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 look at the first category:
Permanent magnet/electromagnetic linear actuator
The permanent magnet or electromagnetic linear actuator is also known as a magnetic latching solenoid or MLS. The design of an MLS is such that the solenoid coil only needs a short current pulse to overcome the force of the return spring system (in Figure 1, this is a spring). This energy pulse moves the solenoid housing from the “deactivated” to the “activated” position (in Figure 1, the solenoid is activated).
Once in the engaged position, the permanent magnet in the MLS will hold the fixture in place, preventing it from springing back to the disengaged position.
When necessary or desired, the fixture can be unlocked to the “off” position by sending a small pulse of electrical energy through the solenoid coil with a polarity opposite to that of the permanent magnet. The pulse of this reverse magnetic energy is sufficient to largely cancel out the holding force of the permanent magnet on the armature, allowing the return mechanism (the spring) to easily return the armature to its original position.
The physical dimensions (the dimensions of the coil, the permanent magnet and the solenoid housing) of an MLS are entirely dependent on the desired release force. The greater this force, the larger all other components must be in order to achieve it. This also applies to the amount of electrical energy that must be supplied to the coil to switch it on and off.
Typical MLS applications
- medical cabinets, cabinet locking
- circuit breakers
- locking mechanisms for solar power stations
- locking of chargers for electric vehicles
- door locks
Advantages of MLS technology
One of the most important advantages of MLS technology is energy saving. After all, an MLS can remain continuously in the “off” or “on” position without requiring any energy. Energy is only required for the actual switching operation. This type of energy saving is particularly valuable in battery-powered applications or other situations where minimal energy consumption is important.
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, part 2:
Magnetic locking mechanism
A magnetic latching mechanism or MLM (Magnetic Latching Mechanism) can exert extremely high counter forces in a much smaller housing than is possible with an MLS.
Magnetic locking mechanisms use the stored energy of the counter spring to achieve high impact and linear movement. This is possible by applying only a very small electrical energy pulse to activate the counter spring.
Unlike an MLS, which can position the housing completely independently, the high impact force combined with the compact dimensions of the MLM require the housing to be manually brought into the so-called “tensioned” state. In this state, the counter spring will be fully compressed, with the permanent magnet holding the housing in this position; the energy in the tensioned spring is retained until it is needed.
Although the MLM coil is far too small to bring the housing into the tensioned position on its own, only a small amount of energy is needed to release it again. This is because the small electrical pulse generates a magnetic field in the relatively small coil of the MLM that is large enough to cancel out the holding force of the permanent magnet. When this happens, the spring-loaded element is activated.
Once the MLM has been manually tensioned, it can remain in this position indefinitely until a situation arises in which the release mechanism must be activated.
The source of the small electrical pulse required to interrupt the holding force of the permanent magnet can be a battery or even a capacitive discharge circuit.
Typical MLM applications
- Fire protection system actuators
- Power interruption locking mechanisms
Key advantages of MLM
- Extremely high counter forces, capable of generating high impact forces
- Compact dimensions (compared to an MLS)
- Extremely low requirements with regard to operating current
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.
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
- 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 located at even a small distance 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).
By adding an extra permanent magnet, this component is able to hold the object it attracts 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 keeps the relevant subject (patient in bed/the head of the X-ray machine) securely in position. In the event of a power failure, both the patient and the operator of the equipment are protected against unexpected movements.
