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| Learn More | Actuator Design: Transmission and Backlash |
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| What components are in a robotic transmission? |
| Backlash and gear trains. |
| Why use a brake? |
| Why use a clutch? |
| Videos |
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| What components are in a robotic transmission? |
A robotic transmission can contain a variety of different devices, just like in an automobile. However, only a few typically appear. Virtually all robotic systems employ some sort of gear train, and many contain at least a parking brake.
A few specialized systems contain a clutch to disengage the motor from the drive train in the case of an emergency, and a few experimental systems make use of direct drive motors that do not contain a gear train at all.
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| Backlash and gear trains |
With the exception of direct drive robotics, all robots employ a gear train to convert the high speed - low torque output of the prime mover into a reduced speed - high torque input to the robotic joint.
This approach introduces the additional advantage into the robotic system of removing the effect of the motor's rotary inertia from the robot dynamics. However, these systems typically have two major disadvantages.
First, they introduce an additional element of inefficiency into the system in the form of lost motion or windup in the transmission. This effect is termed backlash. Second, they introduce a certain amount of compliance into the system.
When backlash occurs, the gear teeth are able to move without imparting motion upon the next gear. This results in energy being wasted in "winding up" the transmission. When the two gears come into contact an impact occurs, resulting in increased wear to the gear train, and resulting in a disturbance that can affect the rest of the robotic structure.
Backlash will also occur when the primary gear reverses direction, as the secondary gear is now wound up in the wrong direction. These effects can be seen in the animation above. The combined effects of wind up and backlash induced disturbances can severely hamper the precision of robotic operations.
However, careful design and manufacture can reduce the effects of backlash within a transmission, and certain types of gear trains (such as epicyclic gear trains) produce significantly less backlash.
Compliance due to the transmission and actuator is a different matter. Since the joints of a robot must be flexible enough to allow the robot to move. Unfortunately this means that they must also be flexible enough to experience compliance when under load. Since this effect is virtually impossible to design out of many systems, it must be dealt with.
Systems have been developed at the University of Texas that allow this deflection to be predicted and corrected for within the robot control system, enhancing accuracy.
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| Why use a brake? |
University of Texas at Austin, Robotics Research Group Four Fold Fault Tolerant Brake Design |
For most applications using gear train transmissions, the inherent friction within the system is sufficient to serve as a brake. However, the robot is still capable of motion when the prime mover is not engaged, leading to the need for a parking brake.
This brake is also potentially useful in the event of a failure in the robot, as a joint can be "locked down" rather than allowing a potentially uncontrolled motion from occurring. Certain applications demand even greater levels of safety from their brake systems.
These systems may employ Fault - Tolerant Brake Designs, such as the Four Fold Fault Tolerant Brake Design developed by the University of Texas for NASA. NASA was particularly concerned about the potential for a robotic arm to begin swinging free during the re-entry phase of the flight.
This design can lock down a joint even if three of the brake modules fail, thus providing significant security to NASA in the event of a systems failure. |
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| Why use a clutch? |
University of Texas at Austin, Robotics Research Group Four Fold Fault Tolerant Brake Design |
Few robotics systems currently employ a clutch. However, a clutch can offer certain advantages within a system. Unlike an automobile clutch, a robotic clutch would be used only intermittently.
In the event of a systems failure, where the robotic arm had to be repositioned manually, the gear train may provide sufficient resistance to make the arm difficult to move, even with the brake system disengaged.
As a result, the Robotics Research Group has designed a compact clutch for robotic applications that would allow the users to disengage the transmission from the joint module, allowing for easy manual manipulation of the robot. |
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