blog; Motor Controller Options for 2015

[FrankJ]

Since I don't speak for the Cross the Road people, this is based on what I know about CAN. CAN is a two wire bus. It is intended to be daisy chained. That would be the reason for two pairs. One in & one out. The legs of the pairs would be connected together in the Talon. I know some people use the star configuration for CAN & get away with it. It is not to the standard & it in not a best practice.

[Oblarg]

Originally Posted by FrankJ

Since I don't speak for the Cross the Road people, this is based on what I know about CAN. CAN is a two wire bus. It is intended to be daisy chained. That would be the reason for two pairs. One in & one out. The legs of the pairs would be connected together in the Talon. I know some people use the star configuration for CAN & get away with it. It is not to the standard & it in not a best practice.

My only problem with the daisy-chain configuration is that it greatly exacerbates the failure mode. It's rather annoying to lose half of your motor controllers instead of just one of them.

It would be nice if there were a supported alternative.

[wilsonmw04]

Originally Posted by Oblarg

My only problem with the daisy-chain configuration is that it greatly exacerbates the failure mode. It's rather annoying to lose half of your motor controllers instead of just one of them.

It would be nice if there were a supported alternative.

There is an alternate: PWM's

[Oblarg]

Originally Posted by wilsonmw04

There is an alternate: PWM's

While in the past I would have wholeheartedly agreed, this is a far less-appealing alternative with the new control system.

[Monochron]

Originally Posted by Oblarg

While in the past I would have wholeheartedly agreed, this is a far less-appealing alternative with the new control system.

Do you mean with the robotRIO, or because CAN benefits are now available. If the former, why is everyone so down on using PWMs with the roboRIO?

[Oblarg]

Originally Posted by Monochron

Do you mean with the robotRIO, or because CAN benefits are now available. If the former, why is everyone so down on using PWMs with the roboRIO?

The former.

AFAIK, in order to use solenoids, you must use CAN.

[notmattlythgoe]

Originally Posted by Oblarg

The former.

AFAIK, in order to use solenoids, you must use CAN.

Correct me if I'm wrong, but I believe that the system is still plug and play with the modules. There is nothing extra needed to make the solenoid module work, the CAN implementation is in the background.

[NotInControl]

Originally Posted by Oblarg

The former.

AFAIK, in order to use solenoids, you must use CAN.

This may not be entirely true. While the new system does have a PCM module that automatically controls the compressor/pressure switch and has support for 8 solenoid channels.

Nothing stops you from using the 4 relay ports on the RoboRio to drive solenoids. Without needing to have a PCM.*

You can use One relay for the compressor, and 3 other relays for double acting solenoids. In fact, this is how we were running our pneumatics system using the RoboRio during Alpha testing when the PCM modules weren't supported yet. Only if you exceed 3 double solenoids, would you need to venture to use the CAN PCM module.

And even if you use the CAN module for pneumatics, that doesn't mean you can't or shouldn't use PWM. In fact, I believe most veteran teams will continue to use PWM on their drive train as a minimum despite the new control system, due to PWMs proven reliability and known failure modes. I believe this will be true even if they choose to use CAN motors elsewhere on their robot. Nothing currently prevents a mix use of CAN and PWM on the Robot.

*This is true as long as the 2015 rules do not prohibit this. Doing this is perfectly legal under 2014 rules.

Regards,
Kevin

[marshall]

Originally Posted by FrankJ

Since I don't speak for the Cross the Road people, this is based on what I know about CAN. CAN is a two wire bus. It is intended to be daisy chained.

Sorry Frank, but I'm really not sure where you are getting this from... In fact, you said it yourself, CAN is a two-wire BUS. Daisy chaining is just an easy method to accomplish it. There is nothing stopping you from putting terminating resistors at each one of your end points and then running everything back to a central hub rather than daisy chaining.

To my knowledge (and I haven't looked at all of the ISO standards and heaven only knows I could be wrong) there is nothing prohibiting the use of a star topology with a CAN network rather than a daisy chain topology.

[Jared Russell]

Originally Posted by marshall

To my knowledge (and I haven't looked at all of the ISO standards and heaven only knows I could be wrong) there is nothing prohibiting the use of a star topology with a CAN network rather than a daisy chain topology.

(Passive) star topologies are more susceptible to reflections and fan-out problems than traditional bus topologies and are best avoided for CAN networks in my experience.

[NotInControl]

Originally Posted by marshall

Sorry Frank, but I'm really not sure where you are getting this from... In fact, you said it yourself, CAN is a two-wire BUS. Daisy chaining is just an easy method to accomplish it. There is nothing stopping you from putting terminating resistors at each one of your end points and then running everything back to a central hub rather than daisy chaining.

To my knowledge (and I haven't looked at all of the ISO standards and heaven only knows I could be wrong) there is nothing prohibiting the use of a star topology with a CAN network rather than a daisy chain topology.

This is not accurate. CAN can not be wired like any other network.

The protocol is named CAN Bus because it should only be used in a bus as it was intented, a bus is a type of topology. CAN can not be used in a STAR or Ring topology or Hub type topology natively without having additional CAN modules, or increasing the complexity of the layout, and even so, in the end, the Ring or Star implementation will only be a cosmetic one, and will not be more efficient then the original Bus topology. You will also loose a lot of link speed.

