delivery system concept
I have an idea for a solution pump that needs some input from the list members who are more familiar with the programming or the mechanics that might be required. This is only addressing a portion of a possible delivery and feedback system. Measuring vAg and mixing is not discussed in this message.
If you tie a stepper motor to a small peristaltic pump unit, shouldn't it be possible to measure out exactly what you need to add to the emulsion based on the amount of steps the motor has to complete? Also, if you tie an optical encoder (a cheap one is a few bucks, or you can take them from an old computer mouse) to the output of the pump shaft, couldn't this be used as a type of "flow meter" to get feedback into the motor control circuit?
The idea is that if the fluid amount in a given length of a particular diameter of tubing is known, the peristaltic pump can be designed to move that same unit of fluid between cam lobes (or 2 one-half cam lobes [not two-and-a-half] if you want to reduce pulsations in the line) over 'x' amount of time.
A peristaltic pump is a loop of flexible plastic tubing, held in a semi circular housing, and an rotor with 'x' number of sides [4,5,6 etc] with rounded corners rotates within the same housing as the tube, squeezing the tubing as the rounded corners of the rotor travel past it, pushing fluid along the length of the tubing. These are physically simple devices, and you can make the pump housing out of plastic using a forstner bit, and use small blocks of nylon, teflon, etc, for the rotor. The caveat here is that you must make the pump rotor as geometrically correct as you can (square, pentagon, hexagon, etc must be precise, not lopsided, and round the corners slightly). The rotor has to travel within the housing, but far enough away from the housing edge to allow for the tubing to be mounted along the edge, and squeezed as the rotor travels past. Too close to the tubing and the rotor will bind and stop.
What I am trying to figure out is if a small piece of slightly larger diameter tubing inserted somewhere in the middle of the original diameter tubing, after the pump output, would act like an 'expansion chamber' smoothing pulsations in the line. Or, would building a pump cam with say, 8 lobes instead of 4, would smooth out the pulsations to a manageable level.
Again, the thinking here is that accurate delivery of ingredients would require feedback of the actual amount of solution delivered to the mixing vessel (if you add the recycling feature mentioned in an earlier post by Photo Engineer, the motor will be running at the right speed when the valve is switched to deliver the solution to the mixing vessel at the right rate and duration).
I know this is adding up to more complexity, but while it is more involved, this may not be expensive to build at all. Another forum member mentioned using free CNC software to control the motors. I haven't looked yet, but this might fit the bill, if we can control more than 3 axis. Where the problem lies is in accurately measuring vAg, and how to integrate that feedback signal to adjust solution flow as desired.
Your comments are appreciated.
1. The solution motion is a series of pulses that have to be turned into a steady flow at the delivery end of the tube.
2. The tube changes diameter (relaxation) during use and changes volume with every use and so must be calibrated.
3. etc... BTDT. It works. It just has several years worth of problems to work through and program. We had a whole team of engineers and $M or more.
Do you mean that over time, the tubing wall itself "weakens" and the diameter increases? Was this effect seen only at the portion of the tubing directly impacted by the pump rotor?
Yes indeedy do. The pump rotor gradually weakens all plastic tubing and the diameter changes gradually by a fair amount.
In addition, any replacement tubing is different in diameter when averaged over even one foot of length as it is not made to very exacting tolerances. Therefore, before each run, the tubing must be carefully calibrated for delivery and of course a correction must be put in for the calibration process itself which further weakens the tubing. You get this as a dv/dr (change in volume per run) in which time of run is part of that polynomial. That term is dv/dt so you might say it is one or two integrals depending on viewpoint. So, how is your calculus?
And then of course, the fluctuations change (pulsations) due to the rotors depending on speed and tubing wall thickness. This also has to be solved. We did this quite elegantly at EK IMHO. I had nothing to do with it. It is unpublished work by that friend who did the scuba diving.
My calculus is extraordinarily rusty. Something else I am going to have to brush up on....
Thank you for your help,
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If you're using a stepper motor, why would you need to use the optical encoder? Surely any feedback you get would just be a scaled version of the number of steps of the motor (which you have complete control over anyway).
PE - my calculus is fine but as an EE student dv/dt means something else and to smooth it one would just add a capacitor... something tells me this wouldn't work here :P
Would flexible (rather than stiff) tubing with a smallish help to give a similar effect?
The idea of the optical encoder was to provide a second source of feedback in the event the pump rotor came loose from the motor shaft, as well as a type of "flow meter" to verify that what the stepper was sending was being "delivered' by the pump. If the two signals didn't coincide ('x' number of steps = arc separated by two adjacent lobes, for example) you can use the event (signal?) to abort the procedure. That was the original intent, anyway.
I recognize that David.
Originally Posted by David Grenet
I meant that the delivered volume in 5' is different today than it was 1 week ago due to tubing changes. All tubing we tested changed. Some was incompatible with the chemicals.
One way is to use a geared pump with stainless tubing that goes on gravimetric flow. There are other ways.
A Syringe is good, but a Peristaltic Pump is too complicated.
Originally Posted by rmazzullo
I've just re-read all the relevant posts on this topic and it has become overwhelmingly clear to me that peristaltic pumping is not the way to go.
Its fraught with problems  that can only be solved by a team of dedicated experts doing it for a living - and that ain't us ! The syringe idea has the most merits and I suggest that we should accept that and move on.
The benefits of the syringe idea are as follows:
- It is proven technology which is available now.
- It is the cheapest automated delivery method.
- You do not need to model a complex system in order to work out how much solution you have just delivered - the delivery volume is solely dependent on the distance the plunger has moved (assuming no air in the line - because at the pressures being used, the compressibility of the fluid is insignificant)
Additionally, I would recommend servo motor control of the syringe's lead screw rather than stepper motor control. (Stepper is the only method discussed so far.) A servo motor will attempt to get to its destination because its position is controlled using a closed feedback loop, whereas stepper motors generally don't use encoders or pots to track their position as they operate open loop (without feedback).
If we used a stepper motor and the syringe exerted a force of greater strength than the motor, then it will interfere with the motion and you won't end up delivering the volume you requested. With a servo motor, you won't miss any steps unless the syringe jams up.
 Even if there is a head of air in the syringe, then it's compressibility can be estimated using the eq'n P1V1/T1=P2V2/T2. I say estimated because the system is not closed and therefore the compression isn't adiabatic.
 PE has recently said that "We were always concerned about developing pulsations when using a peristaltic pump at low flow rates." and that
"It works. It just has several years worth of problems to work through and program. We had a whole team of engineers and $M or more."
The replies I have received from both PE and you have been very helpful. The syringes are inexpensive, and are available from www.smallparts.com, as well as others, so I will look at using that method of delivery. The largest syringe I can find so far is 60cc. I was trying to come up with an idea that would have more capacity than 60 cc, but still be considered small scale.
Any further thoughts are most welcome.