Replicating an Encoder

Yes 4096 is likely the encoder resolution. It means 2¹² either 12 bits. Three wires are likely serial connection to controller [RxD TxD and common]. Either particular protocol might be applied. It's electrical and time sampling part could be hacked by skilled person.

I'm using the encoders in my work which I do make myself as I also do not prepared to purchase these stuff for $200. Unfortunately it's common price for industrial good brand encoders.

Thanks for your response Caramba.

I checked the encoder inventories at DigiKey, Inc. and as close as I can guess, these types of encoders sell for about $25 each here in the US.

I need more certainty however, before I go and attach new ones to the microprocessor.

I do not want to save a few dollars on new encoders only to damage expensive circuitry.

Thanks

L. J.

The 4096 is probably the resolution or maximim count for 1 revolution

The J35193 is probably the part or model number of the manufacturer.

The high cost is related to the high resolution. In that small item there is probably 4096 holes on a miniature disk that a LED is shone through to a sensor.

Try here

or

Google low cost optical encoder to find many manufacturers.

Here is an example of one

http://www.quantumdev.com/?gclid=CP7PhIvG4JUCFQEGQQod5EY7WQ

Another option is to combine multiple encoders of lower resolution. i.e one encoder of resolution 4 (coarse) and one encoder of resolution 1024(fine). Combined, these = 4096

Thanks Techno., I looked through the links. Unfortunately, I do not know enough about these devices to know what I am looking at and compare them to what I need.

Mine seems to be the simplest of the lot. One thing I did not mention is that it apparently looses its positional references when the power is off. I suspect this as the telescope needs to be aimed at a known target and an indexing button pressed to lock the X,Y position. Then a second target is done too.

From what I gathered, there are encoders that loose data when power is off and mine are apparently of that type.

I appreciate the support but need help in choosing.

L.J.

It looses data because the encoder can turn continuously. It is basically counting pulses as it turns. Each pulse represents a certain amount of movement. In other words the encoder will use the same spot on its wheel for many different positions of the telescope.

If the system is physically locked before it is turned off, the system is pretty sure it can start where it left off. A well designed system should not have to re-calibrate unless the alignment is disturbed.

Techno wrote: "A well designed system should not have to re-calibrate unless the alignment is disturbed."

In an ideal world, a lot of things would be different. This system is the one I have and I must deal with it.

As far as I can tell, there is no way of saving positional information when the microprocessor is turned off. No juice. No memory. If my assessment is accurate than fixturing the shafts won't help.

In addition, since it's probable that I would move the unit to another telescope, any stored encoder data from the earlier platform would be of no value anyway.

It's also possible to loose encoder registry if one turns the scope mount too quickly. Some encoders can deal with that but I doubt these can.

I'm content that installing the suggested encoders will be sufficient. All I need do is to maintain the same drive ratio on all the scopes so equipped, when coupling the mechanicals.

Thanks for the support.

L. J.

Right,

To save positional information an a stable platform, it is simply a case of storing the co-ordinates as variables on the hard drive. If the system is mobile, these coordinates are useless.

When I wrote this, I also forgot that the Stars (reference points) are constantly moving (in relation to the telescope). Even if you stored the physical deflection of the telescope, the data would be useless, unless it was compared to a time referenced star map.

As I could understand your system has lost referencing between db indexing and encoder's code.

I'm wondering how was formed your db. Whether it was pre-defined on factory or it could be build gradually by user?

In any case it might be look as follows[Excuse me,I'm not a power user of astronomy]

Index on sky map->Coordinate X->Coordinate Y->Date->Time->Code1->Code2 etc.

Code x --- is code transmitted by encoder which consists: number of full revolutions in respected direction(CW,CCW) and remains of code after last full revolution [up to 4095].

Have you ability by any means to direct your telescope to known object [I don't know for instance North Star] and let your data base know that it should be referred with this one in connection with generated encoder code? What is the initial db's entry when you push Reset knob? I.e. has your Tscope predefined mechanically locked position for encoder calibrating?

I'm assuming there should be something like as adjustment procedure. But I am not sure is it acceptable for end-user.

The database was pre-defined by the factory and can also be expanded by the user.,

"Have you ability by any means to direct your telescope to known object?"

Yes. When the system is first turned on, it first tells you to point to a known target in the database. After indexing, it tells you to point to a second known target in the dabase.

I assume that there is some triangulation protocol that then calculates the position of the telescope.

The process must be done quickly. If too slow, the system times out and asks you to start over.

L. J.

I've spent many hours trying to learn enough about encoders to identify the one I have and buy several more. Thus far I have not been able to achieve the comfort level needed to spend the $25 each. In fact, I'll bet they are even less than that. I'll need two.

I have a suspicion that this unit I have is as simple as they get. It spins in both directions. It has no switch and has four wire male connector and has a resolution of 4096.

L. J.

Maybe its something like a Hall effect sensor?

I've worked with Hall effect sensors. There may be different types but all those I worked with used magnetic fields to time the ignition system.

These are definitely optical

Thanks for trying

L. J.,

These look a bit like the one in your picture:-

http://www.motioncontrol.com/products/index.cfm/Incremental-Encoder--Incremental-Modular--Kit-Encoder--Mounting-Kit-Required

Randall wrote: "These look a bit like the one in your picture. . . "

Yes they most certyainly do.

Thanks!

L. J.

I think you may be looking at this back to front.

When you switch the computer and telescope ON and you set to a known position, program sets the encoder position count to zero or 1.

When you want to move to a particular star, the program looks into the database and knows that stars co ordinates. Those co ordinates are likely to be a number of steps, whether clockwise or anticlockwise form the current position - lets say zero, and drives the 2 motors to the required position.

