5 Things to Know Before Buying Robot Joint Actuator

Today’s robot joints have highly integrated motors, encoders, gearing solutions, and sometimes brakes. It is common to use a direct drive frameless torque motor kit coupled to a precision high ratio gear system. While these two terms “direct drive” and “precision gear” tend to conflict, the combination is successful in optimizing for size and performance.

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The five tips below focus on how to select a direct drive frameless torque motor kit and provide system engineering requirements for the drive motor selection.

1. SYSTEM ARCHITECTURE

Selection of control methods, feedback requirements, and mechanical attributes really drive the selection of the motor.

a. The most common motorized joint includes a brushless permanent magnet frameless torque motor kit, an absolute encoder kit, and a high ratio zero backlash gear.

b. In most cases, a high precision absolute encoder is also required on the output of the gear to handle any lost motion and wind-up due to the low stiffness and lost motion in the gear systems.

c. System voltage levels tend to be in the low voltage range < 50volts. This drives low impedance motor characteristics and defines the input speed range. d. Field oriented control sinusoidal servo drives with proper safety functionality and serial communication interfaces, such as EtherCat, are being used. Drives are getting smaller and located closer to the motor. e. Force and torque sensors may be necessary if the robot is used in a collaborative environment.

f. If the robot needs to hold a pose, fail-safe power-off brakes may be required. 

2. MOTOR SELECTION

The most important attributes in a frameless motor kit are mechanical form factor, motor constant (Km), and torque vs. speed characteristics when operating within the power limits. Do your homework, know what motor constant, Km, you need.

a. There are no standards in the industry regarding motor form factor, i.e. Diameter, Length and through hole size.

b. Motor Constant, Km, is the only true indicator of a motor’s ability to output torque under thermal constraints. Km is a calculation = Kt/sqrt(R), where Kt is the torque constant NM/Amp, and R is the resistance in Ohms. Take care in making sure units are consistent and always calculate this because datasheets are notoriously lacking.

c. Cogging torque can greatly impact smoothness of operation. Cogging torque is not normally shown in a motor datasheet. The absolute value and the frequency of this cogging torque is important to the dynamics. Less is always better in this case, zero is preferred.

d. Sinusoidal torque versus angle curves and phase balance are critical to smooth motion. These two items are not shown in datasheets. It is only through experience and testing a problem will be uncovered. Ask for this data from the supplier.

e. Thermal resistance and thermal time constant are also important to know. Again, not always provided in a datasheet, and often assume unrealistic test conditions. Some robots have very specific temperature requirements almost disconnected from how motors are typically rated.

f. Electrical time constant is usually overlooked during selection. However, servo system response and servo driver PWM frequency play a large role in system performance. A low inductance motor, (typically a result of smaller size, zero cogging, weight optimized, low voltage motor kits), requires higher driver PWM frequency to minimize current ripple. Current ripple can cause electrical noise as well as additional heating.

3. MOTOR INTEGRATION

Integrating a frameless motor kit into a mechanical joint is a complex mechanical design, with rotating mechanics, dynamics, and thermal considerations.

a. Joint size reduction is achieved by a high level of mechanical integration. Simplifying the number of bearings, eliminating couplings, and sharing rotating shafts and housings. This is the best way to improve reliability and reduce components.

b. Direct drive brushless frameless torque motors provide the highest Km, and therefore the highest torque output within the thermal budget. Torque requirements are not always known, if in doubt, look for the highest Km motor you can find for the available space. Supplier ratings for continuous torque are for reference only due to thermal conditions. A high Km motor will allow flexibility in electrical, mechanical and thermal attributes.

c. Motor Km increases with diameter faster than it increases with axial length. Always select the largest diameter frameless motor kit available with the shortest length to achieve the Km required.

d. Large diameter, high Km motors also typically have a large through hole. This through hole can be used for mechanical integration with bearings, or it can be used to house a holding brake coaxial to the motor. This will reduce the axial length of the joint.

e. Radial and axial magnetic forces are present in all motors. Again, not shown in datasheets, so you need to ask the supplier.

f. Cogging torque is undesired in all cases for a robot joint. Find the lowest cogging or zero cogging and it will eliminate much of the down stream diagnostic and vibration issues. Cogging frequency is also very important, a high frequency is generally better than a lower frequency and can be more easily tuned out. You will need to ask for this data.

4. TEMPERATURE AND THERMAL LIMITS

a. The thermal resistance of a motor when mounting into a robot joint is unknown. This means that whatever the datasheet says for torque output isn’t really relevant to the robot joint. Getting the highest Km in the size available will allow the most flexibility.

b. If a power off brake (mostly the case for safety reasons) is required and integrated, the brake is heating when the motor is moving. This heat reduces the available thermal budget for the motor.

c. A thermal model is necessary to predict heating for all sources and consider the available thermal budget for each joint. Each joint needs to be highly engineered.

d. In most cases the motor will have to be sized to perform under less than optimal thermal conditions essentially derating the motor from what it could produce.

5. OFF THE SHELF, MODIFIED STANDARD, OR CUSTOM MOTOR SOLUTION

a. Well, everyone wants to use “off the shelf” standard product… right? They are readily available and lower cost…sounds good. Realistically, if you are designing a precision robot to be manufactured in a highly controlled quality system, off the shelf may not be the best solution.

b. Tight control of design and revisions directly conflicts with the “off the shelf” supplier mentality. Buying off the shelf means that the supplier can change whatever they want at any time, especially if there are no industry standards controlling design and interface compatibility. Without control, internal materials, design, or form factor can change without notice. Forcing a supplier to control the design and changes, turns the product into a custom product that is not “off the shelf” any longer.

c. Some low-cost hobby motors from global distributors posing as manufacturers are finding their way into advanced systems. In this case, it is unknown who the actual manufacturer is, and there are no controls in place to guarantee that materials and design will not change on the next delivery.

d. Buy from a reputable supplier with an open book policy allowing factory visits, sub-supplier identification, and customer included change control. At minimum, purchase a modified standard product, where the modifications include control/approval over design revisions.

e. If volumes approach a few hundred units per year, a custom designed solution may be the best option. It gives complete control over the design, materials, and manufacturing process. It will get you to the center of the requirements spectrum in Figure 1 below.

