Common joint types include revolute joints (allowing rotational movement) and prismatic joints (permitting linear movement). The number of joints in a robotic arm determines its degrees of freedom (DOF), representing the range of possible movements.
Each joint in the robotic arm is actuated by a servo motor. A servo motor is a specialised type of electric motor that incorporates a feedback control system, consisting of a position and speed sensor and a current sensor. These sensors allow the servo motor to rotate to specified positions and provides feedback to the controller to confirm its actual acceleration, speed, and position.
The feedback loop from the servo motors is crucial for precise control. The control system continuously compares the actual joint positions received from the feedback with the desired positions and adjusts the control signals to minimize any discrepancies. This closed-loop control system helps maintain accuracy and compensates for external disturbances or changes in the environment.
The servo motor will be coupled to a gearbox inside the joint to reduce the rotational speed and increase the torque output of the system. When moving the robotic arm to a target position the servo motor controller attempts to create a torque in the arm to accelerate it to a target speed, then measuring the movement through a position encoder it decelerates it to a stop in the desired position.
Because the forces on the arm are highly dynamic depending on the load, orientation, acceleration, and speed of the system control is difficult, engineers use PID control of the current and acceleration. There is a trade-off between the required operating speed of the robot and positional accuracy. More aggressive acceleration profiles mean faster movement from point to point, and generally for a robot faster is better as this means a more productive system overall. But faster movements are harder to control. Very slow movements of the robot can also be hard to control with a position sensor-only feedback system, robots even sometimes struggle to maintain a stationary position without an element of servo drive torque jitter. Jitter in robotic servo motors can be caused by several factors, including:
- Control System Instability: The servo control system may become unstable due to improper tuning or design, leading to oscillations and jitter. This can occur when the control gains are set too high, causing the system to overshoot and undershoot its target position.
- Mechanical Resonance: Mechanical resonance occurs when the natural frequency of the motor or the mechanical system matches the frequency of the control signal. This resonance amplifies vibrations, resulting in jitter.
- Mechanical Backlash: Backlash is a small amount of play or clearance between mechanical components, such as gears or linkages. When the motor changes direction, the backlash can cause a momentary delay and result in jitter.
- Electrical Noise: Electrical noise in the power supply or control signals can affect the accuracy and stability of the servo motor, leading to jittery movements.
- Load Variations: Sudden changes in the load on the motor can cause fluctuations in the output, resulting in jitter.
- Temperature Effects: Changes in temperature can influence the motor's performance and introduce jitter, especially if the motor's properties are sensitive to temperature variations.
- Encoder Resolution: The resolution of the encoder used to provide feedback to the control system can affect the accuracy of position sensing. Low-resolution encoders may not capture fine movements accurately, leading to jitter.
- Communication Delays: If the control signals experience delays in transmission between the controller and the motor, it can lead to instability and jitter.
- Friction and Wear: Over time, mechanical components in the motor can experience wear, leading to increased friction, which can cause jitter in the motor's movements.
Many of these control issues stem from using the motor current as the primary motor control parameter. Motor current is proportional to output torque and the controller makes a torque estimation based on pre-determined motor characteristics. With speed and position feedback the controller measures the position, speed, and acceleration of the arm and will attempt to control the input current/torque accordingly.
However, because the torque-current relationship varies depending on several key factors some of which are difficult to measure, for example, the temperature of the motor rotor magnetics, and the dynamic response of the system between a measured current input and torque output response it is hard to accurately control the system with current and position sensing alone. The system will also have to consider gearbox friction and backlash.
As a result, modern high-performance robots are implementing torque sensors into their joints to provide improved feedback and control. Utilising a torque sensor, it is possible to directly measure the output torque of the motor and gearbox.
Integrating a torque sensor into the control system of a servo motor can significantly improve the overall performance and control of a robot. A torque sensor measures the amount of torque or rotational force applied to the motor's output shaft or joint. By providing real-time feedback about the motor's torque, the control system can make more informed and precise decisions, resulting in the following benefits:
- Force and Torque Control: With a torque sensor, the servo motor can be controlled based on the actual force or torque applied to the robot's end effector or joints. This enables the robot to interact with its environment more delicately and safely, allowing for tasks that require precise force control, such as assembly, picking up delicate objects, or handling fragile materials.
- Compliance and Force Limiting: By monitoring the torque, the robot can exhibit compliance and adaptability in response to external forces. This is particularly useful in collaborative robot applications, where the robot can sense and react to forces applied by humans or other objects to avoid accidents or damage.
- Stiffness Control: A torque sensor can be used to adjust the motor's stiffness in real time. This means that the robot can be either stiff or compliant depending on the task requirements, making it more versatile and adaptable.
