"Best" robotic arm geometry?

The 6DOF (Excluding gripper!) articulated robot arm is the most universal commanly type, but there are some (commercial) robots that add an extra 3rd arm segment. This can be handy for example for installing a dashboard or a seat into a car, where access into the car is restricted because of the surrounding metal of the car frame.

The "palletizing" robots (Based on parallelograms) are simpler and therefore cheaper (quite often just 4DOF) and when you only want to move items in a horizontal plane it may be adequate. The "EEZYbotARM V3" is a nice DIY example of this. This plywood version from fablab ruc:

SCARA has the advantage that most of the motors do not have to carry the weight of the payload. This type is relatively common for big magazines for CNC machines.

Then there is of course also the stewart platform. It seems to fit the build of (airplane) simulators well, but it's relatively stiff and I've also seen some examples of it being used for CNC milling.

Then there is the Delta robot arm, which is a "simplified" version of the stewart platform, with only 3 DOF. These can be very quick and are commercially used for pick and place of lightweight objects (such as picking up cookies and stuffing them in boxes).

And of course the XYZ cartesian version. These can easily made to custom size by bolting linear stages together. It is also much like most of the commercial CNC machines.

There are some (probably) good books about robotics, but I never looked much into them. I suspect they are easily to generic, or too detailed.

Of all the (quite many) DIY robot arms I've seen, they nearly always have a quite low gearbox ratio, combined with stepper motors. I guess that a gear ratio of around 1:30 to 1:40 will be optimal, and this is difficult in a high efficiency single stage gearbox (both cycloidal and strainwave can do it, but those are expensive). Nema23 motors run quite well up to about 600rpm, and it's nice if your motors can reach this when they move at their maximum speed. Note that if you double the gear ratio, the stepper motor has to run twice as fast, and it's output torque wil be significantly lower (maybe even near a factor or two), but this is compensated because the input shaft of the gearbox only needs half the torqe. The main gain is in higher resolution, and in higher payload capability for slow movements. All robot arms based on hobby servo motors are quite garbage. Too much backlash, low torque, and mostly not enough resolution to control a robot arm properly. I would never build such a robot myself. Using a single hobby servo in a gripper may be acceptable though.

 The combination of motor size and gearbox is quite important and has to made early in the design. I've seen too many DIY robots that can barely hold their own weight, or even don't work at all without a significant re-design. You can either do some calculations (which can be inaccurate due to loss of torque for stepper motors at higher rpm, (unknown) inertia and acceleration, and other factors. You can also start with some motor and gearing, and add a broomstick with an (approximated (final arm + payload)) to the end. This can give you an idea if your motor setup is "adequate" for your case quite quickly.

But what type of robot arm you want to design / built is mostly a personal question. For a lot of the DIY projects, the software seems to be a big stumbling issue. Chris Annin does put a lot of effort into software for his (open sourced) AR3. That may be worth checking into. There are quite a lot of youtube video's about building DIY robot arms and the first movements, but more elaborate examples of doing something useful with them, or even fluid motions are quite rare.

For myself I also do not know yet how I'm going to make it. Some of my designs are based on flat wood panels that can be lasercut, other models are designed to be made from steel tubing. Before you decide on a model to build, think of some long term things you could do with it. A lot of the DIY robot arms probably end up in some closet quite soon. A long term use of a robot arm could for example be something simple as a desk (reading) light. Or maybe a (fixed position) microphone or (macro?) photo camera that swings into position as you start some software on your PC.

The "palletizing" / parallelogram based version can be quite nice for this. I'm thinking of making it dual use as a tapping arm.

