Electric and magnetic fields are too important and too common to be neglected by nature in its grand design. Many animals and organisms have found ways to take advantage of these fundamental forces for sensing and actuation. The electric field in particular is used for both sensing and actuation. Almost all rays and sharks can sense electric fields produced by prey, as can some catfish, eels, and the platypus. Electric fields are sensed through use of special gelatinous pores that form electroreceptors called ampullae of Lorenzini. Sensing can be passive or active. Sharks and rays use passive sensing; prey is located by sensing weak electric fields produced by the muscles and nerves in the prey. Some animals, such as the electric fish, can generate electric fields for the purpose of active electrolocation of prey. The same basic sensory system is used by young sharks for protection by freezing in place when electrolocation fields are detected. But perhaps, the best known example of electrolocation is the platypus, which uses electroreceptors in its bill to hunt by night. Actuation is just as common and is used primarily to stun prey, and also for protection. The torpedo or electric ray (genus Torpedinidae) is one of some 70 species of rays that can produce electric charge and apply it in a manner similar to a battery. The charge is produced in a pair of electric organs made of plates connected to a nervous system that controls them. In rays, these biological batteries are connected in parallel to produce low -voltage, high -current sources. The range is between 8V and more than 200 V, with currents that can reach a few amperes. Another example is the electric eel (Electrophorus electricus). Since it lives in freshwater, which is less conductive than seawater, it has its plates in series to produce higher voltages (up to 600 V at perhaps 1 A, in short pulses).

Chapter Contents:

• 5.1 Introduction
• 5.2 Units
• 5.3 The electric field: capacitive sensors and actuators
• 5.3.1 Capacitive position, proximity, and displacement sensors
• 5.3.2 Capacitive fluid level sensors
• 5.3.3 Capacitive actuators
• 5.4 Magnetic fields: sensors and actuators
• 5.4.1 Inductive sensors
• 5.4.1.1 Inductive proximity sensors
• 5.4.1.2 Eddy current proximity sensors
• 5.4.1.3 Position and displacement sensing: variable inductance sensors
• 5.4.2 Hall effect sensors
• 5.5 Magnetohydrodynamic (MHD) sensors and actuators
• 5.5.1 MHD generator or sensor
• 5.5.2 MHD pump or actuator
• 5.6 Magnetoresistance and magnetoresistive sensors
• 5.7 Magnetostrictive sensors and actuators
• 5.7.1 Magnetostrictive actuators
• 5.8 Magnetometers
• 5.8.1 Coil magnetometer
• 5.8.2 The fluxgate magnetometer
• 5.8.3 The SQUID
• 5.9 Magnetic actuators
• 5.9.1 Voice coil actuators
• 5.9.2 Motors as actuators
• 5.9.2.1 Operation principles
• 5.9.2.2 Brushless, electronically commutated DC (BLDC) motors
• 5.9.2.3 AC motors
• 5.9.2.4 Stepper motors
• 5.9.2.5 Linear motors
• 5.9.2.6 Servomotors
• 5.9.3 Magnetic solenoid actuators and magnetic valves
• 5.10 Voltage and current sensors
• 5.10.1 Voltage sensing
• 5.10.2 Current sensing
• 5.10.3 Resistance sensors
• 5.11 Problems

Inspec keywords: actuators; magnetic sensors; electric field measurement; magnetic field measurement; electric sensing devices

Other keywords: electrophorus electricus; electroreceptors; passive sensing; electric sensor; electrolocation; genus torpedinidae; nervous system; current sources; actuators; electric organs; magnetic sensor; current 1 A; electric eel

Subjects: Sensing and detecting devices; Sensing devices and transducers; Measurement of basic electric and magnetic variables; Voltage measurement; Magnetic variables measurement