Our team is running low on PWM ports, and Me and a teamate have gone through the rules several times, and it leaves this quesiton we have very gray. We want to know if it is legal to hook more than 1 servo motor to 1 pwm port. (Meaning have a Y-type cable to split the PWM to run 2 servo’s, etc).
We are in a crunch, like you are too, a quick response would be greatly appreciated.
I don’t have time to look through the rules right now and decide if this is legal or not, but I do have another suggestion for you.
Teams are provided with 2 Digital I/O modules for the cRIO and 2 Digital Sidecars. Are you currently using both? This would provide a total of 20 PWM ports if memory serves which would be an awful lot to be using on one robot.
yes we are using both. we have A LOT of servos and pwm suckers.
I would say it’s legal if it’s not expressly forbidden in this years rules. This has been legal in the past.
You can’t split the output of a victor or jaguar but you can split PWM signals to drive more than one device. We have multiple conveyor belts but they use the same type of motor and always work the same direction so we are splitting the PWM signal going to the victors.
There are electrical limitations on how many servos can be run off a PWM, but I don’t know of a specific rule prohibiting this. We have used servos to shift our transmissions in the past using a “Y” PWM cable, since they have to shift at the same time. You can always use the Q & A to get an official answer, and I will look over the diagrams and rules for anything specific. 
I would be very interested to know what all you are using.
FIRST lists in the manual that each sidecar PWM output has a max of 15 mA of current. You can go to this link to see for yourself on Page 30.
http://www.usfirst.org/uploadedFiles/3-FRC%20Control%20System-Component-Datasheets-Rev-0-5a.pdf
I could not find an amp rating on the HiTec servos, only voltage and speed on the HiTech site. You can call HiTec at 858-748-6948 and speak to their tech dept on feasability and specs. The drawings of the sidecar outputs show one servo per PWM. Remember running amperage will be much lower than stall amperage, which is what will cause you problems (aka blown) in the circuit.
When you place a jumper over the supply-enable pins for the PWMs (which must be done for those running servos), the 6V power supply is tied into the cable going to the servo. This has a much higher allowance, and the data sheet for it is found on page 31 (IIRC).
The 15mA refers to the drive strength of the signal. Jaguars and Victors have opto-isolators on their signal input, which means they draw current. I believe that most servos don’t have an opto (though I could be wrong).
The servo 6V supply has 3A available for actual power.
I do not actually know the rules on N servos per output. Please Q&A it.
I gotta admit I’m curious as to what you could possibly be doing that would require 20 PWMs… we’ve never had more than 16 available to us up to this year, and even then I don’t think my team’s ever used more than 10 and that was a complex robot.
Last year our team has split the pwm signal to two shifting servos and the inspectors were ok with it. In '06 we split the signal to two victors and I don’t think that the inspectors were not so happy with it.
You might want to consider using some relays to run motors instead of just speed controls to free up some pwm ports.
I’m wondering the same thing. If I counted right, there are 15 motors available for use. Even if you power all 15 through speed controllers you would still need to be adding 6 or more servos to run out of PWM ports. That is one heck of a complex bot to have that many moving parts.
Can you post a picture of your bot, I am very curious to see why you are needing all thoose PWM ports!
~DtD
Hotwings,
In the past the RC controller manual allowed a “y” cord to establish a PWM signal to be split to two speed controllers. Since Eric has pointed out the DSC is current limiited to 15 ma, I would not suggest you split a PWM signal on this year’s control system. You run the risk of over current on a single PWM output with two opto couplers so connected. Also as Eric pointed out, the jumper to allow power to a servos is limited to 3 amps. You will need to determine the current rating on each servo type you will be using and make sure your design would remain below the three amps.
Oops, sorry I wasn’t clear.
Signals:
If the servo’s aren’t opto-isolated (and therefore have a high impedence input), they will draw a negligible amount of signal current. If this is the case, you could theoretically run many servos off of a single signal.
If they are opto-isolated, they will draw current. If they draw too much current, they won’t work reliably. However, the DSC won’t be damaged. It is internally protected with series resistors.
Power:
Each pin is rated for 750mA, the total supply is 3A.
If you require more than 750mA, you will need to use more than one power and ground pin. If you draw more than 3A total you will run the risk of triggering the internal over current protection. If this occurs, no damage to the DSC will occur but your servos will stop working.
In short:
The DSC is relatively well protected.
If you need more than 750mA, double up. Failure to do so might cause damage.
If you try to go out side of the specifications, things might not work.
That being said, there is an unfortunate difference between “can” and “may”. Even if you “can” safely and reliably double up your servos, you will have to check to see if you “may” under the rules this year.
I have revisited my analysis of the technical feasibility of using two servos off of one output. My musings are at http://forums.usfirst.org/showthread.php?t=11864
Long story short, the damage threshold is higher than originally thought (I was using the wrong part ). Bear in mind that there is still no guarantee that it will work, and the determination of whether or not it is worth the risk is left up to the team.
Again, there are three gates to pass:
My comments only pertain to gate 2.