Solenoid variable voltage/position problem

[alardin]

I am attempting to build a device that pulls against a spring at variable tensions, based on how much electricity is passed to it.

I have read that solenoids don't "like" being charged for long without going to a closed position. That they burn out. The specific problem is that the spring must extend or retract to different positions without burning out, and when the power is off, it needs to reset completely (open position).

Any help?

 

[JimAlexander]

I would be interested to know a little bit more about what you are trying to build. What you described reminds me of a plunger type variable position valve. In my field experience we often see them replaced with other types of valves that offer better control. Often the load (fluid of some type) is itself giving variable pressure against the valve bobbin or spool so keeping the valve in a constant position isn't always possible. This would produce a chattering sound and the pressure would be rapidly changing on the controlled side of the valve.

As far as burn-out, with regular industrial solenoid valves, the valve doesn't burn out often if it is properly specified for the work it has to do. We have pneumatic and hydraulic solenoids in our machinery that are on most of the day, all day, every day. There can be quite a few things to consider, but if done right they hold up just fine. Things to look at would be holding current, temperature and suitability for the specific application. They can get hot to the touch and still be operating within their spec.

It ain't much, hope it helps.

 

[alardin]

"I would be interested to know a little bit more about what you are trying to build"

My goal is to create an electronic interface for automobile's throttle cable/pedal assembly.

I want the throttle cable to be pulled with different force, based on different electrical inputs, and released completely if the electrical input stops. The ability for the throttle to release completely is an absolutely essential fail-safe. If the electrical input failed, and the throttle cable were still engaged, that could be disastrous (as you can imagine).

I have looked at solenoids, but they appear to not allow for the functionality I need. The throttle could be held in a fairly low pressure position for hours, which would create intense heat, reduce the holding force, and potentially burn out the solenoid - from what I've read and the companies I have consulted with.

I have looked into actuators, but they don't seem to work by increasing pressure against a spring, but more about setting the actuator's position. That would be manageable but significantly less desirable, and when the power stopped it would not return the actuator to zero-position.

So I am stuck.

 

[dlgoff]

Okay. What you are trying to do is similar to http://en.wikipedia.org/wiki/Cruise_control" .

The vehicle will maintain the desired speed by pulling the throttle cable with a solenoid, a vacuum driven servomechanism or by using the electronic systems built into the vehicle (fully electronic) if it uses a 'drive-by-wire' system.

With "drive-by-wire" systems, one of the functions is http://en.wikipedia.org/wiki/Electronic_throttle_control" .

Electronic throttle control (ETC) is an automobile technology which severs the mechanical link between the accelerator pedal and the throttle. Most automobiles already use a throttle position sensor (TPS) to provide input to traction control, antilock brakes, fuel injection, and other systems, but use a bowden cable to directly connect the pedal with the throttle. An ETC-equipped vehicle has no such cable. Instead, the electronic control unit (ECU) determines the required throttle position by calculations from data measured by other sensors such as an accelerator pedal position sensor, engine speed sensor, vehicle speed sensor etc. The electric motor within the ETC is then driven to the required position via a closed-loop control algorithm within the ECU.

And here is a US patent #6646395 Electronic throttle body drawing that shows the motor coupled to the butterfly valve via gears.

[PLAIN]http://upload.wikimedia.org/wikipedia/en/3/3d/USPatent6646395.png

Hope this gives you some ideas.

 

[JimAlexander]

Generally speaking, Solenoids are either on or off, not so common to want a variable (continuously variable) one. I'm not as familiar with automobiles, but I can mention some things we do in industry.

In a control situation like this, we often use motors and servos. As a basic rule of thumb, motors are best for speed control applications, and servos are better for position control and tension control. Servos for position control, with the ability to adjust to the amounts of torque needed at anyone moment may be one of the best options. They can come pretty small, although once you miniaturize you may have to deal with some limitations.

There is another family of products we have started to use but are not yet as familiar with called linear actuators. A motor or servo drives a shaft, like a ballscrew. One of these may not be sensitive to pressure, but if the input and processing stages of your device are fast enough it may not be necesary.

Good Luck.