Linear Actuator Question

The Reason for the project is so you can learn these things.

why would you be restricted to linear? the most expensive sort of actuator [ unless you only need 2 positions]

A air cylinder will work as a linear.

How fast does the application need to move?

How far?

How much force?

How much can it weigh?

How many cycles does it have to last for?

How acurate?

What sort of feedback?

What source of power

the possibilites are endless, solutions are are limited by budget & your ability to understand & implement.

Well actually I was not assigned the project. The team is very advanced and as a first year I just kind of jumped on. Even the very advanced mechs and professors don't know if this exists so I wanted to ask the general engineering community. The device will be used for throttle/brake actuation on a fully autonomous vehicle. In the end we need control at 1mpg increments. The device will push a standard gas pedal.

We are researching two methods. One is a rotary system with cables and pulleys. The other is a direct linear actuator. With the rotary we can use a electronic clutch so that we can switch between full auto and manual human driving. With linear actuators maintaing their final position we could not do this. Also, in case the auto system fails an electronic clutch allows the actuation to halt whenever power is cut (an e switch) to just the rotary actuator. To do this with a linear that hold final position we would have to cut the engine. So we are looking for a linear actuator of sorts that will not hold its final position when shut off, but can readily be moved. Power is a will be a 12 or 24 volt battery system. Precision is key, but only to 1mph. We know how to do everything with the actuator option except the powered-down manual.

One way you could go is to use air pressure if you have it or can make it. If you use a valves that opens to atmosphere when off then the air cylinder would offer minimal interference when not being pushed one way or the other. Hydraulics is another possibility but more involved and more expensive. You must vent those to a reservoir but you can get more power.

I presume you are using the acutator to connect directly to the trottle linkage. Why not use a servo motor connected directly via an adaptor plate to the axis of the mechanical trottle acuator and therefore ignoring the linkage and the linear element of the motion. You can rotate a servo when powered down and accuracy should not be a problem with the right encoder and motor.

For position control stepper motors are always worth looking at. You can get motors that will step and hold position in increments as small as 1.5º and they will pretty much free wheel when the power is off. Steppers are also really easy to control with a μ controller. If you engineer them properly you don't need any positional feedback, all you need to do is keep track of the number of steps you have mad them do.

I don't suppose you've thought of using an off-the -shelf cruise control mechanism with maybe a stepwise relay system? Don't re-invent the wheel. Use any type actuator you want - pneumatic, electromechanical, c3po. I accept beer as payment.

Buenos dias a todos. Bmw's use rotational transducers on the gas pedal so the actual position of pedal is feed to the computer governing the gas trotle on the engine. When driver release the pedal the motor goes to idling speed. for autonomous driving the algorithm controling the process does not need the pedal position( in robotic mode, gas/pedal position is desabled and viceversa ).

if you are looking for low power actuators you may want to see how spool valves work or go to vickers or bosch servo valves ( they have a proportional valve- tre way valve with infinite positions- wich uses small/low power solenoid actuators). I'm positive you can reengineer some of this ideas on your project.

In Europe they have a program call formulastudent where they design, build and test small race cars and I'm sure they have seem some of this problems, good luck.

check out electric or electric-over-hydraulic brakes for trailers. Some goose neck and pintle hitch trailors have a box with electric motor driving hydraulic brake master cylender, look at one of those. Most other trailers have electric brakes, if this vehicle doesn't need to meet DOT specs and a bunch of other restrictions, you might be able to get away with using electric brakes only. Then just feed break pedal signal into processor for manual mode... A lot of this stuff is out there, keep looking.

as for a powerful actuator that is affordable, power seat adjustors can create lots of force, but thier typicly slow. If you want a "non locking" linear actuator, you should teach yourself about "over driving lead screws" these are just lead screws that most likely have multiple starts and a realy steep pitch and usually no gear train. The gear train would be hard to over drive. You should also learn about "Rack and pinion" they are typicly overdrivable.

Or you could keep your linear actuator idea and design a linkage that could be locked or unlocked. Not always is the case where one component can be used to "do it all". Keep the end objective in mind, keep it simple and practice thinking out of the box.

However, if, as you say, you are "researching actuation devices" as the means to an end, given the resolution of 1 mph, you'll need to find one that can handle thousands of small movements per minute. The automobile does not make a very good closed loop system for a servo control system. The drivetrain has too much backlash and the inertia input is varied and comes from many places. For example: Going downhill.

