Can Springs Effectively Reduce the Torque Required for a Robotic Arm?

[Mrj005]

Hello!
I have to build a robotic arm and I want to use servo motors but the problem is that the strong servos with high torque are quite expensive. So, my question is, can I use springs to reduce the torque required?

 

[berkeman]

Hello!
I have to build a robotic arm and I want to use servo motors but the problem is that the strong servos with high torque are quite expensive. So, my question is, can I use springs to reduce the torque required?

Welcome to the PF.

Can you UPLOAD a PDF or JPEG sketch of your idea? Just as there is no Free Energy, there is usually no Free Torque, although leverage and stored energy can be your friends. I can think of one moderately complicated mechanism that could be used to store energy at a lower torque and deliver it back at a higher torque. Can you make some guesses on the kind of mechanism that I'm thinking about?

 

[Mech_Engineer]

Springs would only be able to balance a constant force such as gravity, but even then you would need to assume a near-constant line of action.

Have you considered gearboxes to increase the motor's torque? What are your motor requirements?

 

[CWatters]

+1
Springs typically only help in one direction. They hinder when going in the other direction. So they can only be used to halve or equalise a force like gravity that only works in one direction.

 

[Mrj005]

I did the math and the torque required for the first articulation is almost equal to the maximum torque that can be provided by the servo motors (and it might stop working). The highest torque is required when the arm is parallel to the ground (but I might be wrong, I'm not really sure about that), so if I add a spring like in the picture will it help or it doesn't change anything?
https://imgur.com/unbrDY8

 

[CWatters]

Take care if loading servos to near max torque as they can start "hunting" (oscillating) causing increased wear and power consumption.

 

[Mech_Engineer]

What kind of motor are you using? Does the motor have a gearbox?

 

[Mrj005]

What kind of motor are you using? Does the motor have a gearbox?

MG995

 

[Mech_Engineer]

Have you considered upsizing your servo? What is your torque requirement?

 

[Mrj005]

Take care if loading servos to near max torque as they can start "hunting" (oscillating) causing increased wear and power consumption.

I calculated the forces for the worst case scenario, in reality it might not be as much force aplyed, but that is why I want to add that spring.

 

[Mech_Engineer]

I calculated the forces for the worst case scenario, in reality it might not be as much force aplyed, but that is why I want to add that spring.

A spring isn't the answer, because your servo will have to fight against it in any orientation where it isn't counteracting gravity. I would recommend up sizing your servo or reconsidering your design to limit torque requirements.

 

[Mrj005]

Have you considered upsizing your servo? What is your torque requirement?

I need about 18kg*cm of torque (approx 1.8 Nm) so I'll use two servos linked in parallel (each motor can provide about 10 kg*cm), the problem is only in the first joint.

 

[Mrj005]

A spring isn't the answer, because your servo will have to fight against it in any orientation where it isn't counteracting gravity. I would recommend up sizing your servo or reconsidering your design to limit torque requirements.

First link is only requir to move between 0 and 80 degrees so the servo won't need to fight against it (not too much :) ).

 

[Baluncore]

The first axis should be vertical = azimuth, so gravity is not involved.

The second axis will be horizontal = elevation, so may use a spring to balance gravity since it has a predictable range of orientation. That appears to be the case you are presenting.

The third and further axes cannot benefit from a spring. But they may still benefit from counter-balancing, where heavy actuators on the arm are moved back to behind the elbow pin in a way that does not restrict the range of movement.

Adding mass to counter gravity should be avoided as it slows the response of the mechanism.
If you increase the torque through a gearbox you reduce angular velocity for the same power.
Try to design a balanced system that requires no power to maintain common static positions.

 

[jrmichler]

Do you really need to move your robot arm 60 degrees in 0.16 second (6.5 radians/sec)? If a slower speed is acceptable, then a gear reduction will help. A 2:1 gear reduction will double the torque at half the speed.

Good sources for small gears and parts are Berg: https://www.wmberg.com/, and McMaster-Carr: www.mcmaster.com.

 

[Baluncore]

There are a couple of other ways to vary force with arm elevation angle.

The first way is to use a 3 bar linkage = a connecting rod between two cranks. By setting the relative phase of the cranks you can achieve the variation in torque required. The motor driven crank can sweep between 0° and 90° but the linkage arm must avoid the infinite speed point, but might work well over the 60° range you require.

The second way is to use a matched pair of non-circular gears. That could give more torque, (less speed), when the arm is horizontal, but less torque, (greater speed), when the arm is vertical. There is an example shown here. https://en.wikipedia.org/wiki/Gear#Non-circular The four lobe gear shown there changes ratio over 45°. Unfortunately, two easily made circular gears, eccentrically mounted, will have 180° between the minimum and maximum ratio. Your application would need a variation over 90°, so it could benefit from a matched pair of centre mounted elliptical gears where the ratio changes from minimum to maximum over 90°.

 

[Ketch22]

The highest torque is required when the arm is parallel to the ground (but I might be wrong, I'm not really sure about that)

It is reasonable to carry a motion to an extreme and consider the overall effect. In your case I believe I see your point in easing the force at extreme limits of motion. Adding a spring to "soft couple" a motor to an appendage can function but most often is done with a resilient coupling.
The quoted sentence is bothering to me. No matter what the coupling does to affect a softer motion the maximum torque that can be developed must be overcome by the motive force. If this is not designed into the machine one will have a machine that works great most of the time. Then "all of the sudden" it gets stuck. As a design engineer we need to identify that worst case scenario and either overcome it, or prevent it from happening.
In your case It is looking like you need a stronger motor or you need to put a hard stop in the hinge that prevents the arm from developing more force than the motor can develop.
I would agree with Baluncore as well. Balance is your friend when trying to reduce force. The least amount of mass that can be added the greater the potential accelerations possible for a given force. Complex gear trains work very well but can be very "fiddly." What is the use of this arm ultimately. A carefully maintained production arm does very well with frequent adjustments. A competitive "BattleBot" needs to be above all robust.