sorry, our customers are cheap LOL. they are used to buying a $12 product and now we're putting something together for them that is 10-14 times the cost.

It may be unrealistic aiming for what you are describing for ~ $12.00?
At least with a little quality?
Max.

 

It may be unrealistic aiming for what you are describing for ~ $12.00?
At least with a little quality?
Max.

Seems he is aiming for $100 to $168 as a target price for this device.. Which should be no problem.. The $12 is the manual device they are looking at changing out from..

That Newegg link dispensing device meets that.. You could even resell them if you don't mind a 10-15% margin

 

I have a bad feeling relying on a centrifugal pump for accurate delivery. In the lab with a new, clean pump, defined fluid, and consistent ∆P, it might look fine. But we all know the real world is dirty, worn, and changing all the time. Look at the pump curve - how the volume pumped varies with ∆P, for instance. Throw in a viscous solution with a few dead insects in it, and worn impeller vanes, and you've got a nearly chaotic situation. What happens if your customer expects the delivery to be 10' above the pump's inlet? What if he powers the pump with a long extension cord and it's seeing only, say, 100V?

I'm just saying. Lots of pumping nightmares in my past.

 

I have a bad feeling relying on a centrifugal pump for accurate delivery. In the lab with a new, clean pump, defined fluid, and consistent ∆P, it might look fine. But we all know the real world is dirty, worn, and changing all the time. Look at the pump curve - how the volume pumped varies with ∆P, for instance. Throw in a viscous solution with a few dead insects in it, and worn impeller vanes, and you've got a nearly chaotic situation. What happens if your customer expects the delivery to be 10' above the pump's inlet? What if he powers the pump with a long extension cord and it's seeing only, say, 100V?

I'm just saying. Lots of pumping nightmares in my past.

"The devil is in the details" is what I say all the time when designing products..
Its the "details" that take 90% of the time too..

Also
Better, Faster and Cheaper... Pick 2

 

I have a bad feeling relying on a centrifugal pump for accurate delivery.

Second that one, unless it is a metering pump, it's not that accurate.
Max.

 

guys we are talking about 3-8 ccs of fluid that will be the "tolerance" its plenty accurate. yes it does drip a little, yes it does change a little depending on if the head is + or -, but so far i Have not be over/under 2.5cc's for a 200cc dose. its "accurate enough"

Welcome to AAC.

I can appreciate where you're coming from. I'm assuming this is a small R&D project you're using as a proof of concept and if and when you move to production, you'll get said device approved by the appropriate agencies if required.

Precisely.

While I agree this forum isn't designed to offer "free engineering services", I'm looking at this more as a small project you're getting paid to learn electronics and expand your skill set. I didn't get the impression you're expecting us to gift wrap this for you, more that you are looking for guidance.

IT certainly was not my intention to give that impression. and that is indeed the best part of my job, i get to wear as many hats as i want and get paid to study

I've learned a great deal myself working for small companies on projects such as this. While there may be a COTS product that does the job, it may not be the right size, have the desired features, etc. Of course, I'm the first to look at something and think I can make it cheaper and I've been told numerous times that probably isn't the case, so you can take my advice with a spoon of salt.

That said, let's have some fun.



Can you elaborate on your requirements?

Example:



Device has LCD screen so user can select/change following:

• Dosage Size (what controls this, length of time power is applied to pump?) That was the idea. Now however, its looking more and more like i may need to use PWM instead of just cutting the pump "on"
• Fluid Viscosity (what controls this, size of pulses to pump?) this would be just another component of the time based variable. we will be including a small device used to measure viscosity. this will also help us offset the change of fluid viscosity due to temp changes, so no need to program in yet another set of variables.

LCD will also display number of times button is pressed.

When button is pressed, PWM signal will be applied to pump for x time and increment will increase by one. For brevity, lets call it a trigger, as not to confuse the activation trigger, from the button on the LCD panel for menu swapping. And yes, that is how it would work. and of course the set time would be referneced from the lookup table.

Anything else?

How many different values for the PWM and times do you foresee needing?
I would like to have a table with a 20x20x10 resolution. and im assuming we could make the microprosseor interpolate the values in between. the good thing is that the LiFePO4 batteries have a rather flat voltage profile so wont need to spend to much time there.

Also, what is the current draw of the pump you're using? I assume it is running from 12VDC?
the pump we currently use has a constant draw of 2.2-2.8 amps depending on viscosity and pump head. im currently looking in a .5 amp pump, and also the afect of shut off valves that will dial the dosage in even further.

You might be able to do this with a PICAXE microcontroller (uC). They are cheap, programming them is simple, and initial investment to get up and running is minimal. They are not fast and thus not ideal where multiple simultaneous operations are needed, but I think you'd be okay for your application if a 1-second pulse is close to meeting your requirements.

that sounds good, but again this would be the point when i start to get out of my element.

You could also use a uC to handle the bean counting and LCD and add a PWM circuit with a selector knob if you don't have too many timing selections.
what i was thinking was a rotary encoder, use the LCD button to select the proper menu, and then the knob to set the numerical value, however, it looks as if it will be more cost permissive is i just stick with up/down buttons

You might be able to skip the uC altogether and use a PWM circuit and a 7-segment display, but we need to know more about how the viscosity and dosage size is selected and controlled. Of course, from a power standpoint, an LCD with uC is the way to go if using a battery.

