Stepper Motor NEMA 17 or 23 or something else entirely? Advice please...

I have an Eccotemp L10 2.6 GPM portable tankless water heater that has two adjustment knobs, one for cold water flow and one for hot water flow. This heater is bolted to the outside of a shed due to venting requirements (uses propane). Inside the shed I have a rudimentary shower set up. Trouble is, based on input water temperatures and ambient outside temperatures the “shower” can be to hot or to cold, and adjusting it requires stepping outside the shed, walking around to the heater, adjusting the hot knob, going back in, and testing. I know there are other ways to solve this issue (cut hole in wall and reach thru, move heater inside and install appropriate vents, etc), but what fun is a simple solution when I can complicate things a bit!

This seems like a perfect opportunity for me to learn about stepper motors! What I want to do is get a stepper with the same shaft size as the knob shaft, couple the knob shaft to the stepper, then wire it to an Arduino I can mount inside the shed, and use a potentiometer or similar “adjustment” tool (maybe even a temp sensor for self regulation) to control the stepper and adjust the hot water knob.

Here’s the concern. I know nothing about steppers. When I look at steppers in the NEMA 17 or 23 range it looks like they are really low torque. The knob on the heater is easily turned by hand, but I have no way to easily determine the torque required to do this, and I have no idea what the usable torque is on these NEMA steppers. It looks like the “holding torque” is in the range of 50Ncm to maybe 2Nm, and that doesn’t seem like much. I figure a stepper is a good choice as I only need to turn the knob around 150°-180° of rotation, so having good control on how far the stepper turns is a good thing.

Any advice on what the usable torque is on these NEMA 17 or 23 motors would be helpful. Do you think they’d “turn a knob”. I suppose I could use gearing or something (belt drive), but I don’t want to create a Rube Goldberg machine (well, maybe I do).

First and formost, NEMA ONLY relates to the mounting size and design to face-mount the stepper motors. Nothing else!

Which brings up the mechanical part of your project. How do you intend to mount the stepper motors to something strong so as to allow them to turn the valves? The shaft connection is only half the design. Have you measured the torque necessary to turn the valves? Without that value, you will never pick a proper motor.

Use vice grips to lock onto the valve shaft. Use a spring scale to measure the pull on the vice grip handle. Measure the distance from the scale connection point to the center of the valve shaft. This is the length of your lever. Divide the scale value by the lever length to get the torque value. Probably double it to be sure. Then find a suitable motor.

Paul

Yep. I have a Wheeler Firearms Accurizing torque wrench with inch/pound measurement that I’m going to get fitted to the shaft of the valve. Once I do that I can hopefully get an inch pound reading of the torque. As far as mounting, the front of the heater is metal and I’ll fabricate a metal mount box and screw or rivet it to the face of the heater, depending on the depth of the stepper I get.

But, I’m trying to understand if the “holding torque” of these steppers is the same torque as the drive or movement torque? It’s not real clear to me at all…

The holding torque is basically the same as the moving torque. I was going to write that once you get a valve set to the position you want, it will stay there on it's own and you can tell the stepper controller to disable the motor. BUT, you will likely need to make a lot of fine adjustments during use, so that option is not worth the bother.

Consider using timing belts and corresponding pullies to connect the valves and the stepper motors. Much more compact and much more forgiving in alignment.

Paul

Get a thermostatic mixer shower. I know it's not a sexy solution - but if your main object is to have an enjoyable shower then "it just works".

...R

First, I appreciated your willingness to give a reply. So thank you. But...

I’m always surprised (and I guess I shouldn’t be as old as I am) that people don’t actually respond to the question being asked but rather suggest a solution to a different problem. I’m asking about stepper motors as I am trying to learn about them and their uses. I’m aware that there are many other solutions to the “cold/hot shower” issue. I could build a real house and use a real shower. I could install a regular water heater. I could get used to cold/hot showers. I could skip taking showers at all. I could move the heater inside and adjust the knobs directly, the list goes on. I could even get a thermostat mixer shower. But interestingly none of those solutions deal with learning about stepper motors. It reminds me of automotive forums where someone asks “I have a x car and it has a small oil leak near the third cylinder, where do you think it might be coming from?” And people reply “x cars suck, sell it and by a y car”. But, I guess if you want to drive your posting numbers up non sequitur answers like that will do. (I know, this posting makes me a wanker...)

It looks like I might be able to use a planetary gearbox Nema 17 (or 23] stepper motor as they generate higher torques than standard steppers. I’m also looking at belt drive with gearing, but I think a direct connect using a flexible shaft coupler is probable less complicated.

MadMacks:
But, I guess if you want to drive your posting numbers up non sequitur answers like that will do.

My Reply #4 was made in good faith because I am in a somewhat similar situation with uneven pressure for my hot and cold water driven from a 12v pump. I endured manual mixing for a few years before discovering the joys of a thermostatic mixer. Admittedly my manual mixing controls were at my shower, and not outside.

By all means use this as an opportunity to learn about stepper motors, but don't expect a quick solution.

I would not be surprised if your flow control shaft is too stiff for an economical stepper motor - if so the choice is between a large stepper motor together with the essential expensive high current stepper motor driver or reduction gearing between the motor and the flow control shaft - perhaps with a toothed belt. Obviously installing gears or a belt drive will make the mechanical installation much more complex.

