How to select a motor for robot joints | frameless motors

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.

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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.

Figure 4 &#; Traditional Robot joint with exposed servo motor

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.

How to Select the Best Motor for a Jointed Arm Robot

Article from | Rozum Robotics

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.

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.

 

 

The content & opinions in this article are the author&#;s and do not necessarily represent the views of RoboticsTomorrow

Comments (1)

At present, there is still room for improvement in the servo motor industry. Whether it is machine tools, non-standard equipment or robots, the whole market continues to rise; However, the increase is not as obvious as in previous years. Servo is the core of the robot, or the soul, but also for. Whether ordinary or cooperative, there are differences in the internal transmission and feedback of the servo motor.

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