Absolute Rotary Encoders

[Mark Harriss]

Adam Anderson wrote:

Mark Harriss wrote:

I'm looking for absolute rotary encoders with at least 18 bit
resolution for a product currently in design, does anyone have
any brands or products they can recommend / not recommend?.

TIA
Mark


Optical encoders at that resolution will fail after a period of time as
the graticule's are too fine, even given small mechanical disturbances.

Geared encoders at that resolution will not perform due to mechanical
backlash.

you need dual speed synchro resolvers and a resolver to digital converter.

Thanks for the information Adam, do you recommend any specific brands?.
The Sick Stegman unit I was looking at has a metal disc of nickel alloy,
something like invar I assume, would it be likely to last longer?.

 

[Adam Anderson]

On 24/03/2010 12:11 PM, Mark Harriss wrote:

Adam Anderson wrote:
Mark Harriss wrote:

I'm looking for absolute rotary encoders with at least 18 bit
resolution for a product currently in design, does anyone have
any brands or products they can recommend / not recommend?.

TIA
Mark


Optical encoders at that resolution will fail after a period of time
as the graticule's are too fine, even given small mechanical
disturbances.

Geared encoders at that resolution will not perform due to mechanical
backlash.

you need dual speed synchro resolvers and a resolver to digital
converter.


Thanks for the information Adam, do you recommend any specific brands?.
The Sick Stegman unit I was looking at has a metal disc of nickel alloy,
something like invar I assume, would it be likely to last longer?.

I couldn't say for sure as I've no experience with that product. If the
atmosphere in the encoder was inert (eg argon) then that would help.
I'd definitely want extended warranty.

Data Devices Corporation (US) manufacture the Resolver to Digital
converter cards & IC chips. There are some export restrictions that can
apply, to prevent their use in missile guidance systems and the like.

Harrowe make resolvers but It's been a while and I'm not sure if they
make dual speed units or not (electrically geared not mech gearing).

 

[Adam Anderson]

A little more on why not optical.

The least significant bit in the optical wheel will (assuming binary
encoding and not BCD etc). will change state 2^18 times per rotation.
this means cutting a slot for the LSB every 2nd place or ever 2^17.

Lets assume our optical encoder is 1 meter in diameter (a bit rediculous
but anyway) the circumference will be 1xPI simplifies to PI.

lets change to millimeters 3141.5mm so our graticule spacing is
3141.5/(2^17) = 0.02mm spacing, pretty bloody fine spacing.

As always when putting maths on the web it's probably full of in correct
assumption and mistakes, so take it with a grain of salt, and do your
own calcs.

Cheers
Adam

On 26/03/2010 10:28 AM, Adam Anderson wrote:

On 24/03/2010 12:11 PM, Mark Harriss wrote:
Adam Anderson wrote:
Mark Harriss wrote:

I'm looking for absolute rotary encoders with at least 18 bit
resolution for a product currently in design, does anyone have
any brands or products they can recommend / not recommend?.

TIA
Mark


Optical encoders at that resolution will fail after a period of time
as the graticule's are too fine, even given small mechanical
disturbances.

Geared encoders at that resolution will not perform due to mechanical
backlash.

you need dual speed synchro resolvers and a resolver to digital
converter.


Thanks for the information Adam, do you recommend any specific brands?.
The Sick Stegman unit I was looking at has a metal disc of nickel alloy,
something like invar I assume, would it be likely to last longer?.

I couldn't say for sure as I've no experience with that product. If the
atmosphere in the encoder was inert (eg argon) then that would help.
I'd definitely want extended warranty.

Data Devices Corporation (US) manufacture the Resolver to Digital
converter cards & IC chips. There are some export restrictions that can
apply, to prevent their use in missile guidance systems and the like.

Harrowe make resolvers but It's been a while and I'm not sure if they
make dual speed units or not (electrically geared not mech gearing).

 

[David]

On 28/03/2010 2:56 PM, Adam Anderson wrote:

A little more on why not optical.

The least significant bit in the optical wheel will (assuming binary
encoding and not BCD etc). will change state 2^18 times per rotation.
this means cutting a slot for the LSB every 2nd place or ever 2^17.

Lets assume our optical encoder is 1 meter in diameter (a bit rediculous
but anyway) the circumference will be 1xPI simplifies to PI.

lets change to millimeters 3141.5mm so our graticule spacing is
3141.5/(2^17) = 0.02mm spacing, pretty bloody fine spacing.

The optical spot size of a Blu-ray disk is 580nm, over four orders of
magnitude smaller. Shouldn't be an issue to get 2^18 resolution.

David

 

[Mark Harriss]

David wrote:

On 28/03/2010 2:56 PM, Adam Anderson wrote:
A little more on why not optical.

The least significant bit in the optical wheel will (assuming binary
encoding and not BCD etc). will change state 2^18 times per rotation.
this means cutting a slot for the LSB every 2nd place or ever 2^17.

Lets assume our optical encoder is 1 meter in diameter (a bit rediculous
but anyway) the circumference will be 1xPI simplifies to PI.

lets change to millimeters 3141.5mm so our graticule spacing is
3141.5/(2^17) = 0.02mm spacing, pretty bloody fine spacing.

The optical spot size of a Blu-ray disk is 580nm, over four orders of
magnitude smaller. Shouldn't be an issue to get 2^18 resolution.

David

A lot of these discs are glass with a sputtered aluminium mask, any
moisture and you'll get corrosion or fungus. I've found a unit which
may work but lower in resolution, it comes with a 2 year warranty.