With their many parts and the need to be able to smoothly rotate all of their axes, jointed arm robots require the perfect actuator to power their specialized movement with the right type and amount of force. Robots with jointed arms are often tasked not only with mundane tasks, but also with performing human-like actions in dangerous or high-stakes environments, so the motor must be perfectly matched to these requirements. There is a seemingly endless selection of DC, stepper, and servo motor products on the market, each with their own advantages and drawbacks. Going into the selection process having answered a few key questions will vastly simplify the selection process.
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There are several factors to consider when selecting a motor to power a robot with a robotic joint
1. What type of robotic joints are used? There are five types of robotic joint: linear, orthogonal, rotational, twisting, and revolving. Does your application use the simpler linear and orthogonal joints, the more dynamic rotational, twisting, or revolving joints, or a mixture of both? This will determine the types of motions and the related nuances of their requirements.
2. How much noise is tolerable in the application? If your application will be used in a factory largely away from people, noise may not be an issue. But if it will be used alongside humans for more than a brief amount of time, you may favor a quieter motor.
3. How much precision is required? When a robot is being used to move shelves in a warehouse, not much precision is required, whereas there is no room for error when one is filling prescriptions. Different motors provide precision in different ways, some with distinct disadvantages; it&#;s important to know which of these may be allowable for your product.
4. How much torque is necessary? Torque can be achieved at various speeds and with varying degrees of constancy. If you need high torque only at a particular speed, you may be able to sacrifice unnecessary torque capability for other motor features.
Now let&#;s review the three types of electric motors most often used to run applications on a typical jointed arm robot&#;DC, stepper, and servo&#;against these considerations.
DC motors come in brushed and brushless varieties. It is commonly thought that brushless DC motors have supplanted brushed ones, but brushed DC motors are still quite popular for some applications. A brushed DC motor is about 75%&#;80% efficient, achieves high torque at low speeds, and is simple to control, but creates quite a bit of noise due to the brushes used to rotate the machinery. On the other hand, a brushless DC motor is quieter, even more efficient, and can maintain continuous maximum torque, but is more difficult to control and can sometimes require a specialized regulator. Although DC motors usually provide low torque, they can achieve high speeds and are good for washing machines, fans, drills, and other machines that require constant circular motion.
There is always the option of adding a gearbox to the system to create more torque for robotic applications utilizing a robotic joint mechanism. Keep in mind, the motor and gearbox should be designed to work together, so purchasing a motor with an integrated gearhead is a good idea in this case.
Stepper motors can control precise movement, have maximum torque at low speeds, and are easy to control, making them popular in process automation and some other robotics. However, they come with several drawbacks: They are noisy and relatively inefficient, and they run hot since they continuously draw maximum current. Finally, since they have low top speeds, they are known to skip steps at high loads, which can be a critical flaw in some jointed arm applications. Despite these limitations, they have proven effective in medical imaging machines, 3D printers, and security cameras.
Servo motors provide extremely precise movement, thanks to a feedback loop that senses and corrects discrepancies between actual and target speed. They can provide high torque at high speeds, and can even handle dynamic load changes. Additionally, servo motors are lightweight and efficient. Downsides of using servo motors include their possibility for jitter as they respond to feedback and their requirement for sophisticated control logic. Despite these drawbacks, the precision offered by servo motors often make them a good option for a jointed arm robot, the sophisticated movement of which is designed to match that of humans!
Your jointed arm robot may perform sensitive tasks and come with high expectations, so you need a motor that not only powers your system but makes your robot maximally appropriate for the environment in which it operates. When selecting a motor, making sure you know exactly what you&#;re trying to achieve and ranking your priorities will help you make smart functionality tradeoffs for optimal performance and suitability.
Which actuators are suitable for your application depends very much on what kind of robot arm you want to build. Once you have decided on what kind of arm you want you can decide on a suitable actuator for each axis.
Assuming from your description that a gantry robot wouldn't be viable, then depending on your specific application, you may want to consider a SCARA arm over an Articulated arm, which is what most people think of when they think Robot arm.
The big advantage of a SCARA arm is that most of it's lifting strength is in it's bearings. The main shoulder, elbow and wrist (yaw) joints are in a flat plane, which means that the motors only need to be strong enough to produce the lateral forces required, they don't need to support the weight of the remaining axes.
The Z axis, pitch and roll (and grip obviously) all have to work against gravity, but the Z axis is easy to gear highly enough to be able to support plenty of weight, and the pitch, roll and grip axes only have to support the payload weight, not the weight of other axes.
Compare this to an articulated arm, where many of the axes have to support the weight of all axes further down the kinematic chain.
Typically a gantry robot will use linear actuators for the main X, Y & Z axes. These could be low performance, low accuracy, high force actuators such as a lead screw with a servo or stepper drive (force and performance can be traded but accuracy will always be limited by backlash), all the way up to high performance, high accuracy direct drive linear motors with precision encoders.
The remaining 3DOF manipulator will usually require precision rotational motion for pitch, roll and yaw, so usually an electric motor (either stepper or servo), will be most suitable. Even a small motor with a reasonably high gearing can resist gravity against quite high loads.
The difference between servo(1) and stepper is a trade-off between complexity and certainty in control.
A servo motor requires an encoder for position feedback, whereas a stepper doesn't. This means that a stepper is electrically much simpler, and from a control point of view simpler if you want low performance.
If you want to get the most out of your motor though (pushing it close to it's limit), then steppers get much more difficult to control predictably. With position feedback on a servo you can tune performance much more aggressively and since you know if it fails to reach it's target position or velocity then your servo loop will get to find out about it and correct it.