If you do choose the route of direct drive I suspect you will be lucky if the stepper motor shaft matches your knob shaft. However it is quite common to use couplers that have different diameters at each end. For example on my small CNC mill the stepper motor has a 5mm shaft and the lead screw is 8mm.

...R
PS ... this overlapped with Reply #6

I really really appreciate your not taking offense at my initial reply. Thank you. It was not intended as a shot. And I appreciate your understanding that.

On the stepper...I found a “Nema23 Worm Gear Stepper Motor L56MM 3A Gearbox Ratio 30:1 Speed Reducer”...

(Amazon.com: Nema23 Worm Gear Stepper Motor L56MM 3A Gearbox Ratio 30:1 Speed Reducer for CNC Router: Automotive)

...that looks like it might be a good fit. With the gearing it looks like it will put out almost 30Nm (I don’t know the efficiency losses) with is a LOT more torque than I need. It also appears to have an output shaft of 11mm so I might be able to use an 11mm to 8mm bore adaptor and actually slip it right over the knob shaft (which I believe is 7.5mm).

It’s a 1.8° stepper (200 steps) so I think that’s enough resolution for my application. If I pair it up to a TB6600 driver (motor uses up to 3amps) I think it all might work. A bit pricey, but a good stepper learning setup?

You should ensure that the stepper driver can supply at least 4 amps to give yourself a bit of headroom. It is never a good idea to operate electronic equipment close to its limit.

...R

PS ... a thermostatic shower would be cheaper :slight_smile: :slight_smile:

In the cost issue, I've priced the components and I think I can get them all for around $100 (US) so I guessing a new thermostatic show would be much more.

I the controller I've identified is rated at 4 amps so it should handle the motor ok. It's really only going to run for a few seconds maybe a few times a week, depending on weather. It's not being used very much. And I completely agree on using a higher rated controller. Maybe I'll post a picture of the set up once I get it up and running. Thx for the advice...

MadMacks:
I've priced the components and I think I can get them all for around $100 (US) so I guessing a new thermostatic show would be much more.

This is the Triton shower that I have. It is priced at £70 and I think there was a special offer with £10 off when I bought mine. These things are usually cheaper in the USA.

But, hey, I'm not one to prevent an Arduino project from proceeding.

...R

MadMacks:
In the cost issue, I've priced the components and I think I can get them all for around $100 (US) so I guessing a new thermostatic show would be much more.

Similarly, I don't want to discourage you, but that's happy path.

If I were doing this, I would likely find that I had to buy three different steppers, two drivers and then a replacement when I destroyed the one I needed. Also five different shaft couplers and some other things that I didn't realize I would need at the start.

I have a lot of spare parts :wink:

I hear you on the 3 steppers and a slew of couplers. But I can use extras on other projects...

Here's the problem with the thermostatic shower mixer solution. There is only a single water feed coming out of the EccoTemo heater, it mixes internally so there is only one pressure output, not two. There is a single pump providing pressure to the system. I live off grid so I don't have "normal" water pipes and electricity. I have a 55 gallon barrel of water that feeds the pump. If the EccoTemo doesn't get enough pressure and flow the heater burner will not fire (all cold water at that point). If I split the pressures water line to feed the Triton with both hot and cold I doubt the pump could provide enough pressure and flow, so add cost of bigger pump and pipe fittings, and a second pipe inside for cold, and the Triton. And high pressure high volume dc (battery based) water pumps aren't cheap either.

The EccoTemo intakes in a single pressured water line and provides a cold and hot internal mixer controlled by two knobs, and outputs a single "warm" water line. I have a bunch of Arduinos laying around so I figured a stepper set up would be cheaper and a learning experience.

Maybe I’ll just install a sprocket on the hot knob, install a shaft thru the wall with a handle on the inside and a sprocket on the outside, connect the two sprockets with a chain, and manually control the hot knob, no Arduino, no stepper, no controller, just manual all the way. But that sounds dull...

I don't see any reason not to go with an Arduino solution if you want to. You will probably learn some valuable lessons along the way which I expect to be worth the price of entry. You might need to be careful about just how hot your water can get - a mechanical or code defect has potential to scald.

MadMacks:
Here's the problem with the thermostatic shower mixer solution. There is only a single water feed coming out of the EccoTemo heater, it mixes internally so there is only one pressure output, not two.

I would be tempted to set the heater to a high temperature and treat that has the hot water input to the mixer and provide a separate cold input to the mixer. In effect, that's how my system works except that the heat comes from the cooling system of a diesel engine.

Bur, to repeat, I am not trying to derail your Arduino project if that's your preferred route.

...R

Robin2:
I would be tempted to set the heater to a high temperature and treat that has the hot water input to the mixer and provide a separate cold input to the mixer. In effect, that's how my system works except that the heat comes from the cooling system of a diesel engine.

Bur, to repeat, I am not trying to derail your Arduino project if that's your preferred route.

...R

What is your plan for a few years from now when you can no longer buy Diesel fuel?
Paul

My system uses propane. My place is remote enough that I can't get any services so I haul 100lb propane bottles (I've got a bunch of them) down the mountain when enough are empty. If I can't get propane I have a much bigger issue than hot water...

I wonder if his diesel system can use old cooking oil or kerosene?

Paul_KD7HB:
What is your plan for a few years from now when you can no longer buy Diesel fuel?

I don't have one. I'm 68 now, I suspect I will have had enough of life before the diesel stops.

And I won't be only one in that predicament.

...R