I can try to ellaborate more if you need.

Found a few PWM circuits you may be interested in trying. I have not used any myself yet, but these look pretty straightforward. This and this are the same circuit - both discuss the operation and one includes a video. SgtWookie provided an even simplier one here some time back.

thanks again for those links.

 

IT certainly was not my intention to give that impression.

I don't think you did and no one meant any offense. Once in a while we get students, among others, who simply expect us to provide an engineering solution complete with documentation. So people are a little leery with some posts and how they are worded. No worries though.

and that is indeed the best part of my job, i get to wear as many hats as i want and get paid to study

Gotta love that.

I'm trying to follow your explanations, but I'm missing something or we're just using different terminology. I'm not sure what you mean by a 20x20x10 table. I'm thinking there will be a PWM value (size of pulse) and a time value (effectively the total number of pulses sent to motor), but perhaps there is more to it?

Let's try this, forgetting about the how, can you go over, step-by-step, what you want this device to do? A quick-start guide for the user, if you will. I'm hoping this will shed some light on the unit's operation and allow us to better "see" your requirements.

Example:


Four buttons: A - select/enter, B - up, C - down, D - trigger

1. Connect battery.
2. Flip power switch to ON position.
3. LCD will show "Dispense XX cc" on top row and "Battery Life: XX %" on bottom row.
4. Press button A to change cc value, otherwise skip to step 8.
5. Press button B to increase or button C to decrease cc value.
6. Press button A to set cc value.
7. LCD will show "Dispense XX cc" with new cc value selected.
8. Unit is ready for operation.
9. Press D button to dispense.
10. Unit will dispense chosen cc amount.
11. LCD will show "Dispense XX cc" on top row and "Number of dispensed amounts is : xx" on bottom row.
12. Press button D again to dispense, otherwise press button A to change cc value.
13. Press and hold button A to view battery life.
14. Flip power switch off to turn off unit. (Is number of dispersions saved if power is cut and/or battery is removed?)

 

sorry i havent had a change to get back o nand answer those questions. ive been working on building a PWM controller with a 555 to try out. But that has led to even more questions. there is so many different schematics for that on the web. I found one that doesnt afffect torque at slow speeds using 2 555s (or a 556) but im not sure i can build it. So im waiting on like 3 more components (mosfet, voltage ref, and pot) to come in at radio shack so i can just finish the circuit ive started on and see how it works. From what i can tell, it adjust the frequency as it adjust the pulse width and i dont like that, but at least its a start getting my hands wet in electronics again.

 

From what i can tell, it adjust the frequency as it adjust the pulse width .

PWM is normally a fixed frequency, just the pulse width varies.
The only way for a speed to remain constant requires a feedback of some kind, usually the simple way is a current sense resistor in series with the motor.
Max.

 

in these particular circuits, f=1.44(r1xc1); in order to adjust the speed of the motor you use the 100k pot to change the resistance value, and thus the voltage on pin 3 of the 555. but because this R1 is directly connected to C1, adjusting the resistance also changes that value in the frequency formula. the values change for pins 3 and 6 and thus changes the output frequency

im not exactly 100% on how the capictors charge and discharge to control this circuit, but this is the circuit im building now. http://homemadecircuitsandschematics.blogspot.com/2012/05/make-this-pwm-based-dc-motor-speed.html

again, these are very simple ciruits not designed for individual control of duty cycle and frequency. If this circuit doesn't tell me what i need, ill order all the parts for the more inclusive design and start over. I would prefer the ability to find the most efficient frequency for this pump

this design also doesnt allow for 100% duty cycle, which isnt to much of a problem since i can just hook directly to power supply for that. id have to do the math to see what the exact ranges are but i think its from like 10-90% with frequency varying from 600-1400hz

 

these are the two other circuits i would liek to build (or attempt to build)

http://1.bp.blogspot.com/-jONLYQ8Ehro/UkD_gzAjA-I/AAAAAAAAFSk/nt6vxMDF9RU/s1600/ELC circuit.png

http://homemadecircuitsandschematics.blogspot.com/2011/12/constant-torque-dc-motor-speed.html

 

Also take a look at figure 12-5 here. I went searching through my books last night and found this one which looked interesting. It is very similar to the first link you posted except the output comes from the output pin, not the discharge. Not sure if that will make a hill of beans difference, but it's something you can play with to give you another option.

You may want to add a resistor in series with the MOSFET gate and possibly consider a MOSFET driver. I can't claim to know much about MOSFET switching, but I have learned that trying to quickly switch MOSFETs on and off can be a problem. Bill goes into detail and solutions here.

Also consider adding the electrolytic capacitor across the drain and source pins of the MOSFET as mentioned in the comments section of the first link you provided. Definitely add a 1N4001 across the the motor leads to protect the MOSFET from back EMF.

555/6's are electronically noisy ICs. Used by themselves they're probably okay, but when you start adding more IC's into the mix, add a 0.1uF ceramic and a 1uF electrolytic capacitor across the power leads to each 555/6 as close to the power pins as possible.

What is the voltage and current rating of the pump you're using?

If you decide on a uC, adjusting the PWM should be pretty straight forward with the uC. Certainly worth testing with your current set-up to verify the pump can be slowed down enough to meet your needs.