The following discussion is part of an occasional series, "Ask the Automation Pros," authored by Greg McMillan, industry consultant, author of numerous process control books, and 2010 ISA Life Achievement Award recipient. Program administrators will collect submitted questions and solicits responses from automation professionals. Past Q&A videos are available on the ISA YouTube channel. View the playlist here. You can read all posts from this series here.
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Is there any advice regarding a temperature control loop in which the thermal actuators, such as a chiller or a heater, work only in either ON or OFF mode?
For “on-off” control to work well, the process control loop must have minimal process dead time, extremely slow load disturbances, and ideally a slow process response time (e.g., near-integrating process dynamics). The classic example is the on-off control of your home temperature by urning cooling or heating on and off via your HVAC unit. Control within one degree is possible because the dead time for stopping the cooling or heating is short and the near-integrating response is quite slow and weather disturbance effects are extremely slow.
For the case of pulse width modulation (PWM), the dead time, which is approximately half of the PWM period or cycle time, must be small compared to the process response time. To expand the scope of the Q&A to deal with many more applications, let us consider the case where other unit operations or valves are being manipulated for PWM. Here is an excerpt from my April 2017 Control Article: “When and How to use Pulse Width Modulation”.
In some applications, throttling of the manipulated flows is difficult or impossible. In the biochemical industry, where precise (good resolution and sensitivity) throttling valves without any crevices (to meet sanitary requirements) are rather limited (there are exceptions such as the Fisher Baumann 83000-89000 series).
Often, pulse width modulation (PWM) is used to turn nutrient and reagent pumps on and off. In the chemical industry, PWM is used to open and close valves whose trim would plug or whose stem would stick if throttled. The sudden burst of flow from on-off action helps flush out the trim and wipe the stem clean. PWM is correspondingly used for small reagent flows, corrosive fluids, and slurries.
It is also used to prevent flashing by a valve position that ensures a pressure drop above the critical pressure drop. PWM is also used in temperature loops to turn heaters on and off. Here, it is commonly called “time proportioning control” but the action is principally the same. Temperature loops for extruders, silicon crystal pullers and environmental chambers often use this technique.
All the applications of PWM have one thing in common; a capacity to filter or dilute the pulses so that they do not appear as measurement noise in the controlled variable. PWM provides a train of pulses that show up as a sawtooth in the measurement unless attenuated. The mass of fluid and metal in a reactor, extruder, or crystal puller and mass of air in an environment chamber must be large enough and the maximum pulse width small enough so that the amplitude of the sawtooth seen is negligible.
The rangeability achieved by PWM is basically equal to the maximum pulse width divided by the minimum pulse width. Since a valve must reach a set position and the pump must reach a set speed during the pulse, the minimum pulse width is fixed by the pre-stroke deadtime and stroking time of the valve or the rate limiting of the speed and acceleration time of pump. Usually, four seconds is adequate for small valves and pumps.
The maximum pulse width is the pulse cycle time when the pulse is almost continuously on. Thus, the cycle time chosen represents a compromise of the desire to maximize rangeability and minimize the sawtooth amplitude seen in measurement and minimize loop deadtime. An additional consideration is the wear and tear on the final control element. Pumps, agitators, and motor driven valves have a maximum duty cycle that must not be exceeded.
Also, heaters in the motor starter will trip for too short a cycle time because the temperature rise from lack of cool down is equated to an overload current. For valves, periodic opening and closing will eventually cause packing, seat, seal or trim failure.
Pulse amplitude modulation can be used in conjunction with PWM to maximize the rangeability and minimize the cycle time and hence dead time. A throttling control valve with good resolution and minimal lost motion is used to set the pulse amplitude. A on-off tight shutoff valve is opened and closed to set the pulse width.
There may be parallel branches of more than one on-off valve in series with the throttling valve to facilitate on-line maintenance to deal with wear and tear of on-off from frequent stroking. The pulse amplitude (throttle valve position) could be reduced instead of the cycle time increased when the demand for the manipulated variable is low.
In highly exothermic reactors, the reactor temperature controller gain is set so large to prevent a runaway reaction that the secondary coolant temperature loop is continually oscillating turning on and off the makeup chilled water flow to a constant jacket coolant flow.
This likely does not help with a heater or chiller, but I think it’s a great story to share.
A good friend who passed a while back had an interesting solution for PWM on relatively fast near integrating loops with no load:
On-off temperature control is typical in extrusion processes in a split range arrangement using pulse with modulation (PWM). In many cases the extruder barrel is divided in various sections with both heating (electric heater) and cooling capacity (cooling water) both pulsed in a time proportioning control fashion. Pulsing of the cooling water is important to ensure an efficient heat removal, sometimes it is so efficient that the cooling process gain ends up being larger than the heating process gain, causing control issues. This unbalance can be corrected by: