Hello,
I would like to build myself a robot, but I am still a complete beginner and am slowly trying to get used to it.
I've already read through various forums, but didn't get the answers I'm looking for, so I'll just ask by myself now.
I imagined building a robot with 6 or 8 legs, each equipped with 3-4 servo motors. So there will be like ( 18-32 ) servo motors.
I don't want to give any specific information about any servos etc., because I'm more concerned with the principle and I don't want to get stuck on any values.
Apparently it is not possible to connect all servos directly to the Arduino and there was always the talk of an external power supply.
Now my questions:
What do I have to understand by such a power supply and how do I connect it to the Arduino?
...or what do I have to pay attention to, when creating the power supply?
How do I calculate the power consumption of the servos and the corresponding battery?
(As far as I'm concerned, each servo uses 10V so it's easier to calculate...)
There are many question marks circling over my head.
I would be really grateful if you could tell me something about that because I'm not very familiar with electronics yet, but I would like to get an Arduino kit tomorrow. Maybe I also get one or the other answer to my questions in the Arduino-tutorial.
I would like to build the robot completely myself and before I can sit down to the CAD to plan it I need the dimensions of the servos, which I can't determine because I'm not familiar with the electrics ^^
The voltage of the servo is not the important item when it comes to sizing your power supply (PS). It is assumed that you will use the correct voltage.
You need to know the stall current of each servo and add them up to get the maximum current draw that your PS needs to be able to supply.
In order to do this, you do need to have specific servos in mind.
In order to decide on the servos, you need to have the specific robot and its mechanical needs in mind.
May I suggest starting with an established hexapod project to get an idea? Perhaps even just review the documentation and get some ideas.
so I thought of 4.8-7.2 Volt servos with 13-20 kg of force.
The robot will weigh max. 2-3kg, probably more like 1-2kg. I already have the Freenove Hexapod as a template, but the components are very cheap so I thought I'd just build the hardware myself.
Ideally the servo power supply(s) and the Arduino supply would be separate to keep motor noise from the logic power.
Servos need power. Small servos need about 1A each. Larger servos need more. Consult the servo data sheet to find the stall current. Each servo will briefly draw stall current every time that it starts. Servo start a lot when trying to keep position. The number one problem that people have when working with servos is not supplying enough current. Your servo supply pretty much cannot have too much current capability.
Hobby servos like those used in RC aircraft have standard dimensions and those should be easy to find on the net.
• Weight: 55 g
• Dimension: 40.7 x 19.7 x 42.9 mm approx.
• Stall torque: 9.4 kgf·cm (4.8 V ), 11 kgf·cm (6 V)
• Operating speed: 0.17 s/60º (4.8 V), 0.14 s/60º (6 V)
• Operating voltage: 4.8 V a 7.2 V
• Running Current 500 mA –
• Stall Current 2.5 A (6V)
• Dead band width: 5 µs
• Stable and shock proof double ball bearing design
• Temperature range: 0 ºC – 4.8 V a 7.2 V – 900 mA (6V) double ball bearing design 55 ºC
You absolutely do not want to use those servos. Projects similar to yours (hexapods, octopods, quadrupeds) have failed because those servos are just too weak... look for something with greater torque - weight ratio, like this one. It's important that you model out the legs of the hexapod so that you can make sure that the effective torque of the motors to hold up the robot is great enough to actually hold it up.
As for power, if you want to keep it simple I'd say to simply get a big lipo battery (something for drones) and use a buck / boost converter to get it to the right voltage, assuming that the motor you use isn't at a nice voltage like 7.4. Most drone lipos can deliver a lot of current (in the 50+ A range) so you likely don't need to worry about that - just make sure that the converter you're using is rated at the right current.
To drive all the servos, you can use a servo driver with multiple channels, like this one. These are popular and cheap.
Ahh ok, sorry I'm still a real beginner xD
So with an 8-legged robot with 3 servos each leg, and it would theoretically move 4 legs at the same time, so I have 4x3 x 2.5A + 4x3 x 0.5A ? Can this be ?
Thanks, I thought the servos were junk. And I'm also aware of the problems with the leverage of the legs, but since I'm going to build a very light robot, I don't want to make too much science out of the calculation of the necessary power of the servos xD I would just take stronger ones to be on the safe side. Unfortunately,those things are really expensive. How many kg should I choose ? 20-30kg actuating force?...I would like to use the servos again for future projects anyway, as long as they are of good quality and durable.
Okay, you're probably right, but the small servos usually have plastic gears...don't you think that's a disadvantage?
PS: the example servos are from a hexapod which is available. (Freenove Hexapod)
Well, don't forget that the servos have weight too! Your frame may be light, but if you put 32 servos on it'll be too heavy before you know it. I'd say to try to keep this project smaller - maybe build a hexapod but with some much smaller 9g servos instead. If you keep the legs small, these servos should be able to do the job and at a low cost too.
As @Paul_KD7HB mentioned above, you should check up on the significance of stall current.
The current rises with demand, and even when one leg is not moving, the servo will still have to provide a fair amount of force to hold the body up. It is not the same as an airplane rudder servo when the plane is in level and stable flight.
Keeping thing light is a good idea.
Metal vs plastic servo gears matter more for control precision at the limits of the servo's capability. In any case, when the limits are exceeded, the metal gear train may not give and thereby cause damage to other structures, whereas a plastic gear set may deform, or slip/give up.
i was no longer able to answer you yesterday for 24h because of my status... so plastic gears are not so bad for a walking robot? because precision is probably more important when building a robotic arm or cnc machine. For example, if I need 50A, do I have to buy a 50Ah battery? Or how do I understand that?
Broadly speaking, you are correct. But a lot more goes into it than just the gears, including quality of the servo amplifiers, the quality of the power supply, etc.
Weight is your enemy! The lighter you can make the robot, the better. A heavier robot needs larger servos which in turn increase the weight.
If your max current need is 50A, then you need a battery that can deliver 50A. Whether it can depends on the battery chemistry. LiPos and LiIon cells are capable of high discharge rates.
Lets say, you find a 5000 mAh battery pack of the correct voltage. And lets say the pack seller says its a 10C pack. What that means is that the pack can deliver maximum of 10x its Amp-hour rating. In this example, that would be 50 amps.
You should seriously consider the suggestions given above, e.g. by @seanboe and start with small servos, and see how it pans out.
Look in the car RC hobby or drone or air RC sites for servos and packs. It may be easier to decide on the servo, then choose the appropriate voltage level.
Thank you very much, I take your tips very seriously, but I wonder about the movement quality of the "light servos"? For example, when I watch the movements of the Freenove Hexapod, they appear very shaky, weak and jerky. I assume that this is caused by inferior servos... or by sloppy programming?
Regarding your example with the C-factor...can you tell me what this model looks like...
No problem. The powerbank has 20 AH capacity, but the outputs are only 2.1A each x 2. So with the USB outputs, max 4.1A at 5V. If you hack the power bank and get to the actual battery, you should be able to get at least 20A. But no guarantees... the data sheet does not list the specs of the individual cells.
They should be very similar to the small servos mentioned above and I imagine the current draw should be lower too.
But do please do a small test run with them. If you don't have a digital multimeter, I strongly urge you to get one. It will help with trouble shooting. A 10A capable DMM should at least allow for measuring the current draw at one servo at a time.
It will also allow for troubleshooting, since servos can bind and draw high current when linkages are not smooth.