Now the weight of the vehicle is causing the drivetrain to become a brake. The engine must be slowed down to a point where the speed is controlled. Your sole method is throttle control and it isn't very good or very accurate. Large trucks use a "Jake brake", a device that closes the valves to the combustion chamber and makes the pistons push against compression to slow down the engine driven by inertia/gravity.

At the peak of it's braking power, the idling engine is not really good for speed control. In addition, during the time the forces acting on the drivetrain are reversed, the slop in the drivetrain (backlash) comes in to play. Without backlash compensation, your test vehicle will jerk and buck quite a bit during level cruise and during transition from level to uphill and so on - at 1 mph resolution!

Auto cruise controls are buffered by design and do not work at such a tight tolerance.

A separate encoder would help get the resolution you need, but would be difficult to implement in a "loose" device such as a vehicle's drive train, especially automatic.

The best setup would be in a hydraulic driven drive train where the forces could be reckoned with in real time, and backlash is physically buffered.

Just some ideas to help your brain to jump!

Is this an electric machine driven by electric motors, or does it have an engine of some type?

Have you considered what a standard cruise-control does?: SET CANCEL RESUME.

Sounds like just what you need. It reverts to manual control when the brake switch is actuated, returning control to the operator.

However, many different combinations of switches can be used to achieve the MANUAL/AUTO mode.Use your imagination and creativity as a budding engineer.

Obeny N. Pebvk

The more I think about it the better stepper motors look. You can get a μ controller kit from Microchip called the PICkit 2 for about US$50-00. It comes with software, cables, manuals, programmer, tutorials on how to use it and a prototyping board. The board has almost everything you need on it to drive a stepper motor, all you would need to do is to add 4 transistors to drive the load. One of the tutorials will just about do what you want to so the programming would be simple. You can find out more about it at this site;

http://www.microchipdirect.com/productsearch.aspx?Keywords=PICkit+2

The other advantage of doing it this was is that you can easily adapt it to drive accelerator, brakes or just about anything by simply re-programming the μ controller .

If this is a modification to an existing gas or diesel vehicle the device you need is likely already on the car and working. You just need to interface with it. The device is called a dashpot and most vehicles have at least 1. They are mounted somewhere around the throttle on the engine intake. These are small linear, voice coil, or vacuum actuators that push on a belcrank attached to the throttle shaft. Their job is to open or close the throttle in varying amounts for different engine conditions. Such as holding a high idle during a cold start, advancing throttle when AC is on, maintaining high revs under load when system voltage is low. Full travel is usually reached before WOT (wide open throttle) but I have yet to see an autonomous vehicle that would be safe at those speeds anyway. If you really need full throttle operation it can be achieved by changing actuator modles.

They are usually operated by the ECU with a 0-5 or 0-B+ variable voltage and its status is usually on the vehicle bus. The TPS (Throttle Position Sensor) is also easilly tapped and used for your own closed loop system or you can also get Throttle% off the vehicle bus.

This eliminates all the linkage and pedal slop, cumbersome interfering hardware in the cockpit and costs associated with doing it the Mythbusters way.

Masu has a good point about using Stepper Motors. However, they typically have a rotational output that must be converted to linear to do what you want.

The typical conversion of rotational to linear actuation is done by what is known as a Leadscrew and Nut. There are even stepper-leadscrew combinations where the stepper motor does not have a conventional shaft, but directly drives an internal Nut, which allows the Leadscrew to actually pass through the stepper motor.

Be careful though. Typical leadscrews in these applications have a single start (single spiral), usually an Acme thread type, and do tend to "lock up", which is usually a desirable characteristic. This is because the thread angle is very small requiring a large linear force, since the inclined plane of the thread is nearly normal to longitudinal axis.

Masu is right, the stepper motor might be free-wheeling when de-engerized, but if the mechanical linkage should lock up or have a very high force requirement, what good is it in your application? Be sure to use multiple start (parallel) threads, as that allows each thread to have a relatively large angle and allow smooth, low-force motion. Also, try to use a low friction polymer (Delrin acetal, teflon, HDPE, etc.) material for the nut instead of metallics (even "oilite" Bronxe). Use of a recirculating ball-nut would lower the friction still.

Rack and pinion was another very good idea already mentioned which could be used with stepper motors, but these might be relatively more expensive and difficult to maintain as compared to the simpler leadscrew designs.