Cheap peristaltic / doser pumps investigation

[theatrus]

eBay is rife with several models of cheap ($10-+) peristaltic pumps. As part of a totally different project, I grabbed two different models for a possible water clock project, but thought I'd do a quick tear down to see how useful they would be in a doser situation.

This is the $10 variety with a "12V" motor. Its quite simple - no gearbox, the motor output shaft is simply friction coupled to the rollers.

This of course means that apply any lubrication to the rollers (there is none out of package) means the motor shaft (smooth metal) is also lubricated against the rollers. You can get the motor reliably running at about 3V, and use voltage speed control to adjust speed.

As it stands, I doubt this design would offer anywhere near reliable enough rotation speed. However, there is a potential work around. In the space between the 1/8" (or metric equiv) tubing, poking a hole in the outer casing and adding a photo-detector looking for reflections of the rollers would be able to count pulses of rotation, giving a closed-loop control scheme.

 

[theatrus]

I also grabbed one of the $35 stepper motor variants. They use a larger diameter tubing, and have a mounted 12V stepper motor. The build quality is quite a bit better, and there is a nice quick release tubing clamp as well. Here it is opened up. I haven't run this motor as I haven't found my stepper controller test board

 

[karimwassef]

https://www.adafruit.com/products/2448?gclid=CMiL5pX91csCFYSAaQodQm4JMw

??

 

[theatrus]

https://www.adafruit.com/products/2448?gclid=CMiL5pX91csCFYSAaQodQm4JMw

??

I have a variant of that... somewhere. I'll find it or just buy another

 

[karimwassef]

$5 ... splurge

Curious to see if the premium is worth it.

 

[theatrus]

Cheap peristaltic / doser pumps investigation

Going on a guess - it will be reasonably step accurate out of the box (the motor is well coupled to the rollers), and it uses a lot more rollers. It claims 42 steps, which is a lot of resolution. The larger diameter tubing will be easier to replace as well, though both pump heads are serviceable.

The issue of wear on the rollers vs the motor shaft on the $10 model is the biggest flaw. Sooner than later that's going to wear enough and not provide any friction.


I'll test how Krytox (pure PTFE grease) works as well.

 

[karimwassef]

But at 4x the price? Will buying 2 cheap motors and keeping a spare not be a comparable or better solution?

Nature tends to go for economy... otherwise, we'd all have the strength of a gorilla and speed of a cheetah.. and eat 5x as much

I wonder how long it would take for the shaft to wear. If it's a couple of months' worth... then it's just not worth it. If it's a year's worth... maybe?

 

[theatrus]

But at 4x the price? Will buying 2 cheap motors and keeping a spare not be a comparable or better solution?

Nature tends to go for economy... otherwise, we'd all have the strength of a gorilla and speed of a cheetah.. and eat 5x as much

I wonder how long it would take for the shaft to wear. If it's a couple of months' worth... then it's just not worth it. If it's a year's worth... maybe?

There is also the added cost of accuracy - the stepper will be predictable, but as the shaft starts slipping your dosing will change with the cheapy.

I may grab a few more of the cheapies and just put one of them on continuous duty at full speed and see what happens.

 

[karimwassef]

Yup. That's what I was thinking.

I have a vested interest in the loss of accuracy over time data - since I use those cheapies myself.

 

[Member No. 1]

Snip--->
The issue of wear on the rollers vs the motor shaft on the $10 model is the biggest flaw. Sooner than later that's going to wear enough and not provide any friction.


I'll test how Krytox (pure PTFE grease) works as well.

For what's it's worth, I have a Litermeter 3. It uses the same style of pump head. You do not want any lubricant on the rollers/shaft/tubing. They need to be squeaky clean and dry. This device is 10+ years old, rollers/shaft/housing still look brand new. I've replaced the tubing twice in that time. On the other hand, the Neptune DOS I have, the rollers are mounted to a holder and in turn the holder has a direct coupling to the shaft. This type does have a small amount of lubricant on the shafts that the rollers spin on.