- Overload Protection: The torque sensor can detect situations where the motor is being overloaded, such as when it encounters unexpected obstacles or tries to handle excessive weight. The control system can then respond by stopping or reducing the motor's torque output, preventing damage to the motor and other components.
- Accurate Position Control: Torque feedback helps in maintaining precise positioning of the robot's end effector or joints, even under varying loads or disturbances. This improves the accuracy and repeatability of the robot's movements.
- Energy Efficiency: By monitoring torque, the control system can optimize motor performance and reduce unnecessary power consumption, leading to increased energy efficiency.
- Adaptive Control: Torque sensing enables the control system to adapt to changing conditions and loads in real time, ensuring the robot remains stable and operates efficiently in different environments and tasks.
- Torque Limiting: Torque sensors can be used to set torque limits to prevent the robot from applying excessive force, thus enhancing safety in human-robot interaction scenarios.
Integrating a torque sensor into the control of a servo motor enhances the robot's capabilities, safety, and efficiency, making it more suitable for a wide range of applications, including industrial automation, medical robotics, and collaborative robotics.
Commonly Strain-Gauge-based torque sensors are widely used in robotic applications due to their accuracy, reliability, and relatively low cost. They can be integrated into the robot's joints or end-effectors to measure torque accurately. Because the robot joints do not rotate through more than 360 degrees it can accommodate the required electronics and processing required for signal conditioning and conversion into a digital format in the joint.
While strain-gauge torque sensors are widely used and offer several advantages, they also have some downsides and limitations, the main issues being:
- Joint Flex: even the best and most sensitive strain-gauge technologies require the joint to have a measurable amount of flex or twist to generate a level of strain that is measurable. This means that the sensor reduces the overall mechanical integrity of the system. The mechanical flex in the joint can lead to bouncing or contribute to jitter issues as mentioned previously.
- Hysteresis: strain-gauge torque sensors can exhibit hysteresis, which means that the sensor's output may vary depending on the direction of the applied torque. This non-linearity can introduce errors in the measurements, especially during highly transient bidirectional torque applications as seen in a robotic arm.
- Temperature Sensitivity: strain-gauges can be sensitive to temperature changes. Temperature variations can affect the material properties of the strain gauge and the bonding adhesive, leading to inaccuracies if not compensated for properly.
- Size and Weight: strain-gauge sensors may require additional mechanical components and housing to protect them from environmental conditions and ensure proper mounting. This can add to the overall size and weight of the robot joint assembly.
- Limited Dynamic Response: the dynamic response of strain-gauge sensors can be limited, especially for high-frequency or rapidly changing torque measurements. This can be a concern for applications where fast response times are critical.
- Electrical Noise: strain-gauge sensors can be sensitive to electrical noise and electromagnetic interference, which can affect the accuracy of the measurements.
Despite these downsides, strain-gauge torque sensors remain widely used and are widely understood, however, other torque sensor technologies might bring advantages.
Surface Acoustic Wave (SAW) sensor technology can be suitable for certain robotic torque sensing applications, SAW sensors have unique characteristics that bring advantages to robotic joint torque sensing, the main advantages of SAW sensor technology for robotic torque sensing are:
- High Sensitivity: SAW sensors can measure far smaller strains than other sensor technology, in the range of +/-200 micro-strain. This means that the joints of the robot can be made stiffer. The sensor can be incorporated into the joint gearbox itself rather than requiring a separate flex plate assembly.
- High-Frequency Response: SAW sensors can offer high-frequency response capabilities, making them suitable for applications where rapid changes in torque need to be measured accurately.
- Harsh Environment Tolerance: SAW sensors are robust and can withstand harsh environmental conditions, including electrical noise, magnetic fields, high temperatures, humidity, and vibration.
- Non-contact Measurement: SAW sensors are contactless, which means they do not require physical contact with the rotating shaft or moving parts of the robot. This means they are easier to integrate into shafts or other components that rotate more than 360 degrees.
- Small Size and Lightweight: SAW sensors can be designed in a compact form factor, making them suitable for integration into robot joints, the wider range of installation possibilities means the sensor can be directly integrated into a motor shaft or gearbox reducing the overall size and complexity of the robot joint.
Surface Acoustic Wave sensor technology is a viable alternative to strain-gauge and other sensing technologies for robotic joint torque sensing, especially those that require a high-frequency response, where robot operating performance is paramount such as higher payload requirements or more repeatable and high-speed operation required, and the unique packaging opportunities bring benefits to the robot design. SAW sensors have not previously been a commonly adopted solution due to a lack of knowledge about the technology and a lack of availability of practical components for the sensing elements and electronics that can be used to implement robust production solutions. Transense Technologies plc has SAW sensing technology that has proven reliability in demanding applications and can be readily applied to deliver improved torque sensing in robotics.