around the set value. The operating conditions are definitely the same, the wiring is fine, and the PID settings as well as other parameters of the temperature controller are also identical. There’s definitely nothing wrong with the electric control valve! It’s strange that for on-off type electric valves, they are either open or closed… But the valve connected to Honeywell stays in a suspended state, not moving at all; it rises slightly when the temperature drops by 1 degree, then stops again! The temperature goes down a little bit at a time·Stop! But K-degree is at most 5 millimeters, and the display shows at most 10 percent! Strange: 2. Identical operating conditions – one TaiDian unit and one Honeywell unit; it is said that they are both produced by the same company. The parameters are the same; TaiDian follows the settings of Honeywell, including PID values and third-level parameters. The valves move back and forth throughout their full range of motion: either they are fully open, resulting in excessively high temperatures, or they are fully closed, resulting in extremely low temperatures. The units do not stop at any point! The situation is urgent·Honey has already placed an order·but I can’t figure out the tactics involved in this! ! ! The Honeywell 1040701000 maintains a stable control within plus or minus 1 degree! ! ! If you know, you can call directly or send a private message~~~
There seems to be a problem with what the original poster described. The situation described in ‘Strange 1’ does not match what was mentioned at the beginning. A on/off control valve can only have two states: either fully open or fully closed. However, the situation described in ‘Strange 1’ suggests that it is an analog control valve, with the temperature controller outputting an analog signal (usually 4–20 mA), rather than a digital signal. The wiring is also not of the relay type; instead, it is a PID control output, which allows the valve to be positioned at any point along its full range. As mentioned in Strange 2, it is estimated that the types of control valves are different.
Master, I think there’s no mistake; what I wrote is correct as well. Both valves are of the on/off type, and their Honeywell temperature controllers together with the valves keep everything under perfect control at one degree! That’s the strange part. Logically, a on/off valve should either be fully open or fully closed, right? But with their valves, when the opening degree of the valve can be used to control the temperature, it stays stationary. When the temperature drops by one degree, the valve moves a little bit – barely any movement at all, not even a millimeter. When the temperature rises by one degree, it moves just a tiny bit as well, still not even a millimeter! ! ! It takes dozens of seconds for it to make an up or down adjustment; the valve moves back and forth from time to time. In most cases, the valve remains stationary, with its opening degree being almost always 10 percent – even when the temperature changes by just one degree, the opening degree stays at 10 percent! ! ! And with our Taiwan-made temperature controllers and valves, the valve opening is either fully open or fully closed! This results in too large temperature fluctuations – the temperature is too low when it’s fully closed, and too high when it’s fully open. There isn’t an error of plus or minus two degrees like with the Honey unit… The parameters of both thermostats are the same; the PID values are identical, and the parameters for the third level are also the same. I really can’t understand why this is the case···
After reading your answer, I don’t know how to respond. You’re not in the instrumentation field, are you? Based on your description, I can tell you with certainty that Honeywell’s electric control valves are not of the on/off type. These valves do not have servo amplification; instead, they receive on/off signals of 220V AC. Such on/off signals are generated by a regulator that does have servo amplification (and can output signals through relays, thyristors, or other means). If the regulator lacks servo amplification, the analog signal it produces must be fed into a servo amplifier, which then amplifies it to generate the on/off signals needed to operate the electric control valve. The speed of operation depends on the settings of the PID parameters. Furthermore, there is no unified standard for setting PID parameters; these need to be adjusted based on the operating conditions and the product in question through on-site tuning. The same parameters may need to be reset when switching to products from different manufacturers, otherwise the system will lose control, just as you described.
Master, your line of thinking is correct; I thought the same way yesterday. It’s a reverse inference approach – the valve motor operates based on the opening and closing timing of the relay, and the opening and closing timing of the relay is determined by the signals from the temperature control instrument. So the problem must lie with the instrument. There’s one more thing, Master: stop doubting it. Both valves are of the on/off type… This is a very special situation; the regulated type can’t even handle the on/off type! I’ve done many experiments – it’s better to use on/off valve control rather than adjustable valve control! With adjustable control, the temperature can be adjusted by plus or minus 2 degrees, while with on/off control it’s plus or minus 1 degree. Today I replaced the TaiDian valve with a Honeywell one and adjusted the PID settings myself; now the temperature can be controlled within 1 degree as well! ! ! ···It’s really frustrating… Is that really the quality of domestic instruments? I’m truly ashamed of domestic instruments. (Is Taiwan’s electronics industry considered top-tier?) ）
For the convenience of discussion, please mention the model of the control valve and the model of the temperature control instrument you are referring to
I’m not sure what the working principle is of the “on-off control valve” you’re referring to – is it either fully open or fully closed? ? ? Based on your description, my understanding is that I already answered it in yesterday’s post, so I won’t repeat it. From a control perspective, if the valve we choose is on-off type (either fully open or fully closed), the regulator should use on-off control rather than PID control. Using PID control when the valve can only be in an open or closed state indicates an error in our design and selection, which needs to be corrected.
The valve selection is correct; the key point is that the Honeywell 1040 unit has a pulse function, which allows it to detect for a certain period of time, with the motor operating for a few seconds at a time. The local power supply also has this function, but it’s not as good as Honeywell’s·
Just trying to spark some discussion – it seems to relate to load control in screw compressors. The PID output is compared with the actual opening degree, and the pulsating output controls the solenoid valve. Also, is there any margin?