Figure 1 – Design Spectrum for a robot Joint, axes converge on the center

A traditional frameless motor is shown below. The motor shown in Figure 2 is a slotless LS Series motor from ThinGap.  These motors have zero cogging and a large through hole. Km of these motors rivals some traditional slotted (or toothed) brushless permanent magnet motors.

Figure 2 – Slotless Frameless Torque Motor Example

Figure 3 below is a compact integrated Robot Joint. It uses a frameless torque motor kit, high resolution medium accuracy absolute encoder on the input. It has a high ration zero backlash precision gear and a high accuracy absolute encoder on its output.

Figure 3 – Highly integrated Robot Joint

Figure 4 below is a Robot Joint with Tradition Servo Motor. It is more than 2X longer for exact same performance. It is also less reliable with more components including a coupling. It has more adjustments and connections than the compact joint above. It does not have a through hole in the shaft.

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Figure 4 – Traditional Robot joint with exposed servo motor

Want to improve your knowledge of robotics? A good first step is to learn the different parts of a robot that might be useful for your chosen industrial applications.

But what are the different parts of a robotic arm?

All robotic arms are similar. They are usually variations on the same underlying design and kinematic structure. Even industrial robots that look very different can be almost identical when you ignore their surface appearance.

This is a good thing if you are still relatively new to robotics. It means you can understand the core design of a robotic arm by just learning about the basic parts of a robot.

Here’s what you need to know about robot parts, and what you don’t need to know.

Just Using Robots? Here’s What You Don’t Need to Know

You might think that you need to know a lot of robotics theory and fundamentals before you can use robots in your business. This is just not true. You can get by with very little basic knowledge about robots and how they work.

There are multiple layers of complexity in robotics.

For example, you could look at robotics from the levels of:

• Top-level operation and programming: Here you need to know very little about the underlying robotic hardware. You just need to know enough to use the robot.

• Mechatronics and kinematic design: Here you would understand the mechanics and electronics of the robotic machine, but not the specifics of each individual component.

• Detailed mechanical design: Here you would understand exactly how the wrist joint is built and controlled. But you might only know the basics of high-level robotic programming.

If you wanted to, you could spend your whole life learning about all the different aspects of robotic mechanical design. Even then, you would only scratch the surface!

Thankfully, you don’t need to know everything about a robotic system to use it. Robots are a genuinely multidisciplinary technology — a good roboticist knows a little of everything but doesn’t need to know all the details.

If you just want to use a robot in your business, you already have the skills and knowledge to do that!

What Are the Basic Parts to Make a Robot Arm?

As robots are such complex systems, they comprise a lot of different parts. You can categorize these parts in various ways.

Common parts of a robotic cell include sensors, control systems, external axes, tool changers, networking, and many more. Most of these are external to the robot but help it achieve its chosen task.

For this article, we’ll only look at the robot itself.

You can split an industrial robot arm into 2 basic parts:

1. The robotic arm itself: You can think of this as “the robot.”
2. The end effector: This is the “business end” of the robot and is the part that actually performs the task.

Let’s look more deeply at the robot arm itself…

The 5 Core Parts of a Robotic Arm

You can think of there being 5 core parts of any industrial robotic arm.

These parts are:

1. Joints and Actuators

The robot’s joints are the “bits that move.” There are various types of robot joints, including rotational joints, linear joints, orthogonal joints, and revolving joints.

Actuators are the mechanisms that drive the robot joints. The most common are electrical, pneumatic, and hydraulic actuators.

Between each of the robot’s joints is at least one physical link. These are often metal tubes, though they can be made of any rigid material — or sometimes even flexible materials.

The links provide the robot with stability and strength. Longer links extend the robot’s reach while shorter links provide more stability.

3. Internal Sensors

Various internal sensors give the robotic system information on the position and orientation of each joint, as well as other properties. The most common example is a rotational encoder or potentiometer positioned at each of the robot’s joints.

Other common internal sensors include motor current sensors — to detect when the robot collides with something — and force-torque sensors for finer force sensing and control.

4. Power Source

The robot arm’s power source provides it with the power it needs to run.

Almost all robots need at least some electrical power for the internal sensors and control systems. Beyond this, the major power source will be the one used to run the robot’s joint actuators — usually either electrical, pneumatic, or hydraulic.

5. Digital I/O and Controller

The robot’s digital inputs and outputs are how it communicates with its controller. These are electronic signals that send low-level control signals to each of the robot’s joints and receive sensor information.

When you program a robot for a particular task, the controller will execute this program.

How to Program Any Industrial Robot With Any Mechanical Parts

With any industrial robot, it’s not enough to simply have mechanical parts.

The controller software and programming options will determine how easy it will be for you to deploy the robot for your chosen application.

Programming robots can be complex and require special expertise. Each robot manufacturer has its own programming language and conventions, which can make it hard if you are not already a robot expert.

But robot programming can also be very easy.

When you are using the right robot programming software, it’s simple to deploy your robot to almost any task in your business.

With RoboDK, it doesn’t matter what mechanical parts make up your robot. The software already supports an enormous range of industrial robot arms and you can start using yours at the click of a button. Even if the software doesn’t yet support your robot model natively, you can add any robot to it with our mechanism wizard.

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