I have not built a robotic arm myself yet, but I've desinged a few in FreeCAD.The 6DOF (Excluding gripper!) articulated robot arm is the most universal commanly type, but there are some (commercial) robots that add an extra 3rd arm segment. This can be handy for example for installing a dashboard or a seat into a car, where access into the car is restricted because of the surrounding metal of the car frame.The "palletizing" robots (Based on parallelograms) are simpler and therefore cheaper (quite often just 4DOF) and when you only want to move items in a horizontal plane it may be adequate. The "EEZYbotARM V3" is a nice DIY example of this. This plywood version from fablab ruc: https://fablab.ruc.dk/robot-arm-v-0-1/ can quite easily be made in bigger sizes (But has a very low gear ratio, see below). Another model of this "palletizing" type I quite like is the uStepper. A lot of the construction is made of standard tubing (Aluminium, or carbon if you like) while the rest is made with a 3D printer, for ease of replication. This one also has a quite low gear reduction ratio though.SCARA has the advantage that most of the motors do not have to carry the weight of the payload. This type is relatively common for big magazines for CNC machines.Then there is of course also the stewart platform. It seems to fit the build of (airplane) simulators well, but it's relatively stiff and I've also seen some examples of it being used for CNC milling.Then there is the Delta robot arm, which is a "simplified" version of the stewart platform, with only 3 DOF. These can be very quick and are commercially used for pick and place of lightweight objects (such as picking up cookies and stuffing them in boxes).And of course the XYZ cartesian version. These can easily made to custom size by bolting linear stages together. It is also much like most of the commercial CNC machines.There are some (probably) good books about robotics, but I never looked much into them. I suspect they are easily to generic, or too detailed.Of all the (quite many) DIY robot arms I've seen, they nearly always have a quite low gearbox ratio, combined with stepper motors. I guess that a gear ratio of around 1:30 to 1:40 will be optimal, and this is difficult in a high efficiency single stage gearbox (both cycloidal and strainwave can do it, but those are expensive). Nema23 motors run quite well up to about 600rpm, and it's nice if your motors can reach this when they move at their maximum speed. Note that if you double the gear ratio, the stepper motor has to run twice as fast, and it's output torque wil be significantly lower (maybe even near a factor or two), but this is compensated because the input shaft of the gearbox only needs half the torqe. The main gain is in higher resolution, and in higher payload capability for slow movements. All robot arms based on hobby servo motors are quite garbage. Too much backlash, low torque, and mostly not enough resolution to control a robot arm properly. I would never build such a robot myself. Using a single hobby servo in a gripper may be acceptable though.The combination of motor size and gearbox is quite important and has to made early in the design. I've seen too many DIY robots that can barely hold their own weight, or even don't work at all without a significant re-design. You can either do some calculations (which can be inaccurate due to loss of torque for stepper motors at higher rpm, (unknown) inertia and acceleration, and other factors. You can also start with some motor and gearing, and add a broomstick with an (approximated (final arm + payload)) to the end. This can give you an idea if your motor setup is "adequate" for your case quite quickly.But what type of robot arm you want to design / built is mostly a personal question. For a lot of the DIY projects, the software seems to be a big stumbling issue. Chris Annin does put a lot of effort into software for his (open sourced) AR3. That may be worth checking into. There are quite a lot of youtube video's about building DIY robot arms and the first movements, but more elaborate examples of doing something useful with them, or even fluid motions are quite rare.For myself I also do not know yet how I'm going to make it. Some of my designs are based on flat wood panels that can be lasercut, other models are designed to be made from steel tubing. Before you decide on a model to build, think of some long term things you could do with it. A lot of the DIY robot arms probably end up in some closet quite soon. A long term use of a robot arm could for example be something simple as a desk (reading) light. Or maybe a (fixed position) microphone or (macro?) photo camera that swings into position as you start some software on your PC.The "palletizing" / parallelogram based version can be quite nice for this. I'm thinking of making it dual use as a tapping arm.

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On this blog, we often discuss industrial robots. For some people, robotic vocabulary may seem simple to understand, but for someone who is looking to buy their first industrial robot, it may be confusing. This article will give you the main vocabulary and a general overview of where to look when buying an industrial robot.

Industrial Robot Applications

First of all, you must know which application will be performed by the robot. This criteria will initially guide you when choosing which kind of robot you need to buy. If you are looking for a compact pick and place robot, you may want to choose a Scara robot. If you are looking more at placing small objects at a fast rate, a Delta robot will suit you best. If your application needs to be done alongside human workers, a collaborative robot should be your robot of choice.

For the following discussion, we will focus specifically on industrial robots. This kind of robot can suit a very large array of applications &#; from material handling to machine tending, as well as welding and material removal. These days, industrial robot manufacturers basically have a robot for every application. You just need to identify what you want to do with your robot and choose between all of the different models.