The program knows the current position so when a new star is requires to be observed, the co ordinates are obtained and simple maths will allow it to drive the telescope to a new position.

Switch the computer off and those calculations are lost. Even if the last position is saved, how can you be sure that the telescope has not been moved so the actual position of the telescope is now different to the saved co ordinates. The encoder is a passive component - it only works when it is asked to.

Does the existing encoder plug unto a module on the telescope or into a black box or directly to the PC. Somehow the number of pulses (1 revolution = 4096) has to be counted and presented to the PC so the PC can do the necessary calculations.

I feel the requirement is a little more than getting another encoder.

Tony

Thanks Tony!

Everything you wrote is correct., However, there is no automated "Go To" instruction given to the motors.

The two encoders have no control over those motors and are part of a seperate aftermarket device. While both the scope axis are driven by stepper motors, they are only used to keep the scope on target when the object is found.

Any time the device is shut down, the encoder position data must first be restored in the microprocessor as I described above.

The user now dials up an object in the database, say Jupiter, and then pushes the scope into position manually until the two digital readouts read Zero. If the original set up was done properly, Jupiter should be visible in the eye piece. The steppers then keep the scope on track.

Thank you!

You are also working with moving reference points - the stars rotate around the north star as th earth rotates. If you turn the machine off, one hour later the stars are not in the same relative position.

It is difficult to tell you the exact model of the encoder you need without knowing the brand of the DSC you are using... However from the picture it is a 99.9% optical encoder. Also who asked $300 for a pair of encoders - that is way too much? I doubt you can get the encoders you need for $25 each unless you get them off ebay. It is more likely you need to spend at least $100.

4096 is the resolution of the encoder after quadrature conversion. The optical encoders are usually characterized by CPR (counts per revolution). To get the resolution of your encoder just multiply CPR by four.

You can most likely use optical encoders from US Digital:

http://www.usdigital.com/products/encoders/incremental/rotary/shaft/s1/
http://www.usdigital.com/products/encoders/incremental/rotary/shaft/s2/
http://www.usdigital.com/products/encoders/incremental/rotary/shaft/s5/

http://www.usdigital.com/products/encoders/incremental/rotary/shaft/s6/

You can also get encoders from JMI:

http://www.jmitelescopes.com/

You can also try:

http://www.skyeng.com/ (go to 'Price list')

You need an encoder with at least 1024CPR. The more popular these days is one with 2500CPR - that gives you better pointing accuracy (360 degrees / 10,000 = 2.16 arc minute resolution).

Best Regards, AD.

Astrodevices wrote: "It is difficult to tell you the exact model of the encoder you need without knowing the brand of the DSC you are using..."

Astro, It's the top-of-the-line JMI NGC-MAX

"Also who asked $300 for a pair of encoders - that is way too much?"

JMI!

However, now that I see what's involved, the price, which is actually over $300, is not that bad. When you figure that they are providing the mounting brackets, cable and connectors, the two gear sets (driving and driven gears) and assume a cost of $100 per encoder, they aren't overcharging. I can see that now.

Still, I simply can't afford to spend that much to convert each scope to take advantage of the existing DSC controller.

And there are six more under construction including a 15" F-4.3 on an all composite equatorial fork with a Mel Bartels, Sidereal Technology drive.

And a 13 inch F-4.6, also on an equatorial Fork drive.

It appears you've gotten the ball into the strike zone. Is there anything else you wish to tell me?

Thanks

L. J.

Laughing Jaguar, All the encoders I mentioned in my post will work with JMI NGC-MAX just fine! They are all optical incremental encoders with 5V TTL levels. Also, you should not have much difficulties with the cables if you want to make them yourself as the manual for NGC-MAX should contain the pinout description. Yes, $300 sounds right for the encoders and the mounting hardware. You can always purchase the encoders and mount them yourself and save a few dollars. Best Regards, Astro Devices.

AstroDevices wrote: "You can always purchase the encoders and mount them yourself and save a few dollars . . . "

That's the plan Astro. Thanks.

Thanks, also, to all those who offered information and support

L.J.

Laughing Jaguar,

It sounds like you have a incremental encoder (position lost when power is removed)as opposed to an absolue encoder (position retained). Have a look at the attached link and it might prove useful.

http://sensors-transducers.globalspec.com/Industrial-Directory/absolute_encoder

Kind Regards

Mr. W.A Snow

Hello, Laughing Jaguar,

In any case you should be aware what a protocol is using by your encoder [it might be CAN or any other]. If you would get specs of using at your telescope encoding protocol it'll be no matter what exactly encoder's brand will be installed as replacing assembly to any your device.

Hack the protocol[electrical and programming parts] or find out any legal ways to obtain encoder specs either all your endures are believed to be useless.

As for bothering you lost of db indexing due to power off, I would advice you to find out the Power Supply pins on your encoder and connect there additional source of power which presumed to be always ON.

Good luck.

caramba

Caramnba wrote: "I would advice you to find out the Power Supply pins on your encoder and connect there additional source of power which presumed to be always ON."

That's an excellent idea caramba however, the lost position information is quickly and easily restored. I'd prefer to keep this simple.

Thank you!

L.J.

It can help either it cannot due to particular specs of this encoder. In a word if your encoder has own "brains" to store position code. Otherwise if it supplied simplest protocol converter chip which can only count revolutions and flag its directions it won't help. Your PC-mouse [no matter ball either optical] is an encoder too, but barely if even you'll feed it by UPS it permit you to save last cursor position on your desktop after power off.

But you can try this option out to check the idea. Stand alone controller for your encoder can be placed on plate or it cannot.

Good luck.

caramba