Servo recommendations

gpierson

Hi wondering if anyone has recommendations on a metal gear, high torque servo I could use with the Basic Stamp.

I know 4.8 volts is my limitation. But can I / should I use a battery pack with more amperage with a higher torque servo?

My goal is to rig up a device to automatically ring a large bell. I need to lift a hammer with the servo so maybe it will need to lift 5 pounds?

Thanks in advance for any advice!

Hal Albach

One person's very humble opinion...
The voltage source for a servo does not have to be the same as the Basic Stamp, the only wires needed between the two are signal, usually white, and ground, usually black. Servo red wire can be connected to a much higher source, limited by servo specs. However, lifting a 5 pound hammer through gear reduction will be very slow. And how do you plan to release the hammer? If by simply not updating the servo the gear trains involved will probably slow the fall of the hammer. Perhaps a magnetic clutch will allow you to uncouple the hammer from the gear train for a good, hard whack on the bell.
Another possibility would be to have the hammer ride a cam with a steep, slightly negative drop. A small gearmotor would probably be better for this application.

erco

+1 to Hal's advise. Your mechanical setup is far moreimportant than anything else. You don't want to try to lift a 5 lb hammer with a servo. Get a sturdy gearmotor and figure out a mechanism (Hal's cam is pretty good!) that will do what you want, then come back and we'll put a limit switch or homing switch in place.

But if you still want a big strong servo and money is no object, go directly to https://www.servocity.com/html/bottom_mount_servo_power_gearb.html#.Vt8-PX0rIps

Another high-power option would be pneumatics. SERVO magazine had a starter article back in November. First four pages at http://servo.texterity.com/servo/201511/?folio=47&pg=47#pg47

xanadu

Here is an interesting way to lift a lot of weight with a weak motor. It's called a twisted string actuator, our makerspace has been playing with them and it is viable for some applications, certainly not all.



Also is it possible to replace the hammer with a solenoid?

gpierson

Hal, erco and xanadu, thanks for the feedback. And right, I don't plan the servo picking up the entire weight of the hammer. I should have explained more. I'm thinking of something like this below. The servo will rotate (speed is not a problem I don't think. In fact I want it to make a full rotation one to two seconds. But I certainly don't want to make the motor spin as fast as this ring maker. I like the simplicity of this design.)

Xanadu, I like the solenoid idea too. But with the solenoid, I would have less to build! I can actually do this "on the clock" at my workplace, so the more I build it, the more parts I get to order, the more fun for me! lol

Hal, I like your suggestion. So lets say I get a servo that allows for 7.2 volts. I wire the stamp up as normal. Including the white and black wire signal to the servo from the stamp. The red wire goes to positive on my 7.2 volt battery pack. Where does the negative of the battery pack go to at this point? Does the black wire go to ground on the Basic Stamp?

NWCCTV

Personally I have found that it works best when everything is grounded to the stamp.

Hal Albach

Also is it possible to replace the hammer with a solenoid?

With a large bell I would think that the hammer striking the bell needs considerable mass in order to gain the full harmonic vibration of the bell. Hitting it with a solenoid, unless it was very large, would probably result in a "tink" sound, rather than an ear shattering "BOOM". Probably in the science of bells there is a relationship between the mass of the bell and the mass of the hammer.

Where does the negative of the battery pack go to at this point? Does the black wire go to ground on the Basic Stamp?

The battery negative lead is connected to all the grounds, ideally at one common point.

erco

The hammer needs to hit the bell and retract/rebound away immediately, right?

Hal Albach

The hammer needs to hit the bell and retract/rebound away immediately, right?

I would think so, too. If it stayed in contact with the bell it would probably make a nasty rattling noise. I wonder if a thin leather cover on the hammer face would prevent that?

gpierson

Erco/Hal, I was thinking of this too. If I design this correctly, with a spring on the hammer, my goal will be to have the hammer hit the bell, then the spring will prevent it from resting on the bell. It's probably going to take some trial and error to keep it from bouncing and hitting the bell twice. I only want it to strike once per cycle of the servo.

I'm going to order my servos today. I'll post back here with my results. Thanks again.