Robot Payload

The payload is the maximum load that the robot can carry in its working space. If you are looking to carry a part from one machine to another, you need to incorporate the part weight and the robot gripper weight into the payload. For further information on payload, you should take a look at the following article that explains the difference between payload and grip force.

Number of Axes

The quantity of axes on a robot is directly related with the its degree of freedom. If you are looking for a really straightforward application, such as pick and place from one conveyor to another, a simple 4 axis robot is enough. However, if your application needs to be executed in a small work space and the robot arm needs to twist and turn a lot, a 6 or 7 axis robot would be the best option. The number of axes is generally dependent on the application. You should take note that having too many axes is not a problem in terms of flexibility. In fact, if you will be moving the robot to another application in a couple of months, you may want to have more axes rather than not enough. The downside of having too many axes though, is that if you only need 4 axes, you will still need to program 2 supplementary axes for nothing.

Robot manufacturers tend to use slightly different nomenclature for the axis or joint names. Basically, the first joint (J1) is the one that is closest to the robot base. The following joints are called J2, J3, J4 and so forth, until we reach the wrist. Other companies such as Yaskawa/Motoman use a lettered nomenclature for their axes.

Industrial Robot Reach

When looking at your target application, you should know what maximum distance the robot needs to reach. Selecting a robot is not all about the payload &#; it also needs to reach a certain distance. Every company gives the work envelope of the robot, therefore you can determine if the robot is suitable for a specific application. The maximum vertical reach for a robot is measured from the lowest point that the robot can reach (often under the robot base) to the maximum height that the wrist can go. The maximum horizontal reach is the distance from the center of the robot base to the farthest point the wrist can reach horizontally. You should also take a look at the different motion range (expressed in degrees). These specifications are quite different from one robot to another and can be very limiting for certain applications.

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Repeatability

Once again, this factor depends on your application. The repeatability can be described as the capacity of the robot to reach the exact same position each and every time it completes a routine. Most of the time, the robot can repeat inside 0.5mm and sometimes even more. For example, if your robot is needed to build an electronic circuit board, you may want to have a super repeatable robot. If your application is quite rough, the industrial robot doesn't need to be that precise. This measure is expressed in plus or minus &#;&#;±&#;&#; because of the 2D aspect. In fact, since the robot is not linear, the tool can be anywhere in the tolerance radius.

Speed

This criteria is relative to every user. In fact, it depends on the rate in which the job needs to be done. The spec sheets always express maximum speed, but you should know that all of the speeds can be reached between 0 and maximum speed. This motion unit is often in degrees/second. Some robot manufacturers incorporate the maximum acceleration rate. 

Robot Mass

Robot mass is an important factor when designing a robot cell. If the industrial robot needs to be sitting on a custom bench or even on a rail, you may want to know its weight to design the corresponding support.

Brakes and Inertia

Basically every robot manufacturer provides information on the braking system on their robot. Some of robots have brakes on all axes and others don&#;t. To have a precise and repeatable position in the workspace, you need to have a sufficient number of brakes. The inertia of certain robot segments can be provided by the manufacturer. In fact, for designing security features this would be a plus. You may also notice the different applicable torques on the axis. For example, if your manoeuvre requires a certain amount of torque to complete the job properly, you need to check if the maximum torque applicable on the axis is correct. If it is not correct, the robot may shut down due to an overload.

IP Rating

Depending on where you want your robot to work you may need to achieve a certain Ingress Protection rating or IP rating. In fact, if the robot works with nutrition related products, laboratory tools, medical tools or in highly flammable environments, the IP ratings will be different. This criteria is an international norm and you obviously need to verify if your application needs a certain protection or if you can use a local rating. Some manufacturers provide the same robot with different IP ratings depending on where the robot needs to work.

I hope this article gives you a few tips to consider for your next industrial robotic investment. We have put together different comparison sheets for industrial robots with payloads from 5 to 10 kg and for industrial robots with payloads from 20 to 50 kg. These grids only include the industrial robots that are specific to material handling, which means that there are no Scara, Delta or welding robots mentioned. I am sure you will find comparing the different robots interesting!

 

 

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