|
|
|
How to get the solenoids on your side
|
| Product |
Solenoids
|
| Description |
solenoid design and applications
|
| Author |
Richard Connell
|
| Printer
friendly Version |
|
|
Richard Connell, Senior Designer at BLP in Newmarket, offers some insights into selecting and designing-in a solenoid while avoiding the most common pitfalls.
The solenoid is a key component in countless products from automotive and safety switches to vending and games machines. Designing-in one of these devices is unlikely to pose any great difficulties, provided you approach the task correctly.
In our experience as manufacturers, the solenoid typically enters the design process far too late. One reason is the difficulty of assigning responsibility - is this the territory of the mechanical or electronics engineer? The answer is, of course, both. The two need to cooperate closely, which nowadays may not be quite as easy as it sounds. Electronics design is frequently subcontracted out while the mechanical side remains in house, so that getting together to specify a solenoid becomes more of a problem.
In fact, many designers never get round to developing a detailed specification for the solenoid at all, relying on back-of-envelope guesstimates followed by trial-and-error selection from manufacturers' samples. This can lead to all kinds of difficulties. Designers frequently find they have provided neither enough space for the solenoid nor a suitable site for its fixings. They discover that the required force/stroke characteristics can't be matched, the duty cycle will lead to overheating and the peak current can't be provided by the existing drive circuit.
In short, they are faced with extensive customisation of the solenoid and re-design of the electronics - or in business terms, increased costs and delayed delivery to market.
The better way to do it
The first piece of advice is obvious enough - include the solenoid as early as possible in your design process! The second is to consult the solenoid manufacturers if you have any doubts or difficulties, because in most cases they should be able to help you design-in a solenoid from their standard range, either unmodified or with relatively light customisation.
It also helps to be armed with some understanding of solenoid technology. A modern direct-acting solenoid is a much more sophisticated device than it seems, and the following is an introduction to the most important features.
The basic format of any solenoid is simple indeed. Its only moving component is the plunger which exerts a pull force, or can provide thrust with the addition of a pin. The cleverness lies in the detailed design and materials.
Solenoids are available in every size from PCB-mounted miniatures to heavyweights capable of lifting kilograms. The vast majority today use DC-operated coils in standard voltages from 5V to 48V. Their main advantages, as compared with AC operation, are that they:
- take the same current throughout their stroke
- cannot overheat when used within their rating
- are more repeatable for a given loading condition
- are naturally quiet when closed.
An AC solenoid may be appropriate where an unusually high force is required in the fully-open position, or where a DC supply is not readily available in the design.
Force/stroke characteristics
The first point to note when designing a solenoid-operated mechanism is that the force supplied by the plunger varies throughout its stroke. The force always increases as the plunger is drawn more fully into the coil because this reduces the magnetic reluctance and therefore increases the field strength.
The characteristics of the force/stroke curve are greatly dependent on the detailed geometry of the plunger and stop. For example, the shape of the plunger end can make as much as a five-fold difference to the force supplied in certain conditions. This aspect of the solenoid's design can be tailored to your application relatively easily - so make sure the manufacturer is prepared to offer this level of engineering support.
Your design will also need to take into account the force/stroke characteristics of the linkage mechanism, which will normally be spring-loaded. Amongst other things, this will affect the closure rate.
Closure rate
The time taken for the plunger to be drawn fully into the coil will be an important consideration for some applications. This depends on the plunger's acceleration at every point on its force/stroke curve, which in turn depends on the difference between the force supplied and the load force - the greater the difference, the greater the acceleration. Fast closure demands high force levels and therefore a high current pulse to the coil. This can be achieved at relatively low overall power by employing a capacitor discharge circuit.
Duty cycle and temperature
All solenoid coils rise in temperature when carrying a current. The published force/stroke characteristics are normally based on a specified maximum coil wattage at a stabilised working temperature of 105(deg)C. However, DC solenoids are capable of supplying a greater force when cooler - which raises the issue of the duty cycle.
The most common operating mode for a solenoid is a repeated pulse cycle. The duty cycle is defined as the ratio of ON time to (ON OFF) time. By arranging the loading to require a short duty cycle, with a relatively long OFF time for cooling, it is possible to obtain the required force from a smaller solenoid rated at higher wattage than would otherwise be possible. In fact, the quoted coil rating can be increased in inverse proportion to the duty cycle - for example, a device rated at 5W on a 100% duty cycle (continuous operation) could be operated at 20W on a 25% duty cycle.
Latching solenoid
If an extended hold time is required, a magnetically-assisted latching solenoid often provides a good solution. In this type, permanent magnets are arranged so as to maintain a small pull. When the coil is energised, the plunger retracts and the force derived from the permanent magnets increases. If properly designed, this will be sufficient to retain the plunger against the return force of the spring-loaded linkage mechanism even after the energising pulse has ceased. A pulse width of 100-150ms is usually adequate.
To release the mechanism requires a pulse of opposite polarity. Clearly, the combination of the coil-derived force and the spring return force must be greater than the force from the permanent magnets. The correct releasing pulse voltage and external resistance can be calculated from the formulae reproduced in.
To sum up, then - bring your solenoid into the design process early, and involve both the mechanical and electronics engineers from the outset. Specify the component as fully as possible, and if it seems to require anything other than straightforward selection from the component catalogues - get on the 'phone to your solenoid supplier's technical support team. Nobody knows more about solenoids than they do, and they should be more than willing to lend a hand.
|
|
|
