24V dc motor project guidline

Hi,
I have a system that I would like it to be as light weighted as possible and match the following criterias:

  1. Use this motor: Pololu - 150:1 Metal Gearmotor 37Dx73L mm 24V with 64 CPR Encoder (Helical Pinion)
  2. The motor has to work 20 minutes a day in total(takes 3A on 24V).
  3. This is where the problems start: I want to use a 12V battery lithium battery pack(let's say 3400mAh that weights ~160g) instead of 24V, for this I have to use a boost converter:
  4. For example this one: Pololu 24V Step-Up Voltage Regulator U3V50F24
  5. The driver and uController are easier to choose because there are plenty options, let's say arduino of some kind and this driver: Pololu G2 High-Power Motor Driver 24v21
    Are all these things realistic and can answer my demands? especially battery capacity which I am a little bit skeptic about.
    Thanks!

The boost converter in Point 4 has a max input current of 5 amps. You say your 24v motor requires 3 amps which means the output from a 12v battery would be over 6 amps when you allow for losses in the boost converter. Also it is very likely that your 24v motor will require a lot more current at startup. The stall current of a motor is often several time higher than the running current.

The correct solution is to find a 12v motor that meets your need. Or use a 24v battery with the 24v motor.

...R

Hi, thanks for the reply. I know this is not the "right" way to do that, but the weight here is very crucial so i'm trying to see what I can with the given opening data.
The stall current according to polulu website is 3A at 24V(No-Load current is 0.1A), is this the maximum current that will be discharged?
If so, will this boost converter fit: Pololu - 4.5-20V Fine-Adjust Step-Up Voltage Regulator U3V70A ?
In addition if I understand correctly I should use 12V battery(again lets say 3400), and for it to have 2C discharge rating.
Will this approximately work for 20 min or a lot less?
Thanks.

When you said

  1. The motor has to work 20 minutes a day in total(takes 3A on 24V).

I assumed you were quoting the running current as that is what you would use to calculate the energy consumption. As 3A is the stall current the motor should work with the boost converter in your Original Post.

but the weight here is very crucial

The Pololu website shows a 12v version of the same motor - why not use that? Avoid using a boost converter if you can as they all waste a certain amount of energy.

If the motor runs for 20 minutes and draws a running current of (say) 500mA then it would require 500 * 20 / 60 = 166mAh so your 3400mAh battery would have plenty of capacity.

Note that the C rating of a LiPo battery is the maximum amount of current it can safely produce. For example a 2C battery with a capacity of 3400mAh can produce a current of 2 x 3400 = 6800 mA or 6.8 Amps

...R

Some thoughts about capacity:
3 amps at 24vdc = 6 amps at 12vdc
6 amps = 6000mAh
The battery suggested is 3,400mAh 2C which should deliver 6,800mAh for 30 minutes.

The Boost Converter will be an issue however, it looks like the maximum output (with a 12vdc input) will be approximately 2,200mAh which will not run the motor at full load.
Additionally the total input current will be approximately 7,200mAh including the 20% loss listed on the website, this does not include any other loads connected to the battery (controller, sensors, actuators etc), the maximum input current is listed as 5 amps.

Hi,
Can you tell us the application of you project?

How often will you be recharging the batteries?

Thanks.. Tom.. :slight_smile:

Robin2:
The Pololu website shows a 12v version of the same motor - why not use that? Avoid using a boost converter if you can as they all waste a certain amount of energy.

If the motor runs for 20 minutes and draws a running current of (say) 500mA then it would require 500 * 20 / 60 = 166mAh so your 3400mAh battery would have plenty of capacity.

Assuming I will use the stall current(which is the current need to be run to use the max torque - correct?),
My calculation in similar to what you did - 6000mA * 20 / 60 = 2000mAh meaning I have some battery left still for other components.

As to why not the other 12V motors - I need the highest torque I can get - which this motor has.

Kiwi_Bloke:
The Boost Converter will be an issue however, it looks like the maximum output (with a 12vdc input) will be approximately 2,200mAh which will not run the motor at full load.
Additionally the total input current will be approximately 7,200mAh including the 20% loss listed on the website, this does not include any other loads connected to the battery (controller, sensors, actuators etc), the maximum input current is listed as 5 amps.

Are you referring to the first converter I put a link to? because I corrected it to this one:
Pololu - 4.5-20V Fine-Adjust Step-Up Voltage Regulator U3V70A which has 8A max output, is this ok?
I did not understand all the way the number you put out 2200mAh for the motor output and 7200mAh for the input, could you please elaborate?

TomGeorge:
Hi,
Can you tell us the application of you project?

How often will you be recharging the batteries?

A little difficult to explain but i'll try :slight_smile:
This is a prototype for someone with a spastic arm, I want to put a motor(with other mechanism attached) that will help the person open his arm in contrast to the muscles "will".
For that, it will be used only a few times a day, and will be charged each night.

Thanks!

newbino:
Assuming I will use the stall current(which is the current need to be run to use the max torque - correct?),
My calculation in similar to what you did - 6000mA * 20 / 60 = 2000mAh meaning I have some battery left still for other components.

As to why not the other 12V motors - I need the highest torque I can get - which this motor has.

This all worries me a lot.

