Traditionally the actuators which move robots are electrical motors, but these aren’t the only way to move robots, an alternative are the electronic muscle, which are this post’s subject.
How they work?
Electronic muscle also are called SMA (Shape Memory Alloy). Are made a type of material which extend or contract with electrical signals. The electrical current heat the wire and it contracts allowing the motion of mechanical parts, this robot is an example of motion with SMA.
The most used material to SMA is the nitinol, a nickel-titanium alloy which contraction can reach 8% of total length, which is much to a SMA. The figure below shows nitinol wires.
The high elasticity of electronic muscles is due the crystal structures which change with temperature. The crystal’s structure in a lower temperature is called Martensite, is where the material can assume any shape. Increasing the temperature, the crystal enter in the Austenite phase, is a transition phase to another structure.
The structure’s behavior follow a hysteresis, a behavior in a physical system which depends of external factors and it’s history.
How to use it?
An excessive electrical current can melt the wire, the circuit to turn on the SMA must provide a constant current. Here we have a constant current source circuit, the current’s value depends of the regulator, input voltage and the R of the resistance. I put the inductor symbol to represent SMA because there wasn’t a proper symbol.
To control with more precision the electronic muscle, you can use PWM circuits. I will talk about this technique in other post.
Other materials and applications
Besides the nitinol there are others SMA alloys like:
Besides move robots, these materials also are used to:
- Medicine to manufacture tooth clips, plates to bending broken bones and reinforce clogged arteries and veins.
- Open solar panels of satellites and space telescopes when it arrive in space.
- Engines, wings and actuators of airplanes. Here is an airplane’s engine with SMA.