The current CAN products we have available in the FRC control system, including 2015, are internally hardwired, such that if a device fails, only the device fails, it does not bring down the entire physical bus. The device CANs act as pass-through so you can communicate beyond a failed device. How the software reacts to an ID that does not exist because it failed is a different story. WPI is currently working on implementing a NON-blocking CAN implementation for 2015, which should help teams have more graceful software failures.

Saying that if one CAN module goes down the entire bus goes down, or saying anything beyond the failed device is unreachable after that module goes down is not correct and shouldn't be perpetuated. However, it is a true statement that if you were to CUT the wires on the CAN BUS, you would loose all communication beyond the cut. This is where PWM differs marginally. If you wired every motor to an individual PWM channel, then you would have to cut every PWM cable to have the same effect, making PWM more robust. However, the reason I said marginally is because most teams I have encountered in my FIRST decade use PWM Y cable or even tri cables to drive up to 3 motors off one PWM channel. In this scenario if you cut the one cable, you loose all downstream communication making it very similar to the CAN problem, although you do not need to worry about what the software does if you loose the PWM connection. (Maybe this will be true for CAN in 2015 as well, I haven't beta tested the new CAN implementation yet).

I am not trying to say one is better than the other, I am just trying to clarify the rumors around these technologies so that teams can have all the proper information when choosing which one best suits their needs, based on robot design criteria and experience.

Regards,
Kevin

[marshall]

Originally Posted by NotInControl

This is not accurate. CAN can not be wired like any other network.

The protocol is named CAN Bus because it should only be used in a bus as it was intented, a bus is a type of topology. CAN can not be used in a STAR or Ring topology or Hub type topology natively without having additional CAN modules, or increasing the complexity of the layout, and even so, in the end, the Ring or Star implementation will only be a cosmetic one, and will not be more efficient then the original Bus topology. You will also loose a lot of link speed.

The current CAN products we have available in the FRC control system, including 2015, are internally hardwired, such that if a device fails, only the device fails, it does not bring down the entire physical bus. The device CANs act as pass-through so you can communicate beyond a failed device. How the software reacts to an ID that does not exist because it failed is a different story. WPI is currently working on implementing a NON-blocking CAN implementation for 2015, which should help teams have more graceful software failures.

Saying that if one ID goes down the entire bus goes down is not correct and shouldn't be perpetrated. However, it is a true statement, that if you were to CUT the wires on the CAN BUS, you would loose all communication beyond the cut. This is where PWM differs marginally. If you wired every motor to an individual PWM channel, then you would have to cut every PWM cable to have the same effect, making PWM more robust. However, the reason I said marginally is because most teams I have encountered in my FIRST decade use PWM Y cable or even tri cables to drive up to 3 motors off one PWM channel. In this scenario if you cut the one cable, you loose all downstream communication making it very similar to the CAN problem, although you do not need to worry about what the software does if you loose the PWM connection. (Maybe this will be true for CAN in 2015 as well, I haven't beta tested the new CAN implementation yet).

I am not trying to say one is better than the other, I am just trying to clarify the rumors around these technologies so that teams can have all the proper information when choosing which one best suits their needs, based on robot design criteria and experience.

Regards,
Kevin

Very useful. Thanks for clarifying.

[Tristan Lall]

Regarding the CAN bus, to some extent, there's always an aspect of star topology on the bus—but traditionally the stub length is short, possibly on the order of a couple of millimetres (in the case of a direct connection to an integrated CAN transceiver). Barring any strange internal arrangement on these controllers, if you leave one pair unconnected, you should still be able to make the connection downstream to the remaining pair, but your stub length will increase.

Here are some general suggestions for optimizing stub length, given other parameters.

[FrankJ]

Originally Posted by Tristan Lall

Regarding the CAN bus, to some extent, there's always an aspect of star topology on the bus—but traditionally the stub length is short, possibly on the order of a couple of millimetres (in the case of a direct connection to an integrated CAN transceiver). Barring any strange internal arrangement on these controllers, if you leave one pair unconnected, you should still be able to make the connection downstream to the remaining pair, but your stub length will increase.

Here are some general suggestions for optimizing stub length, given other parameters.

I think of a star topology as the line length being short (or 0) relative to the stub length. Very different than what the CAN spec recommends. Without doing the math, I think you can get away with something approaching a star topology in FRC because the entire bus length is short. I have also run just one terminating resistor without issues. But if I were to start having problems, one of the steps would be to make the bus wiring completely to spec.

Here is what the CIA-- the organization that maintains the CAN standard, not the American government entity with the same name has to say.

Network topology

This clause is most interesting for system designers.

Electrical signals on the bus are reflected at the ends of the electrical line unless measures against that have been taken. For the node to read the bus level correctly it is important that signal reflections are avoided. This is done by terminating the bus line with a termination resistor at both ends of the bus and by avoiding unnecessarily long stubs lines of the bus. The highest possible product of transmission rate and bus length line is achieved by keeping as close as possible to a single line structure and by terminating both ends of the line. Specific recommendations for this can be found in the according standards (i.e. ISO 11898-2 and -3).
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