I’m about to buy the RobotShop M100RAK Modular Robotic Arm Kit. But it comes with no electronics (which is the part of robot projects that I always get bogged down in.) The one company that seems to cater to folks like me is Phidgets. You just plug their cards in and start programming from the PC without any additional parts or wiring. And Phidgets does sell a “Phidgets Advanced Servo 8-Motor”. So I was about to buy it for use with this arm, but I see this phidget powers servos up to 1.5A. But the arm documentation says “each servo can potentially draw up to 1.8A under full load, so we suggest a power supply capable of at least 4A at 4.8V and 5A at 6V.”
So if I use this controller with this arm, will I just get a lower holding power, or will I burn out the controller (or blow a fuse if I become clever enough to know where to put one)?
Is there a better set of electronics to use with this arm? Money is not a constraint, but simplicity is (a solution that gets me programming quick is a good one; a solution requiring me select more components to get this thing connected to a PC is an inferior solution; a solution requiring me to solder is a danger to me and those near me).
Thank you
Greg
(Can you tell I am a software guy, not an electrical engineer?)
I should note that I intend to run the entire contraption including the arm and the pc controlling it from a car battery. So I think I will have enough total amperage from the supply. (I know I will need voltage regulation, etc)
And I modify my previous statement: "a solution requiring me select more components to get this thing connected to a PC is an inferior solution. However, if someone can help me figure out exactly what electronics to buy to make this arm functional and controlled from a PC, that is an adequate solution (provided there is no soldering involved).
Coleman,
Thank you for the response. I will purchase the arm and card tonight. I do intend to use 6V, its just coming thru either a transformer or inverter from the 12V battery. I still have a few questions and comments:
I am confused by a note in the scc-32 user guide. It says “Caution! The onboard regulator can provide 250mA total. This includes … and any attached peripherals.” I dismissed this card earlier because I thought that means that it would not work with the 1.8 Amp Servos of the M100RAK. Am I just misinterpreting that line?
Additionally, you say “you can get away with [using an under powered controller] by using the servo controller for the signal only and connecting the voltage lines to the power supply directly.” It is unclear to me if you are saying that I might get away with doing this with the Phidgets board or if you are saying that I need to do this even with the ssc card you recommend. Can I hook the motors directly to the power row on this SCC card?
Also, do the servo leads on the robot reach the base where I would install the scc card or do I have to purchase some extenders of some kind to make these wires reach?
Can I get support libraries (DLLs?) for the scc for use on the PC C++ code? (From what I can see from the scc manual, I will have to program the serial line protocol instead of just making function calls right out of the box. This is why I love the Phidgets, you just add their library to your program and start making function calls.)
Thanks for the warning about the arms strength. I think I will be ok in this respect. For now, the arm’s task will holding a camera and an electrical cord. When the mobile bot that carries it notices its power is low, it will use the camera(s) and some vision processing on the PC to find an electrical outlet and move the bot close enough that the arm can plug the bot’s recharger in.
Welcome to the RobotShop Forum. Some servo motor controllers are quite small and are intended to run only small or standard sized servo motors. As such, the physical traces on the board are only so thick and the maximum current they can handle is low. However, you can get away with this by using the servo controller for the signal only and connecting the voltage lines to the power supply directly. The Phidgets servo controller is a good choice, but the traces may not be wide / thick enough to handle the necessary current. at 1.5A per servo, it’s borderline and would need to be tested. We tested the arm with the SSC-32 servo controller which worked well, and the commands are straightforward.
You can add a 10A fuse anywhere along the positive wire coming from the battery, but it should be able to handle spikes (startup current can be very high but only lasts a few milliseconds). A resettable fuse may save you money. Don’t push the arm too much; it may look robust and be significantly more powerful than hobbyist arms, but it’s not as strong as a human arm.
A car battery operates at 12V and would immediately fry all the motors. You need a 6V lead acid battery and use a multimeter to check the voltage when it is fully charged (if it’s charged to above 7V, be wary that is the absolute maximum operating voltage of a servo motor and you may want to discharge it a little or create a simple voltage divider circuit to reduce the voltage).
The SSC-32 has jumpers to allow you to power the microcontroller (ATMega168) from the same power supply as the servos and vice versa. VL (the logic voltage) can only provide so much current; enough for say, 2 standard servos. If you want to use additional servos, or the 785HB servos used on the M100RAK, you would need to remove the VL=VS1 and VL=VS2 jumpers and power the servos separately.
The SSC-32 has two rows for servos and screw terminals for each row as well as a screw terminal for VL. If you only provide power to VL (not VS1 or VS2) all the current used by the servos is being routed through the onboard voltage regulator (which can only provide up to 250mA). The designer included the VS1 and VS2 screw terminals so you can provide power directly to the servo motors and not worry about the onboard voltage regulator. Our suggestion is to connect the servos to one line (VS1 for example) and keep the jumper between VL = VS1 and provide the 6V power to the VS1 screw terminal. Many servo controllers (especially the smaller ones) rely on the microcontroller to provide the current used by the servos. For whatever reasons, the Phidgets servo controller can only handle up to 1.5A.
The M100RAK includes the necessary high current servo extension cables which reach to the base.
What you see is what you get with the SSC-32. The commands are very simple ("#1P1500" - send a signal of 1500 to servo connected to pin #1), and setting the baud rate is a matter of adding/removing jumpers. The choice is yours - there are a number of other servo controllers which would do well, so take a look through the documentation to see which you are most comfortable with (and can handle enough current).
Very cool. We hope you will post some images and video.
