FeedBack and correction mechanism for stepper motor on a linear acftuator

In fact you confirm what I wrote on the other thread the best soultion will be yours! Congratulations and good luck

Hi,

I responded to your reply and gave you more detailed information that you enquired about.

But I have received no suggestions or recommendations from you in this matter.

Regards,

DougRH

Read my answer on the 1st thread Best wishes Nick Name

Link to "other" thread

http://cr4.globalspec.com/thread/48343#newcomments

Hi,

did you look on LVDTs - two inductive coils surrounded by soft ferromagnetic material and an air-hole in the center where an iron core of half the length of one coils is moving.?

Very rugged, can be made to any temperature that the isolation is surviving, proven since pre-transistor times. Maximum stroke of mm to meter - no problem

But your temperature range will (maybe) necessitate a very special solution, 300°C is a tough limit for organics.

You will need an external electronics (carrier-frequency amplifier).

There is a simpler (electronically) version with 3 coils, named differential transformer.

Guiding the inner core in its movement may be a problem, sometimes solved by using airbearings (LION-Precision, St.Paul).

The stepper will have to have a fine interpolation to give the required resolution and a compensation for flux/temperature behaviour of permanent magnets.

You can design this (measurement and actuator) either rotary or linear (with a cam), both ways have some pros.

Have success

and a Happy New Year to everybody, peace, health, love, happiness!

RHABE

Hi RHABE,

Thanks for the information and suggestion.

I was unaware of and had no knowledge of LVDTs. I haven't had a chance to take a close look at them as of yet but will do so in the near future but wanted to acknowledge and thank you for the information and will give you more feedback and perhaps have some more questions on them if you are knowledgeable about them when I am able to.

This is the second thread that I started on this subject. I wanted to get a take and focus on a purely linear solution as opposed to a rotary to linear one as well. Most of the interaction is takong place in the initial one of : http://cr4.globalspec.com/thread/48343#newcomments If you are willing to do it, I would be pleased to have you post this same suggestion along with any other thoughts and suggestions that perhaps you have as there are quite a number of people active in the original one that are fairly knowledgeable and thus can perhaps give me some input and take on the application and useage on LVDTs as well.

Thanks once more (< 8)

DougRH

In fact one of the problems you have to fight is the ratio between effective stroke of a sensor and minimal overall length. For an LVDT the core travels the effective stroke but it is connected to a rod which has at least same length. This means that for a 250 mm stroke you should have at you disposal more than 500 mm of free space!
Do you have it?

The solution with chain or belt and 2 wheels is shorter and more adapted to the space available in an engine room.

Hi again Guru,

As I said, I have only glanced at the LVDT suggested solution but very quickly came up with the notion of using two separate ones that slightly overlap. Each one would sense half of the length of the shifting range

As with the LVDTs themselves, I have only given your suggested solution if utilizing LVDTs a quick cursory glance so far but can see the solution in the design that helps to overcome what we both determined to be an obstacle that needed to be solved if taking this route. As with the LVDTs themselves, I will give your suggested solution much more thorough consideration. Another aspect of this approach also immediately comes to mind is the (perhaps unnecessary) added complexity and greater cost. As many including myself are cognizant of, the cost/benefit ratio is of vital importance to the chances of success.

Thanks again,

Keep on Keeping On! (< 8)

Space is definitely an issue as the sensors will have to be inside the transmission itself. There is never any 'empty 'real estate' space' in such environments.

Doug -

I have a similar application for linear motion (with lead screws). My client has been using steppers without feedback or correction from an earlier design phase and now they realize it dow not work, especially for initialization sequence. The three axes each have a "park" position, and this position is essentially at "home".

I have been looking for sensor or encoder which would provide an absolute reference so that upon start-up the system would immediately know where it is, without having to try and find "park" (and getting jammed half the time).

Mike

Hi Mike,

Does this machine utilize a CNC (Computer Numerically Controlled) controller system?

All such machines have an necessary initialization sequence on start up. Machining and minor positioning errors aside, it sounds like you have a programming problem and NOT a hardware problem. Some controllers are able to keep the positioning information when it is shut off, others are not. This is influenced by the controller itself, but also if the machine is 'turned off' by the CNC controller or if the power to the entire machine is turned off.

If so you need to get the proper installation and programming manual for that particular make and MODEL of the CNC CONTROLLER! These will likely be available on the wWw and downloadable. If these recommendations are applicable, they are your easiest, fastest, best and SAFEST way to overcome your problems. Also if applicable, you will need this documentation to be able to utilize it (fully and properly) anyway.

Zeroing the movable table by it going to the 'home' position is SOP (Standard Operating Procedure) for such devices. You command it to 'go home' which it will most likely do anyway when you start it up unless you have a program loaded that specifically tells it not to. The controller should show an alarm on the readout if it overshoots its 'Home' position. If so, back the controlled table off until it is out of the overshot alarm position and then turn the machine off and then restart it to reinitialize it. Once it is at the 'home' position, you 'zero' the positions and readout so that the machine knows where it is at. You can then select other locations and zero it to that temporary home position. This is generally used so that time is not needlessly wasted by going to the absolute home position between machining segments and routines when it is unnecessary. These actions can be at the start of the NC program, or done manually by the operator. Regardless as far as the controller is concerned it's 'home' position is its 'absolute' reference point. The technical installation manual and perhaps the operators manual may have the procedures for adjusting and locating the home position. But generally you want it to be as far away from the cutter so that you can machine the largest workpieces that are possible on the machine

A couple of things to watch for:

1 - Most such controllers have an 'absolute' and a 'relative' movement modes. You need to be sure which you are using and program it accordingly.

2 - If this is a system that was originally designed for CNC machining, some are programmed in the 'Diameter' mode, while others utilize the 'Radius' mode. Some are switchable and selectable, some are not. Obviously this is going to be a very significant factor in programming and operating the controller and machining.

What sort of equipment is this now being utilized for? ALL CNC machines have fairly standardized procedures for 'qualifying' a new program. Putting a new and unqualified program into an NC machine and just hitting the 'cycle start button' is just looking for trouble and can do significant damage to both the machine and anyone in proximity to it at the time!

The tolerance problems of unwanted movement can be (partially) compensated for by always approaching and machining from the same direction. Unwanted' kick back' by erratic machining forces can be lessened and with some discretion by lightly and partially locking of the clamps that are sometimes used to lock the machine into a fixed position in the plane that it is not being machined in.

(8 >) . A Happy, Healthy and Prosperous New Year to You and Yours . (< 8)

There are 2 types of encoders:

- incremental most used

- absolute

In many machines the electronic module keeps the values measured by th incremental sensor for a accidental loss of power supply. But if during the absence of power the "slider" did move the position is not any more the saved one. For such situations are the absolute sensors good since the signal is alwways related to the actual position. Incremental systems need a "zero" position in order to initialize the counting always from same position.

In your system you NEED an absolute sensor since you cannot accept in case of interruption to give the wrong shift.

Hi again Guru,

An excellent point and duly noted. A 100 % 'Fail Safe' mode IS a necessity with this design. Another and/or additional option is for the CVTs emergency default position to being one of 'stuck in neutral', which is readily accomplished with this design

Thanks again,

Hi Guru,

FYI: Further info in regards to the 'Fail Safe' aspect in case of loss of power, the CVT shifter will just remain in the position that it was in when the power was cut. Your point does address the issue of 'reinitializing' it again once the power is restored though.

Thanks again,

DougRH (< 8)