Prototype Power Distro /Robot Operating Amperage?

[Brayton]

Okay so I work mainly on the mechanical end of things for our team, so bear with me here as I muddle my way through my explanations. My electrical knowledge is limited to essentially things that run off of 5 and 3.3V (i.e. micro controllers).

I am working on designing a power distribution board for prototyping during build season, to avoid me looking like this when we try to power up a single motor: https://i.imgflip.com/k06v7.jpg

Anyway, I was trying to decide what switches to use, I wanted a simple Enable/Disable, possibly a polarity swap (which yes would require multiple switches), and maybe even a voltage readout. So the functionality is relatively irrelevant it is the rating of the switches I need to know. I understand the main power distribution board on the robot has 30A and 40A fuses but I can't believe we are running things at 12V DC at 40A, it just seems insane to me. I can barely find anything rated for that.

TL;DR
- Do FRC Bots actually run at 30-40A?
- Do I need an extremely heavy switch to use as a pass through from battery to a motor (i.e. Full CIM)? (enable/disable)
- If the above is true where can I get these? Would a simple light switch from Home Depot work?


If you have any questions feel free to ask, thanks for the help in advance!


Edit, Please Read:
So I have gotten a lot of great feedback and advice from all of you, thanks so much! It has made me realize how much more complex this project actually is, I was hoping to just toss some switches in a box and and pop in a battery, but it seems that will not be the case. My questions are as follows taking into account my new found knowledge:

- Can I create a variable speed controller using a high amperage potentiometer, or a drill trigger assembly? Like this one: http://goo.gl/yo78ny

- Can I use a 120VAC 20A cherry switch like this: http://goo.gl/sWVQf2 as a passthrough or is the amperage rating to low?

- Would a 15A hardware store light switch work instead? (Wouldn't make much sense to me because if the cherry switch doesn't work 15A is a lower rating already...)

- Finally my last resort is the more expensive route, and that is interfacing a motor controller, such as the talon, with an arduino and running my inputs into it to act as the control module. If this is possible (which I believe it is) resources for how would be greatly appreciated, also would I be able to step down the battery voltage to power the arduino that way I wouldn't have to have a separate power source for it?


Thanks Again!

-

[DonRotolo]

12V @ 40A = yes. The main breaker, rated at 120A, can carry twice that for short time.
No, you don't necessarily need a switch rated for 40A.
Yes, a plain light switch will work for several hundred contact closures before it fails.

You also do not need more than 1 switch to reverse polarity; you need a DPDT (Double-Pole Double-Throw) switch instead.

All that being said, your very best bet is to not power a CIM with full battery voltage. Not at all harmful, but full speed isn't always the best operating point. Better is to power it through old FRC hardware, available from many teams and on eBay cheap. Look for older Power Distribution panels, Victor/Jaguar motor controllers, cRio or IFI robot controllers and so on.

[Ari423]

Individual robot motors definitely run much upwards of 40A, just not for very long (otherwise you have a problem). A CIM in a drivetrain can easily draw 80+ amps for a few hundred milliseconds, which is too short to trip the breakers. Remember whenever you start up a motor it is initially stalled, so it will draw stall current. As it spins up, the speed will increase which will decrease the current draw. Make sure whatever breakers you decide to use give enough lead time to not trip whenever you try to start a motor.


To respond to your overall question, I know a lot of teams will take the triggers from broken 12V drills as a basic motor controller. You have to remove the drill motor and gearbox and the battery. Then connect a breaker and then Anderson connector to the input of the drill (where the battery went) and leads with your favorite crimp connector on the output side (where the drill motor was). Now you have variable speed control (via the trigger) and reversible direction (via the CW/CCW switch). You also get a handy disable switch by moving the CW/CCW switch to the middle position. If you do this, make sure the drill you are using is rated for at least 12V and less than the max amps the breaker allows.


EDIT: Don, I think the point of this low-functionality control system was to decrease the number of wires needed to drive one or two motors for a prototype. Using an old cRIO or IFI control system will probably do the opposite of that.

[VacioArconte]

Originally Posted by Brayton

Okay so I work mainly on the mechanical end of things for our team, so bear with me here as I muddle my way through my explanations. My electrical knowledge is limited to essentially things that run off of 5 and 3.3V (i.e. micro controllers).

I am working on designing a power distribution board for prototyping during build season, to avoid me looking like this when we try to power up a single motor: https://i.imgflip.com/k06v7.jpg

Anyway, I was trying to decide what switches to use, I wanted a simple Enable/Disable, possibly a polarity swap (which yes would require multiple switches), and maybe even a voltage readout. So the functionality is relatively irrelevant it is the rating of the switches I need to know. I understand the main power distribution board on the robot has 30A and 40A fuses but I can't believe we are running things at 12V DC at 40A, it just seems insane to me. I can barely find anything rated for that.

TL;DR
- Do FRC Bots actually run at 30-40A?
- Do I need an extremely heavy switch to use as a pass through from battery to a motor (i.e. Full CIM)? (enable/disable)
- If the above is true where can I get these? Would a simple light switch from Home Depot work?


