next topic »

[on:]

Dear all,
I hope that the subject outlined my problem well enough.

I want to non-destructively add some automatic control to a water with a single handle. It can be moved up/down for the flow, left/right for the temperature, and I'd like to control just the temperature. I also want to be still able to move the handle manually.

My idea is to use an off-the-shelf RC servo with enough torque to move the handle sideways. I'd attach an arm of about the same length as the handle, then use a bar between the ends of the handle and the arm.

I measured the force needed to move the handle while the water is flowing with a small dynamometer, it's around 2,5kg. I thought the arm should be at least 5-6cm long to have a good control range, therefore the servo must provide a torque not less than 13-15kg*cm, 20 to be sure. Is that correct? Should I look for a servo with such torque in the specification?
Somewhere in the datasheet I've read "stall" torque; how that is different to "operational" torque (that's a definition of my own) ?

Best regards,
Enrico

[Reply #1 on:]

Stall torque is the amount of force it requires to keep the servo from moving when power is applied.  At stall, your servo draws the greatest amount of power, produces the greatest amount of heat, and if it's going to catch fire, that's when it'll happen.  If you're regularly meeting your stall torque (you won't be able to exceed it) then you need a stronger servo. 



Stray

[Reply #2 on:]

You might need to build your own "servo". running a motor through a gearbox slows down the motion and increases the torque. You could hook up a potentiometer and feed that back to your Arduino (just like a servo, just closed loop).

If you are monitoring the water temperature you could use a temperature sensor as your feedback and rely on hte slow motion of your motor/gearbox to keep you from overshooting too much, or use some PID code to control the temperature.

A bracket with a vertical slot that is driven side to side and you could control the volume. Perhaps a switch near the off position to disable the Arduino so it doesn't keep driving the valve from side to side when no water is flowing.

[Reply #3 on:]

The below servo has a lot of torque at a reasonable price.

http://www.hobbyking.com/hobbycity/store/__18861__Hobbyking_S5040D_X_Large_Digital_Titanium_Gear_Servo_HV_177g_0_18s_39_8kg.html

[Reply #4 on:]

If you're using a thermistor or similar for feedback, this would be even cheaper...

http://www.pololu.com/catalog/product/73

$10.95, and it's geared down 336:1 so you aren't likely to try to burn out your Arduino by exceeding the 40mA rating.

[Reply #5 on:]

Thanks for the kind advises guys.

Yes, I'll plan to add a temperature sensor (a DS18S20) and a PID that tries to keep the water temperature constant.

I think I will go for another mechanical, more compact solution: the motor placed behind the tap with a small gear against a bigger gear, attached to the base of the handle. The small gear is shaped like a cone with spherical sides, in order to cope with the horizontal rotation of the handle. With this setup I need less torque but a faster motor, anyway I need to do some other measures.

Another question: do you think that sensing the rotation with an encoder is enough to check for reaching the rotation limit? If the Arduino drives the motor but sees no movement, then the limit for that direction is reached. I may use this effect to self-calibrate the Arduino at startup...

Thanks and best regards,
Enrico

[Reply #6 on:]

Another question: do you think that sensing the rotation with an encoder is enough to check for reaching the rotation limit?

Might I suggest using limit switches? (maybe simpler/cheaper)