With a stepper you have to tune the system so that you can guarantee that it can always make the step, irrespective of the desired speed of move or weight of the payload. Note that some people will suggest adding an encoder to detect missed steps on a stepper motor, but if you are going to do that then you might as well have used a servo motor in the first place!
With a SCARA arm, the Z axis is probably the only linear axis, while the remaining axes can all be done with rotational motor, so again stepper or servo motor. Sizing these motors is relatively easy because the weight carried is less important to many of them. The motor required to overcome the inertia of a load is rather less than sizing it to overcome gravity.
With an articulated arm the calculations are more tricky, because most axes will need actuators sized depending on both moving the load and lifting it, but again an electric motor is the easiest to control and use.
Finally there is the gripper. This is where I have seen the most variety in actuators. Depending on your applications you could easily use any number of different actuators.
I've used systems with traditional motor driven grippers, linear actuated grippers, piezo flexture grips, pneumatically actuated grippers, vacuum pick-ups and simple slots or hooks amongst others, many of which were specific to the application. What your typical payload is could significantly change the actuator which is best for you. (2)
As Rocketmagnet suggests ultimately you are going to have to break out your calculator.
You will need to take into account kinematics of your system, the maximum load on each motor (taking into account the worst case with arm fully extended if you are using an articulate arm design), the speed (a smaller motor with higher gearing might give the force you need without the speed, but a beefier motor might give you a higher torque with lower gearing and a higher speed etc.) and the positional accuracy you need.
In general, the more money you throw at the problem, the better performance (speed, accuracy, power consumption) you will get. But analysing specifications and making smart purchasing decisions can help optimise the price/performance of your robot.
(1) Note that my experience is with industrial servos, typically a brushed or brushless DC motors with a rotary encoder, so this may or may not apply with hobby RC servos.
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(2) I would suggest posting another question on this.
When you're choosing actuators, it's instructive to start by calculating how much power you need at the end effector. When you say 'not too slow' you should have some idea what this means, especially under different load conditions.
For example, you might say: 6kg at 0.2m/s and 0kg at 0.5m/s
Now add in the estimated weight of the arm: 10kg at 0.2m/s and 4kg at 0.5m/s
Now calculate the power: 100N * 0.2m/s = 20W and 40N * 0.5m/s = 20W
So the peak power output at the end effector is 20W. You're going to need an actuator which can comfortably produce more than 20W.
I'm going to make assume that you end up deciding to use an electric motor as your actuator. These are still the actuator of choice for powerful electric robots systems. (If you successfully get this robot working with muscle wire without burning down your workshop, I'll eat my mouse).
Since you're using an electric motor, you'll almost certainly be using some kind of gears. Assume the gear train on the motor is about 50% efficient. This means you'll need an electric motor rated for at least 40W. If you want this to be a reliable arm, I'd spec a motor rated for at least 60W.
Next you need to spec the gear train. What's the torque needed? 100N*1.25m = 125Nm. But as usual, you need to spec more torque than this for the gear train, not least because you'll need some spare torque to be able to accelerate the load upwards. Select a gear train which can take more than the rated load.
Lastly, make sure that the motor's torque multiplied by the gear ratio multiplied by the efficiency exceeds your torque requirement, but not the maximum gear load.
There are two more factors to consider: Complexity and cost.
industrial robotic arm http://halcyondrives.com/images/robotic_arm.png
Image from http://halcyondrives.com
normally use torque from the gearbox to direct drive the joint, now think about the torque the gear reduction should support and the size/weight it will be? It simple huge and expensive, their materials need to support a huge torque.
Let's take your example with the arm fully horizontally extended. Lets consider just the load of 6Kg at 1m, you have $ 600Kgf/cm $. This is not including the robot arm own weight (easily more 4Kg), and considering just 1m.
Some solutions industry uses
Image from http://commons.wikipedia.org
To get lighter gear reduction, with high reduction ratios, most uses Strain Wave Gearing or Harmonic drive. Its low-weight, robust, and according to wikipedia can have a reduction of $ 200:1 $ (wikipedia says more) where a planetary gear can archive $ ~ 10:1 $.
But this type of gearing is very expensive and complex.
Image from http://www.globalrobots.ae
Other even simple solution is adding counter-weights like you see in the image. This has a linkage to act both on the forearm (I forget the name) and in the arm. Springs will help too, and if mounted on the same axis of the joint but a bit offset, it will put more force as the arm gets more extended.
Now for less cost and less complex solutions, what I should think is removing the high torque at the gear drive, so you can use less expensive materials. For a pure electronic drive, this would be a linear actuator.
There is a variety of linear actuators. But the idea is that it will take less force (depending on what points of the arm it is attached).
This type of actuator has many sub-types, and that affects efficiency, wear, force, and much more. But in general, they have a high force and a relative slow-to-medium speed (this will again depend on the type, it can be vary fast, like the ones used on some motion platform simulators).
6 dof motion platform with electric linear actuators http://cfile29.uf.tistory.com/T250x250/195BAD4B4FDB0AF104C30F.
Electric linear actuators are substituting hydraulic linear actuators at this application, and they need to be fast and strong, some simulators easily weight more than 2 tons.
For more speed and other simple method are the belt or chain drive like that
Image from http://images.pacific-bearing.com
This is of course an industrial made one, this is a DIY one, and it has more for the application: (yes it has space for much improvement, but is a good form to show how fast and strong it can be even in this design). http://bffsimulation.com/linear-act.php
This allows you to use less gearing, if a motor could output $ 50Kgf/cm $ and you use a 2 cm diameter pulley, you would get $ 50Kgf $ (not considering losses) on the full stroke.
Also the bearings at this actuator will need to support most radial forces, where in a "leadscrew and nut" the bearing will take most axial force. So depending on the force you need to use a thrust ball bearing.
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