Stepper motor properties versus battery life

[philip100]

Say, two different stepper motors has moving and holding torque that exactly suits my needs.
The difference between them (despite sizes) are Amps / resistance per winding and operating voltage.

I need 3 of those motors and I'll power them from a 4S battery pack (4x18650, 16.8-11.2V, 3Ah each).

I'll drive them with Pololu DVR8825's, set to 1/8 microstepping. Holding/move ratio will be about 80/20%.

Should I choose motors with lower resistance or amperage per winding, to keep my batteries run longer?

[groundFungus]

Choose the motor that has the torque required.  A motor with lower current draw that won't move your load is pretty worthless.

Steppers draw the same current while stopped as they do when running unless you disable the driver, but then they can lose position.   That makes sreppers less than ideal for battery powered applications.  Would a servo or gear motor with encoder work in place of the stepper?

Stepper motor basics.

[jremington]

Motor torque is proportional to current, so you need a certain amount of current to generate the required torque.

In any case, steppers are usually the last choice, when battery life is important.

[MorganS]

You can control the DRV8825 to reduce the current when you don't need it. Sometimes you can get away with totally de-powering the stepper, using the reset pin.

If the motors have about the same output power then they will have about the same input power. Even though the voltages and currents are different. The DRV8825 acts like a DC-DC converter and is highly efficient. Power input is very close to power output. It will convert the input voltage to the voltage that the stepper needs to make that power.

Given a choice, I would pick the one with the lower nominal voltage. That lets you drive it "harder" to make each step. The difference will only be noticeable at higher speed.

[philip100]

Thanks for replies!

(...)  Would a servo or gear motor with encoder work in place of the stepper?

Yes, and no.
It would be nice to have possibility to move things by hand without powering up the motor, but I could live without that.
The main issue for me is that I can't find any good geared DC motor with encoder in reasonable price (the same for the servos). Looking at the gearboxes used in typical constructions I wonder how much rattle would they get after a while...
I've seen some reviews of typical constructions on youtube and I've noticed that there's even a play on shaft in "locked" position on most of them.
Another issue is that even if I could afford good geared motors with good encoders, they're nowhere near the size and weight of a stepper motor. It would probably be more justified if I had something like 30kg of load, but I need max of 5kg, which I think would be easy to handle with a 0.4A Nema 14 stepper + 4:1 reduction based on 48T / 12T pulleys (it would probably be also more stable in "locked" position)

I have to admit that I have no experience in geared motors or servos at all. The above are just my theoretical considerations and conclusions based on what I found on the internet :-) so I might be wrong.

Motor torque is proportional to current, so you need a certain amount of current to generate the required torque.

In any case, steppers are usually the last choice, when battery life is important.

I assume that it's not regardless of size? I've noticed that Nema 17 can be nearly twice as strong as nema 14 with the same voltage, amperage and resistance.

Going back to steppers. I think about one of those:

14HS10-0404S
1.8°
14Ncm(20oz.in)
0.4A (per phase at 12 V)
30ohms
12-24V

14HM11-0404S
0.9°
11Ncm(15.6oz.in)
0.4A (per phase at 10 V)
25ohms
12-24V

17HS08-1004S
1.8 °
13Ncm ( 18,4 oz.in)
1.0 A
3,5 ohms
12-24V

[jremington]

I assume that it's not regardless of size?

Torque depends on permanent magnet strength as well as winding current, so it is likely that a larger motor can deliver more torque for the same current.

I have to admit that I have no experience in geared motors or servos at all.

You should get some experience before making the decision. Gearmotors are much better for battery operation.

Pololu has a good selection of gearmotors, but since you have not told us what your torque and power requirements are, it is impossible to offer specific recommendations.

[philip100]

I have tested a 1704HS168A stepper (Nema 17) and it was more than enough for my needs in every scenario.
So the motor should handle max 5kg.cm

Just a rough sketch (3x Nema 14, 34x34x28, 1,2kg.cm + 48/12T pulley reduction)

[jremington]

With three of those 1704HS168A motors, two 1.7A windings each, a rough estimate suggests that you should expect a 3Ah battery pack to last for less than one hour of continuous operation, depending on your actual setting for the winding current.

