Encoder selection to use with NI PCI 6221 for an Inverted Pendulum Project

[Geank]

Hi, I'm a mechanical engineering student is his final years, for my final project I'm doing an inverted pendulum system, the university already provided me that data acquisition card which is a NI PCI 6221, and I have to get the other components (dc motor, encoders, cables, servoamplifiers, etc).

My concern is about selecting the right encoder so I don't have any problems... I've been told that the best way to measure the cart position is using an absolute encoder attached to the motor shaft in some way and incremental encoders for measuring angles of pendulum, but I don't know if the 6221 can handle that type of data of an absolute encoder, and if it does, what are the main parameters for its selection? like format, how many bits.

In the case that it doesn't I'd have to go with the incremental encoders option for both measuring the cart position and pendulum angle, I know that the 6221 can handle quadrature encoders inputs, there´re plenty of examples about it, but since the models of incremental encoders are pretty wide there´re some features that worry me: frequency response vs. sample rate, and the output type.

I found a catalog that has two types of digital incremental encoders says their models have 300 KHz of freq response, being the only differences the output and power supply, anyway the 6221 can handle that freq resp? because the card's sample rate is 250 KS/s, Would it be any conflict?

They offer two output types: TTL/74LS04 and Line driver, and even if I go further with the hohner encoders they have the following outputs/freq resp: RS422 (TTL compatible)/300 KHz, Push-Pull Differential/200 KHz, NPN Open Collector/100 KHz and Push-Pull without complementaries/200 KHz.

Any help would be well received 🙂

PD: I don't know if a similar topic has been posted already, I'm kind of new on this... I searched in other posts, but found nothing, 

[Abel_Souza]

Hi Geank,

The Sample rate of 250 KS/s is for analog input only. On digital input you have on port 0, with 8 lines, an 1MHz update clock. The others digital ports are software timed only. You can see it on the following link, this link is the specifications manual to NI-6221.

http://www.ni.com/pdf/manuals/371290g.pdf

With this DAQ board you problably only can read an absolute encoder until 8-bit resolution.

Regards,

Abel Souza

NI Brazil

[Geank]

Thanks Abel, now I think I'll have to desist from using an absolute encoder, and I get that with the 1 MHz update clock I wouldn't have problems reading an incremental encoder  with a 300 KHz frequency response right?, anyway, what about the incremental encoders output? which one of the ones I mentioned suits better for the digital inputs of the PCI 6221??

And another question, I've seen the counter terminal charts for the PCI 6221 as is showed here:

...and it seems like it can only support two incremental encoders, is there a way to connect a third one?? and if not can I use an incremental encoder with sine/cosine analog output and use any of the PCI 6221 analog inputs available?

Thanks in advance.

[Abel_Souza]

Hi Geank,

PCI-6221 is a M Series DAQ Device, according the following link the best option for M series is the Push-Pull Encoder.

http://digital.ni.com/public.nsf/allkb/0C5091E9099059BC86256FC1007947AA

You cannot connect a third incremental encoder on PCI-6221 and analog input doesn´t have speed to read encoders. The options to you are using 2 DAQ boards or use a Counter Board.

The following link is the user manual for your PCI-6221.

http://www.ni.com/pdf/manuals/371022k.pdf

Regards,

Abel Souza

NI Brazil

[Geank]

Thanks Abel, I read that article about what encoder should I use with an M series board, the problem with the push-pull encoder (at least the ones on the hohner catalog) is that the output signal is < 2.5 V for low level and > 8 or 27 V for high level, while the max digital input on the 6221 is 5 V, which leads me to the following question: the use of a push pull encoder is just a recommendation for better or best performance OR it would work too -maybe with a not so good performance as with the PPencoder- if I use a RS422 (TTL compatible)? I ask this because it's the only one with an 0-5 V output. (I attached a snapshot of the output characteristics so you can have more detailed information)

Maybe there're push pull encoders with a 0-5 V output signal (I'll check out), but the hohner ones are way easy to buy in Ecuador, at least for now.

As for using 2 DAQ's boards, I have an USB-6009 which I think has a counter but not suitable for bi-directional measurement, a NI 9435 and a NI 9403, but these two have no counters...I´ll have to check if we have more boards available :-S

[Geank]

Update: I've been told that we have 4 PCI-6221 so we can use all the ones I need (2 in this case), so now: would it be any problem if I use 2 PCI-6221 at the same time on the same computer?

[erick.yamamoto]

If you have enough slots on your motherboard. Its ok.

[Geank]

Thanks Erick, and what do you think about the encoder output signal type? a NI article says that for M series boards is better to use Push-Pull encoders... but the ones from hohner have a 9-27 V max output, the only one that is between 0-5 V is the RS422 (TTL compatible), is there any problem if I use the latter one? I provided a screenshot of the characteristics of each type of output signal available, thanks in advance.

[Abel_Souza]

Hi Geank,

The better option is use an Push-pull encoder, but you can use yopur board to acquire data from the encoder types.

In the following link have an 5V push-pull encoder.

http://www.theencodercompany.co.uk/pdf/encoder_outputs.pdf

Regards,

Abel Souza

Applications Engineering

NI Brazil

[Geank]

Ok, but just to make it pretty clear (sorry if I'm tedious about it, but since a good amount of money is gonna be spent I need to be totally sure of what I´m gonna buy hehe :)) , in the hypothetical case that I couldn´t get the 5 V push-pull encoders, would it be any problem if I use the RS422 (TTL compatible) ones?? I ask this because the ones of hohner that I posted here are the easiest to get here in Ecuador.

Anyway, I'm gonna try to get the 5 V push-pull ones even if I have to buy them outside Ecuador.