LinuxCNC Support Forum :: Topic: Which of these cards to use for a five Axis Cartes (1/1)

Piscis OFFLINE Senior Boarder Posts: 45 Karma: 0	Can someone please share some knowledge, maybe some experience and provide me with your opinion on which of these cards to use for a five Axis Cartesian robot project using LinuxCNC? Step-Dir PWM +-10V Analog I need to know the advantages of using one over the other. The servo driver’s models I have to choose from offer these three options. Thanks Piscis
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Re:Which of these cards to use for a five Axis Cartes 30 Jul 2012 21:59 #22619

jmelson
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Piscis wrote:

Can someone please share some knowledge, maybe some experience and provide me with your opinion on which of these cards to use for a five Axis Cartesian robot project using LinuxCNC?

Step-Dir
PWM
+-10V Analog

I need to know the advantages of using one over the other. The servo driver’s models I have to choose from offer these three options.

Thanks

Piscis

First, you need to decide what kind of motors you want to use to move the machine. Or, is this a retrofit
to an existing robot that already has motors? Or, does it also have the motor drives? If so, then find
out what type of signals these drives require.

If you will use stepper motors, then step-direction is the most logical control method.

If you will use servo motors, then all 3 options are possible. Do you ever want to E-stop
the machine without having to re-home all the axes? If so, then step-direction is
a bad choice, as it may lose a few steps every time it is E-stopped.

If you are using motor drives that take a digital PWM signal, then you need a controller that
provides this type of signal. If you are using drives that take analog velocity signals,
than you need a controller that provides that.

Jon

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Re:Which of these cards to use for a five Axis Cartes 31 Jul 2012 07:53 #22635

Piscis
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I just register with the forum and this was my first post. Please excuse my too simplistic question for it was intended solely for knowing why and experienced integrator will prefer +-10V Analog over

Let imagine you are building a 5 Axis Cartesian Robot from scratch (All five servo motor has Absolute Encoders on it) and will like to use LinuxCNC as the controller would you use +-10V Analog, Step-Dir or PWM?

I’m having a little trouble understanding the option of +-10V Analog input into the drive. Most of the literature I have read mentions Velocity and Torque modes while robots are mostly based on positioning. Please excuse my lack of knowledge.

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Re:Which of these cards to use for a five Axis Cartes 31 Jul 2012 09:01 #22637

jmelson
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Piscis wrote:

I just register with the forum and this was my first post. Please excuse my too simplistic question for it was intended solely for knowing why and experienced integrator will prefer +-10V Analog over

Let imagine you are building a 5 Axis Cartesian Robot from scratch (All five servo motor has Absolute Encoders on it) and will like to use LinuxCNC as the controller would you use +-10V Analog, Step-Dir or PWM?[\quote]
OK, how will you read the position from these absolute encoders?

I’m having a little trouble understanding the option of +-10V Analog input into the drive. Most of the literature I have read mentions Velocity and Torque modes while robots are mostly based on positioning. Please excuse my lack of knowledge.

If the motors have tachometers on them, then a velocity servo amplifier compares the tach signal
with the commanded velocity signal and makes them match. If you do not have a way of reading
the motor speed (velocity) then you can't use a velocity servo amplifier. (It is possible to
derive velocity from the encoder, but it is not optimum.)

I already said this, but the choice of servo amplifier determines what signal you send to it.
If it needs +/- 10 V, then that will be what you send to it. If it needs a digital PWM
signal, or SERCOS, of FieldBus, or some proprietary command, then that is what you would
send to it.

Jon

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Re:Which of these cards to use for a five Axis Cartes 31 Jul 2012 09:02 #22638

PCW
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With either analog, PWM or other (SSerial/RT Ethernet whatever)
velocity or torque commands, LinuxCNC can control the position
with real time PID loop that reads the encoders and outputs torque or velocity commands to the drives. This has the advantage that tuning and following error management are all done in LinuxCNC instead of the drive.

Normally systems with step/dir input do not have feedback to LinuxCNC so they are not fully closed loop systems. Step and dir servo systems can signal LinuxCNC of a drive following error but thats about it.

Last Edit: 31 Jul 2012 09:03 by PCW. Reason: sp

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Re:Which of these cards to use for a five Axis Cartes 31 Jul 2012 09:20 #22639

Piscis OFFLINE Senior Boarder Posts: 45 Karma: 0	According to your answers these two Mesa cards should be capable of driving any Servo Amplifiers which accepts a feedback signal of +-10V Analog? 5I25 Superport FPGA based PCI Anything I/O card 7I77 Analog servo interface plus I/O daughtercard Is this right? Can these cards also accept Absolute Encoders?
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Re:Which of these cards to use for a five Axis Cartes 31 Jul 2012 09:58 #22640

andypugh
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Piscis wrote:

According to your answers these two Mesa cards should be capable of driving any Servo Amplifiers which accepts a feedback signal of +-10V Analog?
5I25 Superport FPGA based PCI Anything I/O card
7I77 Analog servo interface plus I/O daughtercard

Yes. Note also the existence of www.pico-systems.com/motion.html

Can these cards also accept Absolute Encoders?

Not directly, and it rather depends on what type of absolute encoder it is and what protocol it uses.
Do you already have the motors and drives?
Resolvers are one type of absolute encoder that requires special hardware. Both Pico and Mesa make suitable interfaces. I am not sure that they are still absolute when used with the Pico quadrature convertor. They still are with the mesa 7i49, but that is not compatible with the 5i25, you would need a 5i20 or 5i23 (or 7i43)

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Re:Which of these cards to use for a five Axis Cartes 31 Jul 2012 10:56 #22643

PCW NOW ONLINE Moderator Posts: 1717 Thank you received: 92 Karma: 116	The 7I77 is designed for incremental encoders but could be made to run some types of absolute encoders (SSI and BISS for example) Also a possible better solution if absolute encoders/PWM control was used would be the 7I85S daughtercard
	Last Edit: 31 Jul 2012 10:58 by PCW. The administrator has disabled public write access.