I suspect you are choosing the motor based on the stall torque quoted on the Pololu page. That is NOT the proper way to select a motor. The stall torque applies (as it's name suggests) when the motor is held stationary (stalled) by a heavy load. Most motors will overheat and get destroyed very quickly if they are stalled for any length of time. You need to choose a motor that can do the job without getting anywhere close to its stall limit.

Put the idea "I need the highest torque I can get" to one side for the moment and figure out, and tell us, the maximum operating torque that your project requires. You will only be able to select a motor when you know that.

...R

newbino:
Are you referring to the first converter I put a link to? because I corrected it to this one:
Pololu - 4.5-20V Fine-Adjust Step-Up Voltage Regulator U3V70A which has 8A max output, is this ok?
I did not understand all the way the number you put out 2200mAh for the motor output and 7200mAh for the input, could you please elaborate?

The first converter data included a chart showing output in mA vs input voltage, this showed the 24vdc converter with a 12vdc would only supply approximately 2,200mAh.
The new converter you are proposing appears to only go up to 20vdc?

The 7,200mAh is based on 6,000mAh Full Load Current @12vdc with 20% losses through the converter (20% of 6000 is 1,200) figures are from the data sheet referenced in the original post.
Yes this is based on FLC and you may not ever reach this load BUT it should be considered.

Have you also considered how hot the converter will get if high current loading? the data sheet mentions it gets hot..

I need the highest torque I can get - which this motor has.(*)

If your gizmo is supposed to work for more than a few short trials, pick a motor that can provide at least twice the torque that you think is needed. And all the other bits have to be chosen to handle that torque.

That is a general design rule.

(*) The Pololu spec is 56 kg-cm "stall extrapolation", which means the Pololu engineers did not want to destroy the motor attempting to actually measure the stall torque.

Ok, so I'll start by picking a better motor for this mission.
If I need a torque of 4.7 Nm(or 48 kgcm) what is the recommended motor for that?
According to what you say is at least with 100kgcm - I couldn't find any at polulu's website, any good lead? Preferably the most light weighted there is out there
Thanks!

How did you arrive at the "torque required" figure? Are you certain it is correct?

Motor speed (or power) are also critical design parameters.

jremington:
How did you arrive at the “torque required” figure? Are you certain it is correct?

Motor speed (or power) are also critical design parameters.

It was not taken by me, so I need to check exactly how, but according to the data I have here the torque of the motor needs to be 4.7Nm and RPM 59.
Thanks

Hi,
59RPM = approx 1 rev per second.

What angle range of the shaft rotation is needed?
What does it do in the 20minutes each day?

This sounds like it needs a power servo system.

Has the mechanics of the device been constructed?

Will you be using the encoder feedback in that motor/gearbox?

I would be going for a motor with twice the needed torque, and if it is short repeated operations, check how hot it will get.

Tom..... :slight_smile:

It was not taken by me

It would be a really, really good idea for you to personally understand how that estimate was made and verify the numbers before building or buying anything.

And of course, constructing the framework with motor mount and shaft coupling will be one of the most difficult parts of the project. Best of all, once the framework is built, you can actually MEASURE the torque required to actuate it. Wouldn't it be fun if the estimate was even close?

Some additional mechanical and functional data:
The required torque measured as follow. We applied force of 7kg to the hand in order to straighten the arm. The distance from the applied force to the elbow is 30 cm. Which give us torque of 210 kgcm. We are adding a worm gear with transmission of 1:65 which requires a torque of about 46kgcm at the inpute (e.g. the motor output).
Range of movement is about 120deg. Time to perform this movement is about 10sec.
Thanks

Since when did 210 / 65 give 46?

Even allowing for the high friction in a worm drive you could calculate 210 / 30 giving 7 kg.cm

Of course it would not be the first time I got my maths wrong.

Note also that if you use a motor with a reduction gear followed by a worm drive the output will be very slow.

...R

Robin2, you are correct.
It is my mistake. The required output torque is given - 210kgcm
Based on the geometry (including the teeth shape) of the worm and the wheel the tangential force on the worm is calculated. From there, based on the worm radius, the required input torque is calculated as 46kg
cm. Since I did not calculate it myself, I'll recheck it again.
Thank you very much

I agree with Robin2 that torque calculation is wrong, but the wording of reply #15 is totally unclear.

Start with power = torque*rpm

From reply #15, applying 210 kg cm for a rotation of 120 degrees in 10 seconds (2 rpm) = 420 kg cm rpm of power in non-standard units. This assumes that the "7 kg force" was applied perpendicular to the 30 cm radius.

Ignoring friction, the motor must provide that power.

With a 65:1 worm gear reduction, the motor will turn at 130 rpm (additional gearing required for that) and must provide torque = 420/130 = 3.2 kg cm. The 24V motor you selected is geared 150:1 so that the shaft turns at 68 rpm, and is too slow.

Now: why was a "7 kg force" required to straighten the hand? Against what force was the applied force working?

A drawing of the setup would help us understand. In any case, DO NOT select components before you have completed and verified the design requirements.

Hi Jremington, Thank you.

You are correct with your calculations. I think that I’ll be able to live also with 20sec per operation. This way, the performance of the motor are within reasonable range.
I hope next week to print an adapter and start to run the system. We will get good idea about the performance and feedback from the user who is the most important in all this project.

jremington:
Now: why was a “7 kg force” required to straighten the hand? Against what force was the applied force working?

The 7 kg force is applied by spastic muscles that want to stay in shrink position which causes the arm to fold toward the body.