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Pneumatic Instrumentation - Self-Balancing Pneumatic Instrument Principles

A great many precision instruments use the principle of balance to measure some quantity. Perhaps the simplest example of a balance-based instrument is the common balance-beam scale used to measure mass in a laboratory:

 

Common balance beam scale used to measure mass in a laboratory
 

A specimen of unknown mass is placed in one pan of the scale, and precise weights are placed in the other pan until the scale achieves a condition of balance. When balance is achieved, the mass of the specimen is known to be equal to the sum total of mass in the other pan. An interesting detail to note about the scale itself is that it has no need of routine calibration. There is nothing to “drift” out of spec which would cause the scale to read inaccurately. In fact, the scale itself doesn’t even have a gauge to register the mass of the specimen: all it has is a single mark indicating a condition of balance. To express this more precisely, the balance beam scale is actually a differential mass comparison device, and it only needs to be accurate at a single point: zero. In other words, it only has to be correct when it tells you there is zero difference in mass between the specimen and the standard masses piled on the other pan.

The elegance of this mechanism allows it to be quite accurate. The only real limitation to accuracy is the certainty to which we know the masses of the balancing weights.

Imagine being tasked with the challenge of automating this laboratory scale. Suppose we grew weary of having to pay a lab technician to place standard weights on the scale to balance it for every new measurement, and we decided to find a way for the scale to balance itself. Where would we start? Well, we would need some sort of mechanism to tell when the scale was out of balance, and another mechanism to change weight on the other pan whenever an out-of-balance condition was detected.

The baffle/nozzle mechanism previously discussed would suffice quite well as a detection mechanism. Simply attach a baffle to the end of the pointer on the scale, and attach a nozzle adjacent to the baffle at the “zero” position (where the pointer should come to a rest at balance):

 

A simple baffle / nozzle mechanism as a detection mechanism

 

Now we have a highly sensitive means of indicating when the scale is balanced, but we still have not yet achieved full automation. The scale cannot balance itself, at least not yet.

What if, instead of using precise, machined, brass weights placed on the other pan to counter the mass of the specimen, we used a pneumatically-actuated force generator operated by the backpressure of the nozzle? An example of such a “force generator” is a bellows: a device made of thin sheet metal with circular corrugations in it, such that it resembles the bellows fabric on an accordion. Pneumatic pressure applied to the interior of the bellows causes it to elongate. If the metal of the bellows is flexible enough so it does not naturally restrain the motion of expansion, the force generated by the expansion of the bellows will almost exactly equal that predicted by the force-pressure-area equation:

 

Bellows showing the predicted force-pressure-area equation

 

A photograph of a brass bellows unit appears here, the bellows taken from a Foxboro model 130 pneumatic controller:

 

Brass Bellows unit from Foxboro model 130 pneumatic controller

 

If the bellows’ expansion is externally restrained so it does not stretch appreciably – and therefore the metal never gets the opportunity to act as a restraining spring – the force exerted by the bellows on that restraining object will exactly equal the pneumatic pressure multiplied by the cross-sectional area of the bellows’ end.

Applying this to the problem of the self-balancing laboratory scale, imagine fixing a bellows to the frame of the scale so it presses downward on the pan where the brass weights normally go, then connecting the bellows to the nozzle backpressure:

 

self balancing laboratory scale

Now the scale will self-balance. When mass is added to the left-hand pan, the pointer (baffle) will move ever so slightly toward the nozzle until enough backpressure builds up behind the nozzle to make the bellows exert the proper amount of balancing force and bring the pointer back (very close) to its original balanced condition. This balancing action is entirely automatic: the nozzle backpressure adjusts to whatever it needs to be in order to keep the pointer at the balanced position, applying or venting pressure to the bellows as needed to keep the system in a condition of equilibrium. What we have created is a negative feedback system, where the output of the system (nozzle backpressure) continuously adjusts to match and balance the input (the applied mass).

This is all well and good, but how does this help us determine the mass of the specimen in the left-hand pan? What good is this self-balancing scale if we cannot read the balancing force? All we have achieved so far is to make the scale self-balancing. The next step is making the balancing force readable to a human operator.

Before we add the final piece to this automated scale, it is worthwhile to reflect on what has been done so far. By adding the baffle/nozzle and bellows mechanisms to the scale, we have abolished the need for brass weights and instead have substituted air pressure. In effect, the scale translates the specimen’s mass into a proportional, analogue, air pressure. What we really need is a way to now translate that air pressure into a human-readable indication of mass.

The solution is simple: add the pressure gauge back to the system. The gauge will register air pressure, but this time the air pressure will be proportionately equivalent to specimen mass. In honor of this proportionality, we may label the face of the pressure gauge in units of grams (mass) instead of PSI or kPa (pressure):

 

self balancing laboratory scale with pressure gauges

 

It is very important to note how the pressure gauge performs an entirely different function now than when it did prior to the addition of the feedback bellows. With just a baffle-nozzle mechanism at work, the pressure gauge was hyper-sensitive to any motion of the scale’s balance beam – it served only as a highly sensitive indicator of balance. Now, with the addition of the feedback bellows, the pressure gauge actually indicates how much mass is in the specimen pan, not merely whether the scale is balanced or not. As we add more mass to the specimen pan, the gauge’s indication proportionately increases. As we take away mass from the specimen pan, the gauge’s indication proportionately decreases.

Although it may seem as though we are done with the task of fully automating the laboratory scale, we can go a step further. Building this pneumatic negative-feedback balancing system provides us with a capability the old manually-operated scale never had: remote indication. There is no reason why the indicating gauge must be located near the scale. Nothing prevents us from locating the receiver gauge some distance from the scale, and using long lengths of tubing to connect the two:

 

self balancing laboratory scale with long distance pressure gauges

 

By equipping the scale with a pneumatic self-balancing apparatus, we have turned it into a pneumatic mass transmitter, capable of relaying the mass measurement in pneumatic, analog form to an indicating gauge far away. This is the basic force-balance principle used in most pneumatic industrial transmitters to convert some process measurement into a 3-15 PSI pneumatic signal.

 

Continue to the next page to about Pilot Valves and Pneumatic Amplifying Relays

Go back to previous page, Pneumatic Sensing Elements

Go back reading to Pneumatic Instrumentation - Introduction page

Go Back to Lessons in Instrumentation Table of Contents


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