> From:         James Tu[SMTP:james@2-BIT-TOYS.COM]
> Sent:         Monday, May 03, 2004 7:50 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE:] Linear Actuators and position sensing

> Hi:

> I'm familiar with the concepts for using a rotary encoder with a motor for
> direction and relative position sensing.

> I will be working with some linear actuators.  What have people used for
> position sensing?

> I'm thinking of the following possibilities...
> (1) linear pots
> (2) limit switches
> (3) sonic or optical range finders

> any others?

> What would you guys recommend?

> I have 3 linear actuators that move a platform up and down (we are moving
> it up and down and the platform can also be tilted.)  I want to know and
> control how high or low each actuator should move.

> James Tu
> james@2-bit-toys.com

The industry standard linear position measuring device that has been around
forever is the Linear Variable Differential Transformer (LVDT). It is made
of three coils (I think) wound coaxially with a hole running through the middle of
all three. A ferromagnetic slug slides through this hole, changing the coupling
between the coils. Think of two coils side by side, with a third one wound
over their junction. The center coil is energized by an AC source. With the
slug at the center point (midpoint of the third coil), both "secondary" coils
receive equal flux and produce equal but opposite polarity output. A
differential amplifier sums the two outputs giving zero. As the slug moves
to either side, the output increases in amplitude, with the phase indicating
the direction.
The manufacturer's name most associated with this device is Schaevitz.
Unfortunately, this is not a miniaturized or light weight device, but it can
be very accurate. Since the mass of the slug must be moved, it is also
not a wide band (fast) sensor.

John Power

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