[MarkT]

Say, two different stepper motors has moving and holding torque that exactly suits my needs.
The difference between them (despite sizes) are Amps / resistance per winding and operating voltage.

I need 3 of those motors and I'll power them from a 4S battery pack (4x18650, 16.8-11.2V, 3Ah each).

I'll drive them with Pololu DVR8825's, set to 1/8 microstepping. Holding/move ratio will be about 80/20%.

Should I choose motors with lower resistance or amperage per winding, to keep my batteries run longer?

The power drain of a stepper is I-squared-R where I is the set current and R is the winding resistance
of a single winding.

You might see a small change in power consumption when the motor moves, but to a first approximation
the power drain will be constant.

In practice the lower the power a motor uses the less powerful it will be.

Anyway if you want any sort of decent battery life, stepper motors are not the answer, they are the worst
possible choice of motor for battery operation.

[philip100]

I'd really love to know weather I could DIY something that could provide me fully freezed "stop" position, and fully repeatable movement, while using less power than just a simple stepper motor.

As I mention - I want it to run on batteries - which means that the battery life is not the only important factor here. It also has to be portable, must be easy to handle, light and as compact as possible.

Another important factor I didn't realize before I've tried a basic geared DC motor - friction and balance. While stepper motor will easily handle any spikes of torque, the DC motor will not. I'd need to provide perfectly linear hardware balance to get a stepper-kind of linear output (what I'm not able to).

I was spending my time trying to design something based on DC motors, encoders and selenoid lockers, but I trashed the design because too much things (and money) was getting involved, and the project grew up to a monster size. As for now, I'd rather carry few extra battery packs with me (I can at least use them as powerbanks to charge my phone or camera ).

I'll look into DC motors / servos for sure, as I can see the potential, but experimenting with them at my stage of understanding this topic, is a way too pricy  

---------------------------

On topic now:  

I changed the idea.

I'll make 2 series of parallel 26650's, 5500 mAh. (It will give me ~7.4V, 11Ah)
I'll use 3x 14HS13-0804S (0.8A, 6.8ohms, 5.4V, 1.8Kg-cm) + 3:1 reduction ratio 16:48T pulley (5Kg-cm, I take off 0.4KG as friction caused by reduction)
Maximum load 5KG (I don't expect more than 3.5Kg, but I like to take margins)
Based on my calculations, single batterypack will give me about 3h work

Am I correct with the above?

[jremington]

While stepper motor will easily handle any spikes of torque, the DC motor will not

For someone with no experience using DC motors, you certainly have some strange ideas.

[MorganS]

If you can carry the 3h battery pack and 3h is enough time for you then there is no problem.

Any gearbox using a worm gear is considered "irreversible" meaning that the output cannot be moved unless the motor is powered. If the steppers don't work out, look for gearboxes with worm gears.

[Paul_KD7HB]

If you can carry the 3h battery pack and 3h is enough time for you then there is no problem.

Any gearbox using a worm gear is considered "irreversible" meaning that the output cannot be moved unless the motor is powered. If the steppers don't work out, look for gearboxes with worm gears.

And if you design in a spring loaded split gear to ride on the worm, there will be ZERO backlash in the setup.

Paul

[MarkT]

I'd really love to know weather I could DIY something that could provide me fully freezed "stop" position, and fully repeatable movement, while using less power than just a simple stepper motor.

A servomotor is what you are looking for.  Only uses the power needed at any point in time.

[philip100]

Thanks again for replies 

Kudos @Paul_KD7HB, I had to google "spring loaded split gear" and it's something I have to look into, thanks! 

A servomotor is what you are looking for.  Only uses the power needed at any point in time.

I don't trust gears (at least not the gears that fits my budget)   

(...)
Any gearbox using a worm gear is considered "irreversible" meaning that the output cannot be moved unless the motor is powered. If the steppers don't work out, look for gearboxes with worm gears.

Theoretically, yes. But still gears 

I'll finish my project using steppers, because I want to try my theory 

https://www.youtube.com/watch?v=Y2WZQaG6Kp8 - this thing uses steppers. I can see a lot improvements there while still using steppers.