When and How to use Pulse Width Modulation of Controller Outputs

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. Since the pulse cycle time also sets the time between successive pulses, it adds a maximum deadtime to the loop that is about equal to the cycle time when the pulse is almost continuously off. For an average controller output of 50%, the deadtime added is about ½ the cycle time. 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, agitator 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.

The consequences and methods of mitigation of pulses are discussed in the 12/15/2014 Control Talk Blog “Controller Attenuation and Resonance Tips”. A simple equation to predict the amplitude of pulses after attenuation by the process or a filter that are seen by the controller is discussed in the 12/02/2104 Control Talk Blog “Measurement Attenuation and Deception Tips”.

The generation of a pulse train is done by special output cards or by the configuration of function blocks on the PID output.  The heart of the configuration is a ramp that resets itself periodically. An integrator (INT) function block is employed to generate the ramp. The configuration depends upon what version of Distributed Control System (DCS) or Programmable Logic Controller (PLC) is used. For an integrator that will ramp up towards a setpoint, the input to the integrator is set equal to 100% divided by the desired cycle time. The integrator setpoint is set slightly larger than 100%. The ramp “on” time or pulse width is determined by comparing percent ramp value (integrator output) to the percent controller output via a high signal monitor (HSM) block. When the ramp value exceeds the controller output, a discrete is see equal to one (true), which opens a discrete output or transfers in an analog output value that corresponds to the closed valve position or minimum speed. If the controller output drops below the minimum pulse width, the pulse is turned off by transferring in a negative value before the ramp value is used as the operand of the HSM block on the output of the INT block. The PID low output limit should be set to be slightly less than this minimum pulse width. The functionality of blocks depends upon the DCS or PLC used and any configuration must be extensively tested before being used online.

For viscous fluids and slurries, a precise control valve may be continuously throttled until the valve position gets so small that laminar flow or plugging can occur. At this point (e.g., below 20% PID output), PWM starts. The throttling valve position then stays open (e.g., 20%) and an inexpensive on-off valve in series with the control valve is open and closed by PWM.

There are many other applications of PWM. Pulsed flows have been shown to increase the yield of reactors, the separation in distillation columns and the combustion efficiency of burners. Pulse reagent flow has been very successfully used to mimic a titration for batch pH control. While many well designed pulsed strategies can work for this application, PWM on a proportional-only pH PID controller retains a conventional operator interface via the PID operator faceplate and tuning via the PID gain setting. Also, the controller output can be transferred in for the analog output to reduce batch processing time by providing pulses that are not only longer but that are also larger (pulse width plus pulse amplitude modulation). The gain of the manipulated variable is now nonlinear and is proportional to the controller output. However, for proportional-only control of batch pH processes, this gain change may be advantageous and offset the low pH process gain from the operating point being on the flat portion of the titration curve at the beginning of the batch cycle moving to the steep part of the curve at the end of the batch. This is an example of how a continuous control technique is also useful for batch processing.

PWM also dramatically reduces the effects of deadband and resolution limit in the control valve or variable speed drive assuming the pulse amplitude is at least 5 times as large as the suspected deadband and resolution limit. This normally is the case if the amplitude is > 5%. However, for valves designed for tight shutoff, the backlash and stiction may be as large as 10% requiring 50% amplitude.

You may want to check your pulse now to see how excited you are about PWM opportunities.