Recommendation for High Torque Servo on Robotic Arm.

I bought a robotic arm kit and am having trouble finding servos that can lift the arm at the base. The arm is 14.75 inches long (from base to tip) and weighs 15.25 oz with servos and without a load. This is the robotic arm I purchased without the servos...

This video shows how to put the arm together and sorta tests it but did not run all the servos or range of movement...

Normally, I would post my question to the video owner and get the servo make and model number. However, someone already asked about the servo and got the wrong answer. I know for a fact that the servo connecting the arm with the base has got to have extremely high torque in order to lift the arm from the base to a vertical position.

I am using a 6 volt 2 amp separate power supply to run the servos and use the USB cable to run the Arduino Uno R3. I have not written any custom code yet, but simply connect the power to the servos, connect a common ground and run the Servo Sweep sketch provided in the Arduino Example library for release 1.6.5 and ERW 1.0.5.

If anyone has already built this type of robotic arm I would really like to know the configuration of the servos used that worked for you. Or, if someone has a strong servo background and has an idea on how to control the base servo, their input is also appreciated.

kikidog:
Or, if someone has a strong servo background and has an idea on how to control the base servo, their input is also appreciated.

A link to the base servo and its specs would be useful

1.5Nm or something like than is probably a reasonable spec given the mass and dimensions)

You'll need more amps, 2A is probably not enough for one high torque servo.

When I bought this on Amazon the "Description Section" suggested that you use a servo like the TowerPro MG995 Digi Hi-Speed servo. So, that is what I purchased.

Here are the specs...

http://www.servodatabase.com/servo/towerpro/mg995

When the arm is in the "down" position (ie resting on the base) the servo does not have the ability to lift the arm and just buzzes/vibrates. It seems to be trying to lift the arm but can't.

Is it even reasonable to expect a "Model RC" servo to lift the arm in the first place? I figured who ever designed and manufactures this robot arm would have a working proto type so it should work. Maybe I should use another type of motor such as a worm drive? Or maybe use 2 servos instead?

No, you need a robotics servo for a robot, designed for running at max torque continuously
(all metal construction, basically). Something like twice that torque is more plausible - do the maths
and you’ll see.

MarkT...do you know of a website that sells robotic servos? The mount for the base is designed to hold the standard RC model servo. Are robotic servos different?? Are you talking about industrial servos or advanced hobby servos??

I have seen all metal servos (they are kinda pricey) with dual bearings etc. Is this what I need?

The 16oz load is ~0.5kg, the 15" arm is about ~400mm. Halve that assuming the load's evenly distributed and we can say it acts at 200mm. So the moment is 0.5kg x 0.2m x 10 for gravity = 1Nm so MarkT's 1.5 is very reasonable with wiggle room.

But let's stick with the crappy units the servo makers use, of kg.cm since that makes it easy to check the specs: 0.5kg @ 20cm is a 10kg.cm or 15 if we factor in some wiggle room.

The servo guys annoyingly quote torque as just kg, so look say at HobbyKing for a 10-15kg servo.

Standard servos seem to have torque about 4-5-6 kg.cm, but the big heavy ones are in double-figures.

edit.... but opening post says that's with no load, so don't forget to add on kg x 0.4 for the load.

kikidog:
When the arm is in the "down" position (ie resting on the base) the servo does not have the ability to lift the arm and just buzzes/vibrates. It seems to be trying to lift the arm but can't.

That could be down to insufficient power supply.
For testing anything to do with motors i prefer batteries anyway.

kikidog:
I figured who ever designed and manufactures this robot arm

Hmmm, maybe.

Boardburner2:
That could be down to insufficient power supply.

His servo is just on the 10kg.cm, or 1Nm Torque T. Speed is about 0.2s/60degrees, let's call that w=5rad/sec, close enough.

So Power = Tw = 1x5 =5W which at 5V is 1A.

In theory then the 2A supply is enough but the servo itself is right on the limit of torque required.

JimboZA:
In theory then the 2A supply is enough but the servo itself is right on the limit of torque required.

Yes but its not moving so it operating at stall where the current is higher, im wondering if the psu might be shutting down.

kikidog:
MarkT...do you know of a website that sells robotic servos? The mount for the base is designed to hold the standard RC model servo. Are robotic servos different?? Are you talking about industrial servos or advanced hobby servos??

I have seen all metal servos (they are kinda pricey) with dual bearings etc. Is this what I need?

