motor for a gripper

Knowing Position - I'm not sure I understand the issue with the servo.  If you set its position, it goes to it.  Why would the ESP32 not know the position?  With a stepper, when overloaded will skip steps and the ESP32 will certainly not "know" where it is.  3D Printers have this problem all the time.

Stops - A servo will only go to its stops.  A Stepper must have stop switches or have feedback via the driver to tell you when it has hit a hard stop.

...or did you have something else in mind for a driving force?  

I'm real curious... how did you plan to make the gripper water proof.  I believe you can purchase waterproof servos... I've never heard of waterproof stepper motors.

Posted by: @erhy

Hello!

Our underwater roboter will have a gripper ( 4 x 4 x 4 inch ).

The operating should be with two screen buttons (open/close) .

So long a button is tapped the action should be done.

By "tapped", do you mean that you want the operation (opening, closing) to be continuous as long as somebody holds the button down ? Or do you mean that when somebody presses the button briefly the gripper opens (closes) by itself ?

If none button is tapped the gripper should persists on his state.

So now there's three buttons ? Open/close/hold position ?

We want to handle the gripper by inspection its state visually.

I'm not sure what you mean by this ?

We think a servo is not practical, because the control (ESP32) don't know the position of the gripper.

Servos operate by your uController telling it where to go and it goes there. If you mean that when the gripper closes on and locks onto an object, then yes, the uController may not be able to determine the angle of the servo. But that (sensing load) is outside of the ability of the servo. You would need some kind of sensor or feedback to determine that the serv could not close (or open) and farther.

Is there some specific reason why you think a servo is impractical ?

On the positions full closed and full open, the activations should stop.

Easily done by measuring the angles where the gripper is fully closed and fully open and using them as the limits in the commands from your uController.

Please give us practical advices.

Please give us more details.

Posted by: @will

Posted by: @erhy

Hello!

Our underwater roboter will have a gripper ( 4 x 4 x 4 inch ).

The operating should be with two screen buttons (open/close) .

So long a button is tapped the action should be done.

By "tapped", do you mean that you want the operation (opening, closing) to be continuous as long as somebody holds the button down ? YES

If none button is tapped the gripper should persists on his state.

So now there's three buttons ? Open/close/hold position ? NO, I mean if none of the 2 buttons touched

We want to handle the gripper by inspection its state visually.

I'm not sure what you mean by this ? I mean, that we look to the gripper and see what is necessary

We think a servo is not practical, because the control (ESP32) don't know the position of the gripper.

Servos operate by your uController telling it where to go and it goes there. If you mean that when the gripper closes on and locks onto an object, then yes, the uController may not be able to determine the angle of the servo. But that (sensing load) is outside of the ability of the servo. You would need some kind of sensor or feedback to determine that the serv could not close (or open) and farther.

Is there some specific reason why you think a servo is impractical ? we want to steer more to open ore more to close independent of the position.

On the positions full closed and full open, the activations should stop.

Easily done by measuring the angles where the gripper is fully closed and fully open and using them as the limits in the commands from your uController. We want do it by interrupting the power line with a contact.

Please give us practical advices.

Thank for your fast answer.
See my comments in quoted text.

Now I see, that there are worm drive motors for our puprpose.
e.g.
JGB370 Worm Gear Motor - in AliExpress JGY-370 Wurm DC Motor Turbo Stick

Erhy

@erhy 

OK, if you can see the gripper, then all you need to do is use the software to increment the claw position servo by, say 1 degree, while the close button is pushed and decrement the angle of the claw servo while the open key is pressed.

For a servo you don't need or want limit switches; they'd be a waste of money, waterproofing, power and would make your physical design more complex to implement them.

I could not tell from your profile and some of your writing indicates English may not be your first language.  I think we are missing something in translation. 

Posted by: @erhy

We think a servo is not practical, because the control (ESP32) don't know the position of the gripper.

On the positions full closed and full open, the activations should stop.

Both @will and I are suggesting servos as being able to handle all the problems of knowing where the position is and to having full open and close stops without any extra circuitry or extra programming.  Maybe we do not understand your concern... Why do you think a servo is not practical?

Posted by: @erhy

JGB370 Worm Gear Motor - in AliExpress JGY-370 Wurm DC Motor Turbo Stick

The motor you suggest has no control whatsoever.  It is on or off and depending on the gearing you select and leverage of your gripper, it could cause serious harm to a person.  If you plan on using vision as your sole feedback, you could control it.

You might want to research controlling it with something that can measure "back EMF" so you can tell if it is loading up by hitting a stop or crushing an arm.

I'm sure you have a lot of people interested on the forum.  I know I am curious.

VBR,

Inq

Posted by: @inq FYI @erhy

 

Both @will and I are suggesting servos as being able to handle all the problems of knowing where the position is and to having full open and close stops without any extra circuitry or extra programming.  Maybe we do not understand your concern... Why do you think a servo is not practical?

I asked that question and the answer was (in bold)

Is there some specific reason why you think a servo is impractical ? we want to steer more to open ore more to close independent of the position.

Posted by: @will

Posted by: @inq FYI @erhy

 

.. Why do you think a servo is not practical

If the system ist turned on, the gripper hast an arbitrary position.

Measurement of the position is unreliable compared with a positional switch.

@erhy 

Servos have a potentiometer built into them. It's what allows them to move back and forth to a fairly precise position.

You can tell the servo at startup to return to full open position and it will do so, without having to use a limit switch or other, similar mechanism.

Presumably, when the power is turned off, you're finished with the gripper and its position no longer matters. If it's power-down position is important, then just move it to that position before you power down.

What do you mean by a "positional switch", I'm not familiar with that term ? 

@will - Yep... Casual reading of quotes within quotes and I missed the answer.  And seeing it now, didn't improve my understanding.

Using a plain DC motor, especially with a worm gear on it would be the last thing I would try.  It's certainly not for picking up eggs or doing surgery.  I'm hoping for a new concept I don't know about.  I'll just sit here and (try to) be quiet until clarity hits me.  

@will - OK... failing at being quiet.

@erhy doesn't sound like a beginner.  I can't imagine someone developing a sophisticated robot with grippers and not understanding the capabilities differences of a stepper motor versus a servo.  He appears to be describing the behavior of a stepper motor... being in some arbitrary position on startup - which they do.  I'm wondering if this is a language translation error swapping them. 

Maybe a picture or two will help.

Waterproof Servo

https://www.amazon.com/Youleke-Torque-Digital-Servo%EF%BC%8CWaterproof-Horn%EF%BC%88270%C2%B0%EF%BC%89/dp/B08739MGPL/

Stepper Motor

https://www.amazon.com/STEPPERONLINE-Stepper-Bipolar-Connector-compatible/dp/B00PNEQKC0/