A sensor employs one or more transducers to detect and/or measure phenomena such as temperature, humidity, barometric pressure, texture, proximity, and the presence of certain substances.
Capacitive Pressure Sensor
A capacitive pressure sensor. When force is applied, the spacing between the plates decreases, causing the capacitance to increase and the oscillator frequency to go down.
A capacitive pressure sensor is shown in above figure. Two metal plates are separated by a layer of dielectric (electrically insulating) foam, forming a capacitor. This component is connected in parallel with an inductor. The resulting inductance/capacitance (LC ) circuit determines the frequency of an oscillator. If an object strikes the sensor, the plate spacing momentarily decreases. This increases the capacitance, causing a drop in the oscillator frequency. When the object moves away from the transducer, the foam springs back, the plates return to their original spacing, and the oscillator frequency returns to normal.
The output of a capacitive pressure sensor can be converted to digital data using an analog-todigital converter (ADC). This signal can be sent to a microcomputer such as a robot controller. Pressure sensors can be mounted in various places on a mobile robot, such as the front, back, and sides. Then, for example, physical pressure on the sensor in the front of the robot can send a signal to the controller, which tells the machine to move backward.
A capacitive pressure sensor can be fooled by massive conducting or semiconducting objects in its vicinity. If such a mass comes near the transducer, the capacitance may change even if direct contact is not made. This phenomenon is known as body capacitance. When the effect must be avoided, an elastomer device can be used for pressure sensing.
An elastomer pressure sensor detects applied force without unwanted capacitive effects.
An elastomer is a flexible substance resembling rubber or plastic that can be used to detect the presence or absence of mechanical pressure. Above figure illustrates how an elastomer can be used to detect, and locate, a pressure point. The elastomer conducts electricity fairly well, but not perfectly. It has a foam-like consistency, so that it can be compressed. Conductive plates are attached to the pad. When pressure appears at some point in the elastomer pad, the material is compressed, and this lowers its electrical resistance. This is detected as an increase in the current between the plates. The greater the pressure becomes, the more the elastomer is compressed, and the greater is the increase in the current. The current-change data can be sent to a microcomputer such as a robot controller.
A back-pressure sensor governs the force applied by a robot arm or other mechanical device.
A motor produces a measurable pressure that depends on the torque being applied. A back-pressure sensor detects and measures the torque that the motor is applying at any given time. The sensor produces a signal, usually a variable voltage, that increases as the torque increases. Above figure is a functional block diagram of a back-pressure sensor.
Back-pressure sensors are used to limit the force applied by robot grippers, arms, drills, hammers, or other end effectors. The back voltage, or signal produced by the sensor, reduces the torque applied by the motor. This can prevent damage to objects being handled by the robot. It also helps to ensure the safety of people working around the robot.
Capacitive Proximity Sensor
A capacitive proximity sensor can detect nearby conducting or semiconducting objects.
A capacitive proximity sensor uses an RF oscillator, a frequency detector, and a metal plate connected into the oscillator circuit (above figure). The oscillator is designed so that a change in the capacitance of the plate, with respect to the environment, causes the oscillator frequency to change. This change is sensed by the frequency detector, which sends a signal to a microcomputer or robot controller. Substances that conduct electricity to some extent, such as metal, saltwater, and living tissue, are sensed more easily by capacitive transducers than are materials that do not conduct, such as dry wood, plastic, glass, or dry fabric.
Photoelectric Proximity Sensor
A photoelectric proximity sensor. Modulation of the light beam allows the device to distinguish between sensorgenerated light and background illumination.
Reflected light can provide a way for a mobile robot to tell if it is approaching something. A photoelectric proximity sensor uses a light-beam generator, a photodetector, a frequency-sensitive amplifier, and a microcomputer (above figure).
The light beam reflects from the object and is picked up by the photodetector. The light beam is modulated at a certain frequency, say 1000 Hz. The amplifier responds only to light modulated at that frequency. This prevents false imaging that can otherwise be caused by lamps or sunlight. (Such light sources are unmodulated, and will not actuate a sensor designed to respond only to modulated light.) If the robot is approaching an object, its controller senses that the reflected, modulated beam is getting stronger. The robot can then steer clear of the object. This method of proximity sensing does not work for objects that do not reflect light, or for windows or mirrors approached at a sharp angle. In these scenarios, the light beam is not reflected back toward the photodetector, so the object is invisible.
In texture sensing, lasers (L) and sensors (S) analyze a shiny surface (at A) and a matte surface (at B). Solid lines represent incident light; dashed lines represent reflected light.
Texture sensing is the ability of a machine to determine whether a surface is shiny or rough (matte). Basic texture sensing involves the use of a laser and several light-sensitive sensors. Above figure shows how a laser (L) and sensors (S) can be used to tell the difference between a shiny surface (at A) and a rough or matte surface (at B). The shiny surface, such as the polished hood of a car, reflects light at the incidence angle only. But the matte surface, such as a sheet of paper, scatters light in all directions. The shiny surface reflects the beam back entirely to the sensor in the path of the beam whose reflection angle equals its incidence angle. The matte surface reflects the beam back to all the sensors. A microcomputer can be programmed to tell the difference.
Certain types of surfaces can confuse the texture sensor shown in above figure. For example, a pile of glass marbles can be defined as shiny on a small scale but irregular on a large scale. Depending on the diameter of the laser beam, the texture sensor might interpret such a surface as either shiny or matte. The determination can also be affected by motion of the sensor relative to the surface. A surface that is interpreted as shiny when standing still relative to the sensor might be interpreted as matte when moving relative to the sensor.