If you have any questions feel free to ask, thanks for the help in advance!

I can't speak to your last two questions since I haven't personally used any (non light) switches rated for the high currents of FRC bots, but I can tell you that FRC bots do indeed run at 30-40A, and technically even higher. If you check the stall current for a CIM (http://www.andymark.com/Motor-p/am-0255.htm), you'll see that they can draw up to 133A. Now, you might ask why we have 40A circuit breakers if CIMs are running at such a high current. The thing is that 40A circuit breakers can allow current higher than 40A, but only for a limited time (http://www.andymark.com/product-p/am-0288.htm). A simple google search returned these switches rated for 50A (http://www.wiringproducts.com/50amp-toggle-switches).

Hope this helped!

[Brayton]

Okay cool, thank you all for the quick responses. I understand I'd be able to use switches like these then: http://www.wiringproducts.com/50amp-toggle-switches or possibly a light switch , but they are awfully expensive for the application, would it be possible to use something like a cherry switch: http://cherryswitches.com/us/product/rocker-yr-series/ , they do not have a DC rating listed but the AC rating is 120 at 20A, is that good enough or no?

Also with the drill trigger, is that simply a variable resistor (potentiometer) and could I implement something like that on the board? Would I need some crazy expensive potentiometer that can handle high amperage?

[Brayton]

Originally Posted by VacioArconte

I can't speak to your last two questions since I haven't personally used any (non light) switches rated for the high currents of FRC bots, but I can tell you that FRC bots do indeed run at 30-40A, and technically even higher. If you check the stall current for a CIM (http://www.andymark.com/Motor-p/am-0255.htm), you'll see that they can draw up to 133A. Now, you might ask why we have 40A circuit breakers if CIMs are running at such a high current. The thing is that 40A circuit breakers can allow current higher than 40A, but only for a limited time (http://www.andymark.com/product-p/am-0288.htm). A simple google search returned these switches rated for 50A (http://www.wiringproducts.com/50amp-toggle-switches).

Hope this helped!

Originally Posted by Ari423

Individual robot motors definitely run much upwards of 40A, just not for very long (otherwise you have a problem). A CIM in a drivetrain can easily draw 80+ amps for a few hundred milliseconds, which is too short to trip the breakers. Remember whenever you start up a motor it is initially stalled, so it will draw stall current. As it spins up, the speed will increase which will decrease the current draw. Make sure whatever breakers you decide to use give enough lead time to not trip whenever you try to start a motor.


To respond to your overall question, I know a lot of teams will take the triggers from broken 12V drills as a basic motor controller. You have to remove the drill motor and gearbox and the battery. Then connect a breaker and then Anderson connector to the input of the drill (where the battery went) and leads with your favorite crimp connector on the output side (where the drill motor was). Now you have variable speed control (via the trigger) and reversible direction (via the CW/CCW switch). You also get a handy disable switch by moving the CW/CCW switch to the middle position. If you do this, make sure the drill you are using is rated for at least 12V and less than the max amps the breaker allows.


EDIT: Don, I think the point of this low-functionality control system was to decrease the number of wires needed to drive one or two motors for a prototype. Using an old cRIO or IFI control system will probably do the opposite of that.

Originally Posted by DonRotolo

12V @ 40A = yes. The main breaker, rated at 120A, can carry twice that for short time.
Yes, a plain light switch will work for several hundred contact closures before it fails.
You also do not need more than 1 switch to reverse polarity; you need a DPDT (Double-Pole Double-Throw) switch instead.

Okay cool, thank you all for the quick responses. I understand I'd be able to use switches like these then: http://www.wiringproducts.com/50amp-toggle-switches or possibly a light switch , but they are awfully expensive for the application, would it be possible to use something like a cherry switch: http://cherryswitches.com/us/product/rocker-yr-series/ , they do not have a DC rating listed but the AC rating is 120 at 20A, is that good enough or no?

Also with the drill trigger, is that simply a variable resistor (potentiometer) and could I implement something like that on the board? Would I need some crazy expensive potentiometer that can handle high amperage?

[Ari423]

Originally Posted by Brayton

... Also with the drill trigger, is that simply a variable resistor (potentiometer) and could I implement something like that on the board? Would I need some crazy expensive potentiometer that can handle high amperage?

The problem with using a large variable resistance resistor is that a resistor's voltage drop is proportional to the current flowing through it (V=iR). So as your motor's current draw changes (when it's starting up or when you put a load on it) the voltage applied will also change. A motor controller or drill tigger does not have this problem.

[GeeTwo]

I remember thinking the same thing years ago. Why can a 40A breaker in a house can power a couple of ovens, but only one CIM, and that for a few minutes? Remember that power = VI (ignoring for now the phase issues in AC), and that house voltage is nominally 120V, or 10x the nominal voltage in a robot (or most automobiles).

One way to get a good 40A switch is to use a home circuit breaker at this rating. They are not really made for switching on and off, but they do work like that if needed -- at my church, there are no light switches in the sanctuary, we literally throw the breakers several times a week.

