A displacement transducer measures a distance or angle traversed, or the distance or angle separating two points. Conversely, it can convert a signal into movement over a certain distance or angle. A device that measures or produces movement in a straight line is a linear displacement transducer. If it measures or produces movement through an angle, it is an angular displacement transducer.
Pointing and Control Devices
A joystick is a control device capable of producing movement, or controlling variable quantities, in two dimensions. The device consists of a movable lever and a ball bearing within a control box. The lever can be moved by hand up and down, and to the right and left. Joysticks are used in computer games, for entering coordinates into a computer, and for the remote control of robots. In some joysticks, the lever can be rotated, allowing control in a third dimension.
A mouse is a peripheral commonly used with personal computers. By sliding the mouse around on a flat surface, a cursor or arrow is positioned on the display. Pushbutton switches on the top of the unit actuate the computer to perform whatever function the cursor or arrow shows. These actions are called clicks.
A trackball resembles an inverted mouse, or a two-dimensional joystick without the lever. Instead of the device being pushed around on a surface, the user moves a ball bearing, causing the display cursor to move vertically and horizontally. Pushbutton switches on a computer keyboard, or on the trackball box itself, actuate the functions.
An eraser-head pointer is a rubber button approximately 5 mm in diameter, usually placed in the center of a computer keyboard. The user moves the cursor on the display by pushing against the button. Clicking and double clicking are done with button switches on the keyboard.
A touch pad is a sensitive plate that is about the size of a business card. The user places an index finger on the plate and moves the finger around. This results in intuitive movement of the display cursor. Clicking and double clicking are done in the same way as with the trackball and eraser-head pointer.
An electric motor converts electrical energy into angular (and in some cases linear) mechanical energy. Motors can operate from ac or dc, and range in size from tiny devices used in microscopic robots to huge machines that pull passenger trains.
The basics of dc motors were discussed in previous sections. In a motor designed to work with ac, there is no commutator. The alternations in the current keep the polarity correct at all times, so the shaft does not lock up. The rotational speed of an ac motor depends on the frequency of the applied ac. With 60-Hz ac, for example, the rotational speed is 60 revolutions per second (60 rps) or 3600 revolutions per minute (3600 rpm).
When a motor is connected to a load, the rotational force required to turn the shaft increases. The greater the required force becomes, the more power is drawn from the source.
A stepper motor turns in small increments, rather than continuously. The step angle, or extent of each turn, varies depending on the particular motor. It can range from less than 1° of arc to a quarter of a circle (90°). A stepper motor turns through its step angle and then stops, even if the current is maintained. When a stepper motor is stopped with a current going through its coils, the shaft resists external rotational force.
Conventional motors run at hundreds, or even thousands, of revolutions per minute (rpm). But a stepper motor usually runs at much lower speeds, almost always less than 180 rpm. A stepper motor has the most turning power when it is running at its slowest speeds, and the least turning power when it runs at its highest speeds.
When a pulsed current is supplied to a stepper motor, the shaft rotates in increments, one step for each pulse. In this way, a precise speed can be maintained. Because of the braking effect, this speed is constant for a wide range of mechanical turning resistances. Stepper motors can be controlled using microcomputers. This type of motor is especially well suited for point-to-point motion. Complicated, intricate tasks can done by computer-controlled robots using stepper motors.
A selsyn can be used to remotely indicate the direction in which a mechanical device is pointed.
A selsyn is an indicating device that shows the direction in which an object is oriented. It consists of a transmitting unit and a receiving (or indicator) unit. As the shaft of the transmitting unit rotates, the shaft of the receiving unit, which is a stepper motor, follows along exactly. A common application is as a direction indicator for a wind vane (above figure). When the wind vane rotates, the indicator unit shaft moves through the same number of angular degrees as the transmitting unit shaft. A selsyn for azimuth (compass) bearings has a range of 0° to 360°. A selsyn for elevation bearings has range of 0° to 90°.
A synchro is a selsyn used for the control of mechanical devices. Synchros are well suited for robotic teleoperation, or remote control. Some synchros are programmable. The operator inputs a number into the generator, and the receiver changes position accordingly. Synchros are commonly used as rotators and direction indicators for directional communications antennas such as the Yagi or dish.
An electric generator is constructed in much the same way as an ac motor, although it functions in the opposite sense. Some generators can also be used as motors; devices of this type are called motor/generators.
A typical generator produces ac from the mechanical rotation of a coil in a strong magnetic field. Alternatively, a permanent magnet can be rotated within a coil of wire. The rotating shaft can be driven by a gasoline engine, a steam turbine, a water turbine, a windmill, or even by human power. A commutator can be used with a generator to produce pulsating dc output, which can be filtered to obtain pure dc for use with electronic equipment.
Small portable gasoline-powered generators, capable of delivering a few kilowatts, can be purchased in department stores or home-and-garden stores. Larger generators, which usually burn propane or methane (“natural gas”), allow homes or buildings to keep their electrical power in the event of an interruption in the utility. The largest generators are found in power plants, and can produce many kilowatts.
Small generators can be used in synchro systems. These specialized generators allow remote control of robotic devices. A generator can be used to measure the speed at which a vehicle or rolling robot moves. The shaft of the generator is connected to one of the wheels, and the generator output voltage and frequency vary directly with the angular speed of the wheel. This is an example of a tachometer, a device familiar to people with automotive experience.
An optical encoder uses LEDs and photodetectors to sense the direction and extent of shaft rotation.
In digital radios, frequency adjustment is done in discrete steps. A typical increment is 10 Hz for shortwave radios and 200 kHz for FM broadcast radios. An alternative to mechanical switches or gear-driven devices, which wear out with time, is the optical encoder, also called the optical shaft encoder.
An optical encoder consists of two LEDs, two photodetectors, and a device called a chopping wheel. The LEDs shine on the photodetectors through the wheel. The wheel has radial bands, alternately transparent and opaque (above figure). The wheel is attached to the tuning shaft, which is attached to a large knob. As the tuning knob is rotated, the light beams are interrupted. Each interruption causes the frequency to change by a specified increment. The difference between “frequency up” and “frequency down” (clockwise and counterclockwise shaft rotation, respectively) is determined according to which photodetector senses each sequential beam interruption first.