 

[perkint]

I have a few of the cheap ones waiting for a project. I'd trust them for an ATU where additional checks are easy but I don't think I'd trust them for dosing

Interested to see what you find

Tim

 

[theatrus]

For what's it's worth, I have a Litermeter 3. It uses the same style of pump head. You do not want any lubricant on the rollers/shaft/tubing. They need to be squeaky clean and dry. This device is 10+ years old, rollers/shaft/housing still look brand new. I've replaced the tubing twice in that time. On the other hand, the Neptune DOS I have, the rollers are mounted to a holder and in turn the holder has a direct coupling to the shaft. This type does have a small amount of lubricant on the shafts that the rollers spin on.

If I remember correctly, the LiterMeter employs a lining between the tube and the rollers of a lower friction plastic?

Some commercial peristaltic pumps specify lubricant on the hose/roller interface, though not all.

 

[d0ughb0y]

There is also the added cost of accuracy - the stepper will be predictable, but as the shaft starts slipping your dosing will change with the cheapy.

I may grab a few more of the cheapies and just put one of them on continuous duty at full speed and see what happens.

using the same voltage and controlling the time will give you a more accurate dosage and volume will be directly proportional to time.

these motors are meant to be used for short periods of time and not continuously at full speed, though I have not actually done that test to see how long it will actually last.

the price has come down from 2 years ago when I built mine, and I had to dispute with the ebay seller because the pump in the listing shows it includes the hose barb but was not included when I received the pump, so I had to buy that extra and get reimbursed by the ebay seller.

 

[theatrus]

using the same voltage and controlling the time will give you a more accurate dosage and volume will be directly proportional to time.

these motors are meant to be used for short periods of time and not continuously at full speed, though I have not actually done that test to see how long it will actually last.

Yeah, for this application all of the dosing would be time based. Voltage is simply to control the nominal rate.

The problem with these motors is there is no easy way to sense the current RPM, nor the RPM of the rollers (if you want to compensate for slippage). I mean you could detect the speed from the motor back EMF as the brushes slide over the relevant contacts (http://www.precisionmicrodrives.com...-bulletins/ab-021-measuring-rpm-from-back-emf for example), but the motor could be spinning all day long and the rollers won't be, hence I initially had the idea of using a photo-detector for the rollers.

Units like the BRS doser appear to use a synchronous motor, so will have a relatively constant RPM (aligned with the AC frequency). I don't have anything fancier (LiterMeter, DOS, etc) to look at their control scheme.

Stepper motors have the distinct disadvantage of tending to be loud (or at least audible due to fixed frequency) if run at speed, depending on how good your motor control system is.

 

[karimwassef]

If you really want accuracy, you could use a quadrature encoder sensor mounted motor. It relays what has turned, not what you asked it to turn to.

 

[damage31]

Call me crazy, but if I've got say $500+ invested in my tank, (and that's cheap compared to some) I would skip the $10 version in favor of the stepper version every day of the week. Just knowing exactly how much is being dosed makes me feel way more comfortable than guessing and hoping... Maybe I'm a control freak like that.

But I do love a good experiment .

 

[theatrus]

Fwiw, I run a Ca reactor (without a peristaltic pump) as my main upkeep for the big doses. It's been very very reliable over the years. Upgrading it to a continuous duty peristaltic feed has been on my list for awhile - I probably wouldn't use one of these pumps though.

My main motivation is to dose a few other liquids (amino, trace, vodka), and even explore dosing refrigerated broadcast food solutions (like coral frenzy).

The cost delta on the stepper is not huge in the grand scheme of things (a low cost frag) - I agree that it's probably worth investing most of the time there. I should get a replacement stepper motor driver by this weekend to test. Even with an occasional missed step, the stepper should have a very fine control of volume over time - I'm pretty optimistic.

 

[Fredfish]

Stepper motors have gotten less expensive since I last looked at them 6 years ago!

Where did you pickup those stepper motors?

 

[theatrus]

Search eBay for "peristaltic pump stepper", the usual host of market resellers from HK and the mainland have them.

 

[gbru316]

Yeah, for this application all of the dosing would be time based. Voltage is simply to control the nominal rate.

Why not use PWM instead?

Arduino (or similar uController) PWM to NPN transistor base, transistor between pump (-) and power supply return (gnd)


Reducing the PWM duty cycle will allow you to run the pump slower than would be possible with DC voltage.These small motors usually suffer quite a bit from low torque and reducing the operating voltage makes things even worse. With PWM, you'll be able to run the motor at Vmax which will yield maximum torque.

I built a fairly accurate doser doing just that with the cheap ebay pumps, but never had a chance to put it into use.