2015 Beta Testing - The Components are Here.

[Mark McLeod]

Originally Posted by Ether

Alternatively, have any of the beta testers experimented with driving multiple (two or even more) motor controllers with one PWM.

We tested driving two Jaguars from a single PWM, but we haven't measured impedance or max current draw of the PWM outputs.

[Alan Anderson]

Originally Posted by Ether

Alternatively, have any of the beta testers experimented with driving multiple (two or even more) motor controllers with one PWM.

We did a brain transplant of the 2015 control system into our Aerial Assist practice robot. The drivetrain motors are paired up with one PWM output driving two Talons for each side, and the four flinger motors are similarly controlled in pairs. It's been working great so far.

We have been working around a Driver Station issue introduced in the previous Beta release and haven't yet had a chance to do the brownout and stress testing that's next on our agenda. New Beta files were released this week, so we'll be updating the RoboRIO image on Monday and watching how the system responds to low voltage and high loads. Once we've gathered the information we want, I suppose we could wire up a slew of motor controllers and see how many we can reliably control with a single PWM output.

[G_rupp]

Originally Posted by Ether

Alternatively, have any of the beta testers experimented with driving multiple (two or even more) motor controllers with one PWM.

We have been running the robot with 2 Talons and 2 New Victor SP motor controllers with one PWM to each pair. We have not had any issues with this configuration and the Autonomous still drives straight with different controllers on each side.

[controls weenie]

>> I had a buddy measure the PWM 6V line from the RoboRIO (Our team does not have RoboRIO and it was 5.7 volts with a fresh 12V battery. Does this concern anyone for driving several servo motors? What would happen to the 6V line if there are 6 servos running at the same time?

>> I measured 6.0V on the older cRio. Is there a current limit on the output of each of these RoboRIO PWM 6V (5.7V ) pins? I wanted to make sure I get all the power I need for driving several servos at the same time.
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I am more concerned with the servos, not the motors. The servos are going to use the middle conductor's 6V (5.7V). I would like to know the current limit imposed by the RoboRIO designers. The servo power is directly proportional to the current supplied by the RoboRIO. It looks like we are already getting voltages lower than the spec.

[FrankJ]

While it needs to be tested, you are not really driving any power over the signal line of the PWM to the motor controllers. It is just a signal. I know that is obvious to most of the commenters, maybe less so to others.

As Controls Weenies said, it is more of an issue with the power line of the PWM driving servos & Vex motors.

[AdamHeard]

Originally Posted by FrankJ

While it needs to be tested, you are not really driving any power over the signal line of the PWM to the motor controllers. It is just a signal. I know that is obvious to most of the commenters, maybe less so to others.

Unless it's a servo or Vex motor.

[Ether]

Originally Posted by FrankJ

While it needs to be tested, you are not really driving any power over the signal line of the PWM to the motor controllers. It is just a signal.

It's just a signal, but the motor controller inputs do not have infinite impedance. They do suck some power from the signal (to light the LED in the photocoupler).

Have any of the beta teams tried driving three of the new motor controllers with one PWM signal? (three CIM gearbox).

[G_rupp]

Originally Posted by Ether

Have any of the beta teams tried driving three of the new motor controllers with one PWM signal? (three CIM gearbox).

Each Beta team only received 2 new motor controllers. We will be at a demo tomorrow with 2 other Beta teams and will see if we can test this.

[Mark McLeod]

Five Jags is the most we tried (briefly) off of one roboRIO PWM output and didn't have any trouble with that, but we weren't driving any other PWMs at the same time.

[controls weenie]

Originally Posted by AdamHeard

Unless it's a servo or Vex motor.

Right....I would like to see the 6V signal when the servos is trying to move a high inertia mass. I am guessing the 6V signal will drop from 5.7 to below 5.0 when there is a load on the servo. Can some of you RoboRIO owners try that ? We are designing a PWM extender (for servos and motors) that needs the 6V supply to stay above 5.3V.

Thanks,

[Greg McKaskle]

I used a DMM to test mine, driven by a weak transformer, and the one on our test machine which is on a 12.7V fresh battery. Both read between 5.93 and 5.97 V.

I believe the center pin is driven by a dedicated supply that limits current to 2.2A. There is not a limit for a given PWM, but for the rail.

The PWM signals from the MXP will need to provide their own power for a servo.

I'm curious to hear what others measure.

Greg McKaskle

[jhersh]

Originally Posted by controls weenie

Right....I would like to see the 6V signal when the servos is trying to move a high inertia mass. I am guessing the 6V signal will drop from 5.7 to below 5.0 when there is a load on the servo. Can some of you RoboRIO owners try that ? We are designing a PWM extender (for servos and motors) that needs the 6V supply to stay above 5.3V.

Thanks,

Fear not. This is an artifact of the back-drive protection on this supply. The current path passes through a diode when the load is low. When the current is detected to be enough greater than 0, a FET is switched on and the diode is bypassed. This means that at no load the output looks low, but as soon as there is a load, the voltage climbs to 6V.

Please see the attached image. This is a graph created using the new Power palette in WPILib (that's right, you can monitor this directly in the controller without external connections). I plugged in a servo, enabled, and then twisted the output shaft with my hand, forcing it to fight me and increase load on the power supply. You can see that under load the voltage increases, not decreases.

[Alan Anderson]

Originally Posted by jhersh

Fear not. This is an artifact of the back-drive protection on this supply. The current path passes through a diode when the load is low. When the current is detected to be enough greater than 0, a FET is switched on and the diode is bypassed. This means that at no load the output looks low, but as soon as there is a load, the voltage climbs to 6V.

That's good information to have.

For teams that just want to use the system as a black box and care little about the details inside it, the engineering of this power supply is great. It's the teams that put a lot of effort into understanding and analyzing that could get tripped up without having a good low-level description of how it works.

[FrankJ]

Originally Posted by AdamHeard

Unless it's a servo or Vex motor.

Don't they pull power from the power line not the signal line? That is why you have to jumper the power on the digital sidecar for servo & not motor controllers.

[jhersh]

Originally Posted by FrankJ

Don't they pull power from the power line not the signal line? That is why you have to jumper the power on the digital sidecar for servo & not motor controllers.

You are correct, Frank. The servos and VEX motors should not draw a significant amount of current from the PWM signal pins. I would actually expect them to be a bit lower than the other motor controllers since they are not isolated, but I have not measured them.