During this build season, we wanted a similar thing, though we were not worried about proportional speed controls, just off/on. We put together a "cable" with an Anderson battery connector at one end, a breaker and a button style automotive starter switch in the middle, and two pair of Anderson Power Poles at the other end. It's an easy way to verify that each motor is running the correct direction, provide prototype power to a pickup mechanism, and we even used it as the initial "proof of concept" of our drive train over the defenses by powering one CIM on each side of our chassis (had to swap one side or the other, of course) with the robot at about half weight.

[Peyton Yeung]

I made a briefcase with the cheap and dirty control system from AndyMark and our team has used it intensely this season. It makes prototyping and testing the robot incredibly easy, plus it has proportional control.

[Doug G]

We've made a couple of simple control boards ready for prototyping with some old FRC/VEX components... It ain't pretty but they're free since we had old Victors and Vex equipment just laying around.. Also makes for a good pre-season project for some students. We have like 4 versions of these around now.

Here's our simplest board meant to control 2 motors...



Here's our larger board that can control up to 8 motors... (get's tricky with programming and control with the Vex controller - but if you run a 4 or 6 CIM drive train, those are just mapped to a couple of PWM outputs)...



Finally our latest version has the modern FRC components... the programmers do a bunch of code testing on this thing... As well as testing the new talon on-board functions for PID and such.

[GreyingJay]

To address the last part of your post, I thought I'd chime in with what my team has discovered. We bought a couple of cheap servo testers from Amazon, specifically this one:

http://www.amazon.com/Andoer-Tester-...s=servo+tester

We attached it to a 4-AA battery holder and power switch and now we have what we call the "Team 2706 Handy Dandy PWM Tester". You can plug servos in directly, of course, but you can also pop off up to four PWM cables from the RoboRio, plug them into the tool, and verify that things are working. This was extremely helpful in diagnosing bad PWM connections or broken motor controllers on the robot.

These will also be useful in the future for prototyping mechanisms. You'll still need a 12 volt supply, ideally with circuit breakers, but you won't need to interface with a RoboRio or Arduino for basic tests. You could drive up to four motors/servos at the same speed so you could use this to control a prototype intake/shooter for example.

[Beeker]

Be careful when comparing load breaking / switching between AC to DC. Watch this video.

https://www.youtube.com/watch?v=Zez2r1RPpWY

Use relays rated for the appropriate voltage and amperage if you have to.

[Mr V]

If you only want direction control and running at full power is not a problem then the way to go is a pair of automotive relays of the 5 pin variety. They are available in a 40a rating, though there are also ones rated for 20 or 30 amps so be sure to check the specs before purchase.

The 5 pins are as follows.

Coil +
Coil -
Common (C)
Normally Closed (NC)
Normally Open (NO)

To wire the "power side"

Connect each of the motor's power leads to the C terminal of one of the relays. In other words the red wire goes to one relay common terminal and the black wire goes to the common wire of the other relay.

Connect the NC terminals of both relays to the negative side of your power source.

Connect the NO terminals of both of the relays to the positive side of your power source.

To wire the "control side"

Connect the coil (-) terminal of both relays to the negative side of your power source.

Connect each of the coil (+) terminal the relays to a separate switch, I'd go with a momentary contact style. It only needs to be able to safely switch 200ma at a nominal 12v. Then connect the other switch terminals to the + side of the power source.

Now operate one of the switches and it will energize the relay it is connected to which will supply power to the motor lead connected to it. The other relay not being energized will provide the path to the negative side of your power source. Operate the second switch and the other relay becomes the power source causing the motor to operate in the other direction.

The one problem with this set up is that it will operate like a motor controller in brake mode. This is because when both relays are are not energized the motor leads are connected together via the ground side wiring going to the battery.

The other caveat is that some relays have a clamping diode to prevent the back emf generated by cutting power from back feeding the device doing the switching. These are typically used when a transistor is used as the switching source. Other relays do not have the diode and those don't really have a coil (+) and coil (-) and thus do not have a polarity.

The most common 5 pin relay is usually referred to as a "Bosch relay" because it uses a form factor that they developed, even though Bosch is not in the relay business anymore and a number of manufactures make interchangeable relays.

The terminals on many Bosch style relays are marked with numbers which are as follows

85 Coil (-)
86 Coil (+)
30 Common
87 Normally Open
87a Normally Closed

All of that said the easier way to do this is with an extra motor controller and a device like Andy Mark's Thrifty Throttle. The Thrifty Throttle generates a PWM signal to control the motor controller so you get speed control as well as directional control.

Also be sure to provide circuit protection appropriate for the size of wire in your system. You can purchase an inline MAXI fuse holder that will allow you to use one of the 40a breakers for the PDP or an ATC/ATO fuse holder that will accept a 30a or lower breaker. The other option that you can find at the auto parts store is a self contained breaker that has a pair of #10 studs and nuts for terminals.

[Knufire]

Just get a broken 12V drill, take out the motor/gearbox/chuck assembly, and wire your favorite connector to the wires formerly connected to the motor leads.