Piston Tank (mechanically simple, no machine shop required)

[TuptubBuilder]

This is a feasibility study and not a finished Product. I am trying to balance power requirements with acceptable dive times. I am testing a NEMA11 stepper motor with the POLOLU controller, Tic T500.
Advantages of an open loop stepper motor over a brushed or brushless DC motor:

Programmable parameters:
Acceleration Rate (ramp up, down)
Start velocity (torque is inversely proportional to speed)
Maximum velocity

As a result, these values can be fine-tuned resulting in no jerk on startup and piston “stiction” is more easily overcome.

[TuptubBuilder]

Final Specs:
375 mL Capacity
<300 mA, 12 VDC, 3.2 Watts
<45 seconds to Fill / Empty
Failsafe Limit Switches
Two Rx Inputs required: one to Fill/Empty, one KILL switch (eliminates holding Torque)

BOM:
(1) YEJMK Non-Captive Linear Actuator, NMA11 Stepper -Amazon
(1) POLOLU Tic T500 Controller
(1) 500ML Syringe - Amazon
(2) E61 micro switches - Amazon
(5) 3D Printed Parts, (1) 3D Printed Hole Template
Misc: double face tape & hardware

Listed Items less 3D Prints & Misc = 75 USD

[Scott T]

Thanks for listing the parts. Helps to see what you are telling about.

[TuptubBuilder]

Thanks for listing the parts. Helps to see what you are telling about.

Scott, thank you for your input. I can elaborate.
Thanks to the boom in 3D Printers, these non-captive linear actuators are dirt cheap.
The center of the stepper motor contains a lead screw nut. This NEMA 11 stepper motor can produce around 24# of thrust. The only caveat of using a stepper is the possibility of "ringing" This is when the Stepper motor loses steps. To prevent this, I limit the Acceleration rate to 100 pps/sec. So this 27 dollar stepper eliminates the gear box that a DC motor needs. The POLOLU Controller provides the RC interface and limit switches for the end stops. All in all, it's a clean cheap setup. The difference in running wet vs dry is 50 mA. I haven't determined max operating depth but am confident that it's good for 1.5 meters.

[TuptubBuilder]

Sorry for the PDF but attempting to post directly triggered an approval requirement update.pdf

[Scott T]

Good information. I wonder what something like rain-x or car wax would do? Thanks for sharing.

[TuptubBuilder]

At the risk of sounding a little too geeky I have been giving this some thought and maybe a soft start is the problem given the overall stiffness of the system. In other words, maybe it initially needs to be hit with a hammer instead of a gentle pull. I hate to take this approach due to the subsequent wear and tear and current draw. The only difference between this system and the German's is the instantaneous acceleration of the Engel Piston Tank. I think the Engel system calls for a silicon grease for maintenance.

if anyone has any authority on this subject, please share it.

[Subculture]

Piston tanks are simple Newtonian mechanics. Engel tanks have more torque than necessary, however a direct drive stepper is pushing it unless very large.

[TuptubBuilder]

Piston tanks are simple Newtonian mechanics. Engel tanks have more torque than necessary, however a direct drive stepper is pushing it unless very large.

Sir, thank you for your input. I did do some due diligence before starting the design and I didn’t want to bore people with the math but here it is:
The selected NEMA11 stepper motor is rated at 6Ncm of torque. A stepper motor’s maximum torque is its holding torque and deceases with velocity. The lead screw is a TR5x1.
So the maximum theoretical thrust is:
F = (2 Pi x T x e)/L
Where:
F = force in Newtons
T = torque = 6Ncm = .06Nm
E = Lead Screw efficiency .2 to .4
L = lead = 1mm = .001m
Fmin = (2Pi x .06 x .2)/.001 = 75.4N [17lbf]
Fmax = (2Pi x .o6 x .4)/.001 = 150.8N [34lbf]
This little stepper produces between 17 and 34 pounds of thrust.
Measured static friction up to 10 pounds and dynamic friction was less than 3.5 pounds.
The problem I’m trying to address is the “sitting overnight” stiction.
I couldn’t measure it because my force meter maxes out at 15 lbf.

