David Hannaford's robot pages.
Motors and actuators
To be interesting our robot is probably going to need to move some part or all of itself in some way. The most common ways of doing this are with:
We also need to talk a bit about how we match the speed of the motor to the required speed of action so we need to know a bit about gears and pulleys and what torque is.
Take apart any radio controlled car or electric aeroplane and you will almost certainly find a few DC motors, gears and servos. They are easy to use and just as easy to reuse in our robots.
Direct Current motors take current from a battery or similar constant voltage supply and produce a rotating movement that is output on a shaft. Internally they have coils of wire and magnets which are attracted to each other to create the driving force.
In a motor which has brushes, the brushes pass the current to a commutator on the rotating shaft which directs the current to different sets of windings so that as the shaft rotates it is continually being pulled in one direction by the attraction between the activated coils and the magnets. The sending of the current to the appropriate coil is fully automatic so all we need to do is provide current. Reversing the voltage will reverse the direction of rotation.
Brushless motors which are becoming more common in high performance models have no brushes or commutators and rely on an external process to direct the current to the set of coils in sequence to generate the rotation. They can usually handle higher currents but need an external chip to drive them so are more complex to use than simple brushed DC motors.
Stepper motors are a particular form of brushless motor which is widely used where precise control of the amount of rotation is required. They have a series of coils, each of which corresponds to one step of rotation and as a series of pulses are applied to the right sequence of coils they rotate at a precisely defined speed. Just as for other brushless motors they need an external chip to drive them.
You can recycle them from old PC printers and fax machines where they are used to feed the paper through in precise steps or to move the printer head across the page.
Stepper motors can be quite heavy as they have larger magnets in them than normal DC motors, although small ones are available.
Servos are widely used in radio controlled models as they are very easy to interface to electronic circuits and provide a limited travel rotational movement which can easily be translated into a linear movement. The ailerons and throttle on your model plane and the rudder on your model boat are likely to be driven by servos.
The beauty of servos is that they are positionally driven by a small control signal which can be easily generated by the radio control receiver or for robots by any microprocessor. All they need is a pulse about 50 times a second where the width of the pulse determines the rotational position that they will move to. Typically a 1 millisecond pulse will move them to one end of their travel, while a 2 millisecond pulse moves them to the other end of their travel. Pulses between these values move the servo to the corresponding fractional part of the travel. The servo also takes a separate supply for the higher current to drive the actuator, which can be connected directly to the battery or power supply. Servo motors usually have just 3 wires coming out of them - red and black for the positive and negative supply and white for the control signal.
Pneumatics and hydraulics
Big fixed robots like those used to build cars use lots of pneumatics and hydraulics as it is an effective way of providing power to their actuators. The actuators can be made powerful and compact with hydraulic or pneumatic pistons and rams. For linear movement they are very effective but the complexity of providing a compressed air or hydraulic oil source and avoiding leaks means that they are not widely used in hobby robots.
If you want to try this route you may find that the hydraulic valves used in washing machines can be recycled, but keep clear of ones that require high or mains voltages for your own and everyone else's safety.
Alternating Current (AC) motors could be used but as the majority of these will operate at mains voltage these should be avoided, not only on safety grounds but also because of the problems of providing an AC supply to a device tha might be moving around.
Shape Memory Alloys or Muscle wire is a wire which can be bent, but which when heated, for instance by passing a current through it, reverts to its pre-bent shape. If the wire has an original shape of a spiral like a spring but has been pulled out straight, then when heated it will exert a uni-directional force as it reverts to its spiral form. It is called muscle wire as it simulates the action of a muscle in that it can only pull in one direction. Where the relatively high heating current and response time of several seconds are appropriate then used in pairs they can provide simple and compact linear actuators.
Coils, relays and linear actuators rely on the principle that a current in a coil of wire will create a magnetic field which can be used to attract a steel bar or magnet. Because this magnetic effect reduces quickly with distance it is mainly useful where only a small movement is needed - often just in or out, with no control of intermediate positions.