For my final project I decided to work on a robot arm joint. More precisely, I want to design and make a very compact robot joint. This joint should be connected to watever prototyping board you use and act like a servomotor. A problem I usually have while prototyping robot is the load the motor can handle. The other common problem is the price of high end actuator. I want to create a precise actuator wich can perform a full rotation whith a bit of torque.
Strain wave gearing (Harmonic drive)¶
Strain wave gearing (Also known as harmonic gearing and harmonic drive) is a type of mechanical system that has no backlash, high compactness and lightweight. They also have other advantages such as :
- High torque
- Coaxial input and output shaft
- Gest reduction/ volume ratio of known reducer
Strain gear offer ratio from 30:1 up to 320:1 in the same space in which planetary gears typically only produce a 10:1 ratio
The basic concept of strain wave gearing (SWG) was introduced by C.W. Musser in his 1957 patent.
Here is the main idea. I plan to use a Nema 17 stepper motor as my main actuator. Nema 17 are very common nowadays and cheap thanks to 3D printer manufacturer.
Harmonic gearing are composed of 3 parts :
- The flex spline ( flexible gear )
- The circular spline ( static gear)
- A Wave generator ( eliptic bearing )
The reduction ration can be calculated this way :
I also want to explore an other kind of reducer named cycloidal gearing. This type of gearing seems easier to make but also weaker… The gearing ratio is lower since it has less teeth. Maybe the solution will be just between cycloidal gearing and harmonic drive.
Usually, kid learning robotics may use easy to use actuators. Modelism servomotor are now mainstream and cheap. There are also very simple to use. Want I like with servomotors is how simple it is to control them. You can basically tell them where to go (angular position) and they will to there best to reach the requested position.
The servomotor control il pretty much the same for all prototyping servos. They are using pulse width modulation to be controlled using only 1 command wire.
This may be a cool feature of my final project.
2D and 3D Modeling¶
I already designed a prototype but I need to make a smaller version … I searching way to make it more reliable and durable…
|1||Plastic spool||20.99 €||Link|
|1||Nema 23 stepper motor||23.76 €||Link|