Basic Stamp and Servo for Clock motor

armysaxman

I've got a woodworking project to build a clock. I have been looking for the right motor setup and came across the Parallax site. The plans for the clock include schematics for power supply and motor controller in order to use a stepper motor, which I would be OK with doing. But once I saw the Parallax site, I'm thinking that maybe I could use the continuous rotation servo motor and a Basic Stamp module. Would be about the same in $, and is already mostly assembled and tested. I wouldn't need anything like the Propeller since I wouldn't need to touch it once I got the correct speed set. The motor will be turning two large wooden gears at 1 RPM "forever".

Since I am brand new to this whole thing, I'm just looking for feedback from the experienced folks as to whether or not this would work.

Thanks ahead to those who respond!

Duane Degn

Continuous rotation servos are much easier to use than stepper motors.

While any microcontroller could easily control a CR servo, I'd think it would be much easier to adjust the timing with a Propeller than with a stamp.

One problem you may have is how precisely you can control a CR servo. I'm not sure if the speed would be consistent enough over a long time period. A stepper has a definite advantage over a CR servo when it comes to speed control.

If you do use a CR servo, a Propeller would be able to control its speed with twice the precision of a BS2. A QuickStart (with a Propeller) costs much less than any of the Basic Stamp options.

Phil Pilgrim (PhiPi)

Using a continuous-rotation servo for this app would be an exercise in frustration. Without some sort of velocity feedback, there would be no way to control it accurately enough to drive a clock. Unless you require a lot of torque, commercially-available crystal-controlled clock movements are plentiful and cheap:

http://www.klockit.com/products/dept-157__sku-aaaag.html

In these, the second hand shaft rotates at precisely one RPM. You can get both incremental and continuous-rotation versions of these kinds of movements.

-Phil

armysaxman

Thanks for the replies!

Phil - yes, it will require much higher torque as it is to move two large wooden gears, not just clock hands. I've attached a picture of the project so you can see what the goal is. Now that I look closer, the motor speed will be much higher than 1 rpm since it will drive the small gear at the bottom which will turn the two large wheels at 1 rpm.

So, can I use a Quick Start to control a stepper motor? The plans I have use a 48 step bi-directional motor.

Phil Pilgrim (PhiPi)

So, can I use a Quick Start to control a stepper motor?

Yes, but you will need to construct a driver board to plug into it: either a unipolar driver for a six-wire stepper, or an H-bridge driver for a four-wire stepper.

-Phil

erco

I've thought about this; if it were me, I'd use standard servos. This guy made a thermometer using a servo:
http://www.youtube.com/watch?v=cwNTKGG3BmA

Most standard servos are good for 180 degree rotation. For a clock, you'd have to make a coaxial 2:1 stepup geartrain to drive the two clock hands 360 degrees. Then all you have to do is drive the servos with standard pulsouts to show the time. Of course, once per hour the minute hand would rewind to zero, and likewise for the hour hand every 12 hours.

Duane Degn

erco wrote: »

I've thought about this; if it were me, I'd use standard servos. This guy made a thermometer using a servo:
http://www.youtube.com/watch?v=cwNTKGG3BmA

Most standard servos are good for 180 degree rotation. For a clock, you'd have to make a coaxial 2:1 stepup geartrain to drive the two clock hands 360 degrees. Then all you have to do is drive the servos with standard pulsouts to show the time. Of course, once per hour the minute hand would rewind to zero, and likewise for the hour hand every 12 hours.

erco, you obviously didn't look at the pdf. This is one of those mable clocks with a row for hours, and minutes.

Instead of a motor rotating at 1 RPM (or faster) a standard servos could be used to move the next minute ball into place.

A CR servo could be used with some sort of feedback, like a magnet and reed switch/Hall effect sensor or even a momentary switch to detect when the minute mark has been reached.

erco

Even better! I'd still use a standard servo to cycle a lifting arm to raise a marble from the bottom to the top. Long story short, once you mod a servo for CR, you just making more work for yourself if you then need to add location feedback. If you can use a standard servo, BY GUM, use it as-is!