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Pneumatic PID Controllers
Many pneumatic PID controllers use the force-balance principle
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The self-balancing mechanical system “tries” to keep the beam motionless through an
exact balancing of forces, the beam’s position precisely detected by a nozzle/baffle mechanism
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This
mechanism does not directly correspond to any particular manufacturer or model of pneumatic controller, but shares
characteristics common to many
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Increasing process variable (PV) pressure attempts to push the right-hand end of
the beam up, causing the baffle to approach the nozzle
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If we wished to reverse the
controller’s action, all we would need to do is swap the pneumatic signal connections between the input bellows, so
that the PV pressure was applied to the upper bellows and the SP pressure to the lower bellows
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Changing bellows area (either both the PV and SP bellows equally, or the output
bellows by itself) would influence this ratio, as would a change in output bellows position (such that it pressed
against the beam at some difference distance from the fulcrum point)
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Automatic and manual modes
A more practical pneumatic proportional controller mechanism is shown in the next illustration, complete with
setpoint and bias adjustments, and a manual control mode:
“Bumpless” transfer between automatic and manual modes is accomplished by the human operator paying attention
to the balance indicator revealing any air pressure difference between the output bellows and the output adjust
pressure regulator
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The
controller output is then at the direct command of the output adjust pressure regulator, and will not respond to
changes in either PV or SP
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The controller output will once again respond to changes in PV
and SP
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To add derivative control action, all we need to do is place a restrictor valve between the
nozzle tube and the output feedback bellows, causing the bellows to delay filling or emptying its air pressure over
time:
If any sudden change occurs in PV or SP, the output pressure will saturate before the output bellows has the
opportunity to equalize in pressure with the output signal tube
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If either the PV or the SP ramps over time, the output signal will ramp in direct proportion (proportional action), but
there will also be an added offset of pressure at the output signal in order to keep air flowing either in or out of the
output bellows at a constant rate to generate the force necessary to balance the changing input signal
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Integral action requires the addition of a second bellows (a “reset” bellows, positioned opposite the output feedback
bellows) and another restrictor valve to the mechanism1:
This second bellows takes air pressure from the output line and translates it into force that opposes the original
feedback bellows
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Indeed, it would render the force-balance
system completely ineffectual if this new “reset” bellows were allowed to inflate and deflate with no time lag
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As this
bellows slowly fills (or empties) with pressurized air from the nozzle, the change in force on the beam causes the
regular output bellows to have to “stay ahead” of the reset bellows action by constantly filling (or emptying) at some
rate over time
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The following
mechanism has been stripped of all unnecessary complexity so that we may focus on just the proportional and
integral actions
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The reset (integral) valve has been completely shut off to begin our analysis:
With 0 PSI of air pressure in the reset bellows, it is as though the reset bellows does not exist at all
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Now, imagine opening up the reset valve just a little bit, so that the output air pressure of 3 PSI begins to slowly fill
the reset bellows
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This forces the baffle closer to the nozzle, causing the output pressure to rise
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With this greater output pressure, the reset bellows has an even greater “final”
pressure to achieve, and so its rate of filling continues
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This creates a constant 3 PSI differential pressure across the reset
restriction valve, resulting in a constant flow of air into the reset bellows at a rate determined by that pressure drop
and the opening of the restrictor valve
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e
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Thus, we see in this
mechanism the defining nature of integral control action: that the magnitude of the error determines the velocity of
the output signal (its rate of change over time, or dmdt )
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Fisher MultiTrol
Front (left) and rear (right) photographs of a real pneumatic controller (a Fisher “MultiTrol” unit) appear here:
The mechanism is remarkably similar to the one used throughout the explanatory discussion, with the important
distinction of being motion-balance instead of force balance
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Setpoint control is achieved by moving the position of the nozzle up and down with respect to the beam
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This being a motion-balance system, an offset in nozzle position equates to a biasing of the output signal,
causing the controller to seek a new process variable value
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Integral rate control is implemented exactly the same way as in the hypothetical controller
mechanism illustrated in the discussion: by adjusting a valve restricting air flow to and from the reset bellows
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The reset knob is actually calibrated in units of minutes
per repeat, while the proportional band knob is labeled with a scale of arbitrary numbers:
Selection of direct versus reverse action is accomplished in the same way as selection between proportional and
snap-action (on-off) control: by movable manifolds re-directing air pressure signals to different bellows in the
mechanism
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The following photograph shows one of the manifold plates removed and turned
upside-down for inspection of the air passages:
The two quarter-circumference slots seen in the manifold plate connect adjacent air ports together
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Foxboro model 43AP
The Fisher MultiTrol pneumatic controller is a very simple device, intended for field-mounting near the pneumatic
transmitter and control valve to form a control loop for non-precision applications
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The following photographs show one of these controllers, with the access door closed (left)
and open (right):
At the heart of this controller is a motion-balance “pneumatic control unit” mechanism
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Foxboro model 130
Foxboro also manufactured panel-mounted pneumatic controllers, the model 130 series, for largerscale applications
where multiple controllers needed to be located in one compact space
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Instead of the
four bellows acting against a straight beam, however, these bellows push against a circular disk:
A nozzle (shown in the next photograph) detects if the disk is out of position (unbalanced), sending a back-pressure
signal to an amplifying relay which then drives the feedback bellows:
The disk rocks along an axis established by a movable bar
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However, if
the fulcrum bar is rotated to give the input bellows more leverage and the feedback bellows less leverage, the
feedback bellows will have to “work harder” (exert more force) to counteract any imbalance of force created by the
input (PV and SP) bellows, thus creating a greater gain: more output pressure for the same amount of input pressure
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Its moment arm
(lever length) of course is always equal to that of the feedback bellows, just as the PV and SP bellows’ moment arm
lengths are always equal, being positioned opposite the fulcrum line
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A selector (movable with a hex wrench) turns an air
signal port “switch” on the bottom of the four-bellows unit, effectively switching the PV and SP bellows:
An interesting characteristic of most pneumatic controllers is modularity of function: it is possible to order a
pneumatic controller that is proportional-only (P), proportional plus integral (P+I), or full PID
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This explains the relative scarcity of full
PID pneumatic controllers in industry: why pay for additional functionality if less will suffice for the task at hand?
External reset (integral) feedback
Some pneumatic controllers come equipped with an option for external reset: a feature useful in control systems to
avoid integral windup if and when the process stops responding to changes in controller output
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If for some reason the final control element cannot achieve the state called for by the controller, the
controller will sense this through the external reset signal, and will cease integration to avoid “wind-up
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After the pneumatic lag caused by the reset restrictor valve and bellows passes, the
reset bellows force will remain fixed
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This “race” caused the output pressure
to wind either up or down depending on the sign of the error
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Thus, the dreaded effect of integral windup – where the integral action of a controller continues
to act even though the change in output is of no effect on the process – is averted
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Not only is a bias adjustment
completely unnecessary with the addition of integral action, but it would actually cause problems by making the integral action “think” an error
existed between PV and SP when there was none
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Thus, a doubling of pressure drop across the restrictor valve results in a
doubling of flow rate into (or out of) the reset bellows, and a consequent doubling of integration rate
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3
In case you are wondering, this controller happens to be reverse-acting instead of direct
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