Stepper motors Stepper motors Stepper motors
A typical stepper motor has four sets of coils, arranged so that the rotor is turned from one position to the next as the coils are energised in a fixed sequence. This is listed in the table overleaf.
The sequence repeats and at any step, two coils are on and two are off. The sequence of pulses needed to drive the motor can be provided by a microcontroller.
Finally, there is the matter of coupling the output shaft to the driven mechanism. The problem that this often raises was mentioned on p. 40. If possible, pick a motor with a shaft that matches. A 4 mm shaft is usually easier to couple. Special couplers are made for joining shafts of 4 mm and larger, but these are relatively expensive and bulky. They add to the robot's weight, which is to be avoided if possible.
A pair of very inexpensive motors running on 1.5 V to 4.5 V. They produce 8 gf cm and 18 gf cm respectively. These motors have no
gearbox.
A 1.5 V to 3 V motor with an attached metal gearbox. Low-cost but
comparatively noisy.
A 12 V motor with built-in gearbox. Very quiet. Output shaft is 4 mm, with a flat for
more secure coupling.
Step
no. Coil 1 Coil 2 Coil 3 Coil 4
0 On Off On Off
1 Off On On Off
2 Off On Off On
3 On Off Off On
To get from step to step, first coils 1 and 2 change state, then 3 and 4 change state, then 1 and 2, and so on. The result of this is to produce clockwise turning of the rotor, 15° at a time. If the sequence is run in reverse, the rotor turns anticlockwise. Six times through the sequence causes the rotor to turn one complete revolution.
The rate at which it runs through the switching sequence depends on the timing of the program. The motor turns one revolution for every 24 pulses. Varying the pulse rate varies the speed of the motor. At any step, the rotor can be held in a fixed position by halting the sequence. If a stepper motor is used for driving something like an arm of a robot, the arm can be positioned exactly by programming the controller to produce the required number of pulses. There is no need for limit switches; the robot always knows where its limbs are. It can move precisely from one position to another simply by working out how many pulses to generate. Once in the required position, the rotor is, in effect, locked there and can not move. The torque required to overcome the magnetic field holding the rotor in position is several hundred gram-force centimetres.
Another advantage of the stepper motor is that its speed is precisely controllable. It is not affected by the load on the motor, except perhaps an excessive load, which might
completely prevent the motor from turning. A stepper motor is less likely to stall, overheat, and possibly burn out its coils, than ordinary motor.
The stepper motor may be made to turn 7.5° per step by using a slightly different
switching pattern. Motors with a 1.8° step angle are also produced. When programming a stepper motor do not feed the pulses to it too fast. At excessive speeds there is the
possibility of dropped steps, which leads to an error in the positioning.
The sequence of pulses for running a stepper motor.
other necessary tasks. Also the motors require large current and at least 12 V operating voltage.
Servomotors Servomotors Servomotors Servomotors
A servomotor is designed to move to a given angular position. The motor has three connections to the control circuit. Two of these are the positive and 0V supply lines. The third connection carries the control signal from the control circuit, which may be a microprocessor.
The rotor of the motor has limited ability to turn. Generally it can turn 60-90° on either side of its central position.
The control signal is a series of pulses transmitted at intervals of about 18 ms, or 50 pulses per second.
The angle of turn is controlled by the pulse length:
• 1 ms: turn as far as possible to the left.
A small servomotor of the kind used in flying model aircraft and robots. The 'horns' (white levers) are used for connecting the motor to the mechanisms that it drives.
Solenoids
A solenoid has a many-turned coil of wire, longer than it is wide, and a soft iron core.
When a current is passed through the coil, magnetic forces pull the core into the coil.
Nothing happens when the current is turned off unless there is a spring mechanism (or gravity) to move the core out of the coil again.
Solenoids are used to provide linear motion, to pull something in a straight line. If the core is well inside the solenoid to start with, the force on it can be quite large. A 12 V solenoid can develop a pull of one or two kilograms force. The problem is that the stroke, the distance that the core moves, is only a few millimetres. This limits the usefulness of solenoids in robots.
• 1.5 ms: turn to central position.
• 2 ms: turn as far as possible to the right.
Intermediate pulse lengths give intermediate positions.
Servomotors are often used in robots and in model aeroplanes and vehicles. Model shops, especially those specialising in flying model aeroplane kits, usually stock a wide range of servomotors.
A miniature solenoid suitable for robotic applications.
operate electrically controlled valves.