electronics help

Farid · Sep 10, 2004

Hi to all,

I have to connect my rotery encoder to my plc inputs (incremental type), my question is: how can I extend the length of the encoder's cable, usually it is too short it has a 100cm length, And I need it to be 500cm,what Can I do to avoid losing the pulses, and what kind of cables do you experts usually use?

I had enough from getting unaccurate positioning caused by lose of pulses, so plz experts help.

thanks in advance.

best regards.Farid

Eric Nelson · Sep 10, 2004

The encoder manufacturer should be able to recommend an appropriate cable, and how far you can safely go. 500cm shouldn't be a problem though.

Here's an excerpt from the Dynapar FAQ about encoders:

How far away can I place my encoder from my system?

"There is no set answer to this question. Many factors play a role in determining the maximum length of cable that can be used to connect the units together. The largest problem with running long lengths of cable is that the cable becomes more susceptible to noise. This is due the capacitance of the cable, the cable acting as an antenna, and the loss of power through the cable. The maximum distance of cable can be achieved by following some basic wiring principles. Do not run the cable near objects that create a lot of electrical noise. This includes AC motors, Arc welders, AC power lines, and transformers. Use twisted pair cabling when using the signal and its compliment, and shielded cabling when running any type of signal. Use the highest voltage available for the output voltage. For example if the encoder will output 5 to 24 volts, then use 24 volts. Use an Open Collector or Differential Line Driver output with a differential receiver so that the maximum amount of current can be sink/soured.

If you are using the encoder as an input to more than 1 controller, use a signal amplifier. This is also a good way to help increase the distance a signal can travel. Typical maximum distances for a Differential Line Driver are around 100Ft. or more when using a differential input, and for an Open Collector the distance is around 35 Ft.

beerchug

-Eric

Farid · Sep 10, 2004

ok thanks Eric,

I will check if I can find a signal amplifier somewhere, and I think it will do the job, you are right 5 meters isn't a real problem, but I am using the signals from the encoder to measure position and I am getting 40mm flactuation (difference between readings).\

I think the cable is the problem...the actual cable is UTP.

and I think that a repeater is required.(or a signal amplifier if this is what its called).

thanks Eric for your help.

regards.

Steve Bailey · Sep 10, 2004

Before you spend money on a line driver, please answer a couple of questions about your system. Is the encoder wired to discrete input points or is it wired to a high speed counter module? How many pulses per revolution of the encoder and how fast is the encoder turning? Standard DC inputs, updated once per scan are generally inadequate for anything but slow encoder shaft speeds.

Your problem may not be the cable.

Farid · Sep 10, 2004

hi Steve,

its connected to HSC digital inputs, and PPR = 500, its not that fast, i.e its in the range of the input frequency which is 10KHZ,

now can you please tell me if this signal modifier product is ok to what I am trying to do.

my doubt is that the encoder which is fixed on the machine 5 meters away from the plc is not being able to send all the signals to the plc and I think that some of the signals are being lost cause the cable is longer than one meter(intial encoder's cable length), therefore I am receiing that unaccurate position..

Steve Bailey · Sep 10, 2004

If the cause of the missing pulses is voltage drop over the length of the cable, then the line driver could be a solution. You could also try increasing the size of the wire in the cable. If the encoder can handle it, increase the supply voltage.

Try looking at the pulse train at the input terminals with an oscilloscope. That should give you some idea of what the true problem is.

elevmike · Sep 10, 2004

Farid,

Without the benifit of refering to the documentation, (so I'm lacking in some details). I'll tell you that we use a standard 100 ppm quad encoder with 24vdc signal/power supply. The encoder has a pigtail cable that is connected/spliced to another sheilded cable with two twisted pairs. The sheild is also reconneted and pulled back over the splice. The sheild is NOT connected at the HSC, and only attached at the encoder end. The encoder case is well grounded. We have gone over 200 ft away from the HSC and have never encounted any problems.

DonsDaMan · Sep 10, 2004

Typically, we drop the encoder signal out via a connector, through about 100 feet (30 meters) of cable, to a terminal block, to another terminal block, to the encoder pick up card. The 15V or 24V power supply is dropped in at one of the terminal blocks.

We typically use 2048 PPR at 2300 RPM (115 Hz excitation more or less).

Pretty much we do this with nothing more than twisted, shielded pairs. The initial cable is typically six twisted shielded pairs, and the terminal strips are connected with twisted shielded pairs. I think we use 4 pairs (A, A not, B, B not, Z, Z not, power supply) and have the other two pairs as spares.

I think we use 18 awg throughout. Never really checked (or cared).

Noise has not been a problem.

Farid · Sep 12, 2004

Ok thanx to all...

I will change the actual cable and try, the current cable is UTP cable , I will try to use sheilded cable and see what will happen.

regards.Farid

Rod · Sep 12, 2004

Farid,

Make sure you connect your shield only at one end of the cable

Rod (the CNC dude)

elevmike · Sep 12, 2004

Rod said:
Farid,

Make sure you connect your shield only at one end of the cable

Rod (the CNC dude)

The encoder end.

Farid · Sep 13, 2004

ok thanks for every body who helped ...its ok now.

thanks .Farid

elevmike · Sep 13, 2004

Glad to hear that...

NEXT.....

electronics help

Farid

Member

Eric Nelson

Lifetime Supporting Member + Moderator

Farid

Member

Steve Bailey

Lifetime Supporting Member + Moderator

Farid

Member

Steve Bailey

Lifetime Supporting Member + Moderator

elevmike

Member

DonsDaMan

Member

Farid

Member

Rod

Member

elevmike

Member

Farid

Member

elevmike

Member

Similar Topics