Magnetic locking mechanisms can be divided into three categories. These are distinguished by the way in which the mechanisms are operated, the amount of energy required to do so, and the unlocking force that can be generated by the spring mechanism. The three categories will be discussed in the coming weeks. This week, we will discuss the third category:
Permanent magnets / electromagnets
A. Conventional electromagnets (EM)
Theoretical operation
Conventional electromagnets (EM) are simple instruments consisting of a wound coil (shown in Figure 1 by the coil and magnet wire), a magnetic structure (shown in Figure 1 as the steel body) and an armature.
When a current flows through the coil, a magnetic field is generated that attracts the armature to the body, explaining the term electromagnet. In many applications, the armature has been replaced by a mechanical system in which the electromagnet is mounted.
Figure 1: Conventional electromagnet with armature
The strength of the magnetic force generated by the EM coil depends on several factors:
- The size and magnetic properties of the body.
- The size and winding design of the coil.
- The amount and duration of electrical energy flowing through the coil.
- The mechanical and magnetic properties of the armature and/or the structure of the mechanism attracted by the EM.
- The distance between the attracted body and the surface of the EM and
- The flatness and surface condition of both the front of the EM body and the object being attracted.
Electromagnets are most effective when used to hold an object close to the surface of the magnet body.
Because the strength of a magnetic field decreases quadratically with distance, an electromagnet is very ineffective when attempting to attract an object that is (even slightly) distant from the EM body.
B. Permanent magnets / electromagnets (PM/EM)
Theoretical operation
A permanent magnet/electromagnet (PM/EM) has the same structure as a conventional electromagnet (EM), but differs in that part of the steel body has been replaced by a so-called steel core with a permanent magnet at the end (see Figure 2).
Figure 2: Permanent magnet/electromagnet (PM/EM) with armature
By adding an extra permanent magnet, this component is able to hold the attracted object even when no energy is flowing through the coil.
As with all permanent magnet/electromagnetic components, the attracted object can be released again by passing a small amount of electrical energy through the PM/EM coil. The polarity of this must be reversed in relation to the polarity of the permanent magnet. This energy pulse weakens the magnetic holding force of the permanent magnet, causing it to release the attracted object from the body of the PM/EM.
PM/EM components are used in applications that require a combination of continuous holding force without the need for a continuous supply of electrical energy. From this point of view, a PM/EM not only saves energy but can also be used as a “failsafe” component.
Example: when a PM/EM is used in a medical imaging system, it can perform the following tasks:
a. locking the object to be imaged (e.g. the patient in bed) during the procedure
b. locking the imaging device (e.g. the head of an X-ray machine) during the procedure.
In both cases, the PM/EM holds the relevant subject (patient in bed/head of the X-ray machine) securely in position. In the event of a power failure, both the patient and the equipment operator are protected against unexpected movements.
Author: David Stockwell of Magnet Schultz
Read also:
Magnetic locking mechanisms explained – part 1
Magnetic locking mechanisms explained – part 2