I think it is unethical to reverse engineer another person’s product (BTW, Arkmodel did it right down to the documentation – copied almost word for word) but I did look at the Engel drivetrain.
Assumptions:
The 390 brushed motor produces .025Nm. Assuming the stated gear reduction of 15:1 does not include the lead screw, the torque at the lead screw is 15 x .025 = .375 Nm. This is over 6 times the torque of the NEMA11 at the lead screw.

So, to your credit, the Engel PT has lots of available torque.

[RCENGR]

The math can only do so much for you, start experimenting.
There are a couple of motion parameters that I didn't see listed. What microstepping are you using? I would start with 1/8 step mode, which moves the stepper in smaller increments. IIRC there are benefits to using microsteps when accelerating.
Next, what starting velocity are you using? This should never be zero.
Your acceleration seems to be very low. In my experience, full velocity should be achieved in under a second.

Here's an example using my CNC mill. Obviously the parameters won't match, but at least it is a motor driving a thread instead of a belt like in 3D printers. The mill bed is quite heavy and it does not accelerate quickly, similar to your situation.
The lead screws are 20 tpi, with 1200 steps per rotation for a final 24,000 steps per inch.
Starting speed is 1000 sps
Max speed is 24,000 sps (60 inch/minute)
Acceleration is approximately (my software doesn't actually use a linear curve) 230,000 step/s^2. Full speed is achieved in 0.101 seconds.

You can determine your parameters in a variety of ways, but here's a suggestion.
1. First warm up your system so you are not dealing with the stiction issue. Once you understand the maximum capability you can start decrementing for that special case.
2. Start by increasing the starting speed until the stepper stalls immediately. Set your starting speed to 1/2 that value.
3. Increase max speed until you are consistently stalling. Set max speed to a fraction of the stall speed, maybe 80% or less.
4. Increase acceleration until the stepper stalls before reaching max speed. Try to get to max speed in under 1/2 second. And then try to get there in even less time.
5. Iterate

I know it took some hours in the shop before I was happy with the performance of my mill. Good luck

[TuptubBuilder]

RCENGR, thanks for your input. I do appreciate it. Right now I’m full stepping to get the maximum torque and it appears to be optimal. No more “next day” stiction. However, I don’t have that warm fuzzy feeling of confidence after all of this fiddling.

Here’s the rub. I’ve determined that a NEMA14 stepper with a TR6x1 Lead Screw would be ideal for the 550mL Syringe. It has twice the torque of the NEMA11 TR5x1. The problem is price and availability. My scheme makes sense only when the stepper costs 26 USD. Otherwise, it makes more sense to just purchase an Engel or Maximus-Modellbau Piston Tank.
Also, I suspect that the YEJMKJ vendor on Amazon is having a fire sale on discontinued stepper motors for the 3D Printer Industry. So this experiment becomes a one-off if I can’t get any more Steppers at 26 USD each.

I plan to continue testing for the sake of a learning experience but my dream of a cheap, simple Piston Tank is quickly evaporating.

[type7]

The sealing rings in a syringe have much higher pressures than necessary for sealing against water. Have you thought about making the piston with standard o-ring seals to lower the force needed for movement?

[TuptubBuilder]

The sealing rings in a syringe have much higher pressures than necessary for sealing against water. Have you thought about making the piston with standard o-ring seals to lower the force needed for movement?

Yes. I did and commented on it. It's buried in the PDF. I used an acrylic tube as the cylinder and found it to vary too much dimensionally.

Frankly, I don't know how Engel can sell his PTs so relatively cheaply. If I were to scratch build, i would use a precision 75mm Stainless Steel tube.
Thanks for your comment.