The standard sizes of servo are the same I believe. Robotics servos do work and are tough and heavy,
RC model servos are as light as possible and cheap and cheerful and expected to wear out - they
just have to waggle wing flaps for a few hours. You are not asking your servo to just waggle a wing flap.

So yes, get a servo that's designed for the job - proper bearings, metal case, built to handle the
loads you are going to put on it. You pay for the build quality and greater power/torque handling.

You may get away with the other servos being cheaper as the loads are less.

(this is a repost)

So I have decided to buy the Power HD 1501MG 17KG Big Torque Winch Multi-axis Robot Manipulator servo (only $16) from www.bigupgadgets.com ...

http://bigupgadgets.com/power-hd-1501mg-17kg-big-torque-winch-multi-axis-robot-manipulator.html?fee=14&fep=279747

The specs are as fowllows...

Torque(4.8V): 15.5 kg-cm (215.3 oz/in)
Torque(6.0V): 17.0 kg-cm (236.1 oz/in)
Speed: 0.16 sec (4.8V) 0.14 sec (6.0V)
Operating Voltage: 4.8 6.0 DC Volts
Bearing Type: Ball Bearing x 2
Motor Type: DC Motor

I am hoping that this will work. There is a YouTube video showing a robotic arm sorta functioning with the MG995 servo but it does not go thru full range of motion and the comments do not answer what servos were actually used.. So, I am questioning a little here...

None the less it is an awsome clip and the guy did a great job.

I will post my findings after I get the servo and do some testing. Eventually, I will make a YouTube video clip and add it to my Arduino series ( please search for "Scott Mon". -sorry about the plug-)

Thanks again for all the help.

If anyone has actually made this robot arm and has it working I am very interested in the servos and code used.

In sensible units thats 1.5 Nm at 4.8V so I think it has a good chance. Larger
robot arms are usually designed with counter-balanced joints to reduce the torque
needed. Clever designs use the motors/servos themselves as the counterweights.
However sometimes that comes at the expense of range of movement.

I just found this servo gearbox at www.servocity.com website…

https://www.servocity.com/html/spg5685a-cm_servo_gearbox.html

It has 1,253 oz-in. of torque!!! Or, thats 8.855 Nm!! Or, 6.53 ft lbs!!! NICE.

So, I am at this point going to ditch the previous servo and look into this one.

Has anyone ever used one of these???

Any suggestions?

Is it possible the gear teeth will just shear off???

It is far more pricey than what I wanted to spend. However, you gotta love all that torque!!!

Any input is very welcome.

kikidog:
I just found this servo gearbox at www.servocity.com website…

Is it possible the gear teeth will just shear off???

It is far more pricey than what I wanted to spend.

Not used that myself but servo city do make good servos.

Price- that’s the problem with decent metal gears, they have to be machined (hobbed) rather than moulded.

I would not expect them to shear unless abused.

Cheap gears are stamped I think, but they are still made of steel not nylon which is what handles greater
mechanical load.

MarkT:
Cheap gears are stamped I think, but they are still made of steel not nylon which is what handles greater
mechanical load.

Yes , the servo city ones however are machined ally or brass.

Their frames are also machined ally for the more powerful servos.

Expensive though.

Aluminium isn't much / any better than nylon I suspect. Yes you'll pay more
for good hardened steel gearing, but then you'll get what you pay for in power
handling and lifetime.

MarkT:
Aluminium isn't much / any better than nylon I suspect. Yes you'll pay more
for good hardened steel gearing, but then you'll get what you pay for in power
handling and lifetime.

The sections are thicker though, i have noticed that it is common to pair a larger ally gear with a brass or steel pinion though.

Best power gear i found was a phosphor bronze worm drive of an old wiper motor.
I think they are alll plastic now though.

Currently the Servo Gearboxes are not available and have not been for several months. I am hoping they will be available soon. I had several questions concerning the servos so I contacted Tech Support at Servocity by actually calling them on the phone!!! What I found out was YES they are expensive. You can buy the gearbox separately for about $50 and assemble the servo/gearbox unit yourself with your own Hitec servo if you wish. These servos are ideal because the load is not resting on the servo gear itself. The 7:1 ration provides the maximum amount of torque by reducing the amount of rotation. This limitation for me is fine since the arm can only rotate a maximum of 180 degrees anyway. The servos range from 4.8 to 6 volts and can pull about 1 amp under heavy load.