automatic weight stack lifter

[jqm]

I would like to make a automatic weight stack lifter that could be used to automagically calibrate hanging weigh scales.

The basic idea is the have a hangar that is connected to a in line hanging weigh scale which is connected in line to a lifter (located at the highest point in the system) .  The lowest position of the lifter would be considered "zero".  The lifter will rise and the bottom hangar plate would come into contact with a pre-positioned calibrated slotted weight that is resting in a scaffold like configuration (lifting that weight off of the scaffold would essentially apply a load to the scale)... the process will continue until full scale is reached.

Vertical Movement (to be able to precisely control up and down placement of weight)

Range of Movement - about 1.5 foot (guessing)

Max Load - 1000 lbs

I would like to be able to create a labview program that would allow the user to easily control the weight that is applied (basically an absolute position controller).  

Does anyone have suggestions on a lifting unit / manufacturer  that could easily interface with labview and allow for "live" control of the lifter ?

thanks for your time

jqm

[jqm]

Does anyone have suggestions on a lifting unit or manufacturer  that could easily interface with labview and allow for "live" control of the lifter ?

<bumped>

regards

jqm

[RavensFan]

You basically need a linear actuator.  It sounds like you'd need something similar to a test stand we have where we've used a linear actuator by Exlar.  It is controlled by a motor controller by Control Techniques.  My LV application uses a serial port connection to issue motion commands to the motor controller driving the linear actuator.

[johnsold]

How many positions do you need?  How far does the scale arm move while a weight is being lifted? Is everything balanced well enough the a single point lift will work without tilt or torque issues?  How fast does it need to move?  How long does it wait between positions? The nature and cost of any possible solution may depend on the answers.

My first thought was a stepper motor connected to a gearbox pulling a chain or cable.  A pantograph mechanism or rack and pinion are other options.  I have built such devices for crystal growth apparatus.  Much smaller loads and probably a lot slower but more precise positioning.

Lynn

[jqm]

How many positions do you need?  

minimum 5pt could be more

How far does the scale arm move while a weight is being lifted?

less then 3 ft

Is everything balanced well enough the a single point lift will work without tilt or torque issues?  

All weights, lifting rod would be centered, so there would be minimal torque.  There will be linkage mechanism between the top and bottom of the load cell to aid in eliminate allignment issues / torques.

How fast does it need to move?  

unknown, for load cell testings: to go from 0 to full scale within a minute, but provided that it doesnt "jarr" things too much

How long does it wait between positions?

All timing could be determined by the output of the cell.  that is why I would like to have control of the linear actuator directly.

I havent checked into Ravens Fan suggestion yet...

Thanks for the responses.

jqm

[johnsold]

I guess I do not understand or I did not ask the right questions.

I was thinking in terms of an old style mechanical balance which had multiple weights that could be hung from the balance arm or lifted off.  The design of the balance specified how many such weights were in the instrument. When the weights on the arm matched the weight of the sample on the pan, the pointer indicated the null position and the weight of the sample was given by the sum of the weights hung on the arm.

You mention load cells and the arm moving 3 feet while a weight is being lifted but in the original post you indicated a range of motion of 1.5 feet.

Can you post a picture (.PNG or .JPG, not .BMP) of the apparatus so we have some idea of what is going on?

Are you measuring the output of the load cells or the position of the arm?

Lynn