GLOSSARY
Archimedes Academy videos >
Actuators are the muscles of robots, converting energy into mechanical motion. They are responsible for moving or controlling mechanisms or systems. There are three main types of actuators: electrical, hydraulic and pneumatic actuators. Within the robotics industry electrical actuators, consisting of a motor, drive and encoder are the standard. If there is no driver and encoder, the actuator is just the electrical motor. Note that in some other industries the definition of an actuator differs.
The Archimedes Drive is a patented traction-based speed reducer. It utilizes traction rollers (also called flexrollers) to transmit rotational forces, combining this with the compound planetary principle for distribution and transmission of these forces. Therefore, the Archimedes Drive offers true zero backlash, high stiffness, and exceptional efficiency, making it ideal for applications requiring precise and reliable motion control. Furthermore, it also enhances the drive’s durability, reducing wear and tear over time. The innovative combination of these properties sets the Archimedes Drive apart as a superior solution in the realm of mechanical drives.
Robots capable of navigating and performing tasks in dynamic environments without human intervention. They use sensors, cameras, and artificial intelligence to make decisions in real-time. Not to be confused with an AGV (Automated Guided Vehicle, which follows a fixed path).
Backdrivability is the ability of a drive system to allow external forces to move the output shaft backwards. This characteristic is crucial for systems that need to interact safely with humans or their environment. In the context of robotics, if a robot arm is backdrivable, it can move in response to external forces, such as contact with a human, thereby reducing the risk of injury.
Backdrivability is a key feature for collaborative robots (cobots) that are designed to work alongside humans without the need for safety cages. This feature ensures that cobots can safely coexist with human workers, as they can respond to unintended contacts by backing off.
In contrast, traditional industrial robots are typically not backdrivable and pose a higher risk of causing harm if they come into contact with humans, necessitating the use of safety barriers or cages around them to prevent accidents.
Furthermore, backdriveability is crucial for force feedback systems. For example, if you have a surgical robot, manually operated over a distance by a doctor one can choose to provide force feedback, making it heavier to move when you move towards a dangerous area. In that case, the doctor is operating by moving the output side of a drive, while there is a motor on the input side providing the resistance.
Backlash (also called slop, play, or free-play) refers to the amount of movement between two mating gears or components, which occurs when the driving member is not directly connected to the load. It represents the gap between the trailing and leading edges of adjacent gear teeth. This gap allows for slight movement of the gears without immediate transmission of motion, resulting in a lag or play in the system.
Backlash is an inherent characteristic of gear systems and can affect the precision and accuracy of mechanical systems, especially in applications requiring tight control and positioning, such as robotics, CNC machinery, and instrumentation. Minimizing backlash is crucial for achieving smooth and accurate motion control. Various design techniques and adjustments, such as using precision gears or preloading, can be employed to reduce its impact.
A drive where extra elements are added, typically to increase the gear ratio or reduce stresses. This can for example be done by adding an extra row of planetary wheels, like in the Nasvytis drive, or by adding an extra layer to a planetary drive with a second annulus and sun wheel, while extending the planet wheels through both layers, like in Wolfrom drives.
Type of drive where the input shaft is mounted to a cycloidal disc via an eccentric bearing. This disk moves around ring pings to transmit the torque to the output. Cycloidal drives are known for their robustness, low backlash and quite high gear ratios they can achieve. Vibrations are a downside of this technology.
Refers to the number of independent movements a robotic joint or mechanism can perform. Basically, the more degrees of freedom, the more skilled and flexible the arm.
For example, a robotic arm with six degrees of freedom can move in six different ways: three translational movements (along the X, Y, and Z axes) and three rotational movements (around the X, Y, and Z axes).
However, robots can potentially have more than six degrees of freedom. For instance, an android designed to mimic human movements might have two arms, two legs, and a head, each with multiple degrees of freedom. This configuration allows the robot to replicate the complex and varied movements of human joints, such as those in the shoulder, elbow, wrist, hip, knee, and neck.
A virtual model of a physical system that replicates its real-world counterparts in a digital environment. These models are used for simulation, analysis, and control of physical systems.
Device used to adjust the output speed and torque of a motor. Typically, they decrease the speed and increase the torque by the same reduction ratio, allowing precise control of mechanical systems. These components are critical in applications where specific speed and torque characteristics are required, such as in industrial machinery and robotics.
Note: Drives are not a synonym for actuators or transmissions, but rather a component of these device. See Actuator or Transmission for more info.
A hollow cylinder used in Archimedes Drives instead of planet gears to provide a backlash free movement.
Friction is the force that opposes motion when two surfaces rub together.
It is always a reaction force that occurs when two surfaces are in contact, and a force is applied such that they slide along one another. If the friction is stronger than the force, the object won’t move. This is static friction. If the force is strong enough to overcome the static friction and move an object into the sliding regime, one speaks of dynamic friction.
The amount of friction is determined by the friction coefficient of the surface and the normal force.
Typically, friction is not desired as it lowers the Efficiency, and one tries to keep it as low as possible by decreasing the friction coefficient or the load. An exception to this is when one wants to make use of traction. In that case, the friction coefficient is typically increased.
ISO/TS 15066:2016 is a technical specification that provides guidelines for the safe design, implementation, and operation of collaborative robots, also known as cobots, which work alongside human operators. This specification builds on the general safety requirements found in ISO 10218-1 and ISO 10218-2, which cover industrial robot safety.
ISO/TS 15066:2016 applies to industrial robot systems as described in ISO 10218‑1 and ISO 10218‑2. It does not apply to non-industrial robots, although the safety principles presented can be useful to other areas of robotics.
You can find out more here.
Since there can always be a failing drive in a production batch and there can be quite some variation between the similar drives in the time they run before they fail, lifetime is typically defined as a statistic. L10 stands for the lifetime where 90% of the drives still work, while the worst 10% has already failed.
A type of robotic joint that allows linear or translational motion along one axis. It is used in applications requiring straight-line movements. Linear Joint is a general term for any joint that provides linear movement, which may include prismatic joints and other types that allow for both linear and limited rotational motion.
Lost motion refers to the amount of deviation or lag in the output of a mechanical system that occurs without a corresponding movement at the input. This phenomenon is typically caused by a combination of factors such as backlash, which is the gap between mating components; compliance in the system, which refers to the flexibility or elasticity of materials; and material windup, or torsional deflection, where materials twist under load. Lost motion can lead to inaccuracies in motion control and is an important consideration in precision engineering, robotics, and automated systems where precise alignment and movement are crucial.
Levers are devices that move loads at a certain moment and always need three elements: a fixed fulcrum around which the device rotates, a force to make the movement and a load to be displaced. Depending on where these three elements are positioned with respect to each other, a lever can be defined as three different classes.
Gears can also be seen as levers. The fulcrum of the driving wheel is its center axis, which at the same time is also used as the force, by rotating it. The load is at the place where the gears mesh. For the driven gear, the place where the gears mesh can be seen as the force, while its axis acts both as the fulcrum and the load that is moved. Within the gearbox industry we however like to talk about the input, output and ground instead of the force, the load and the fulcrum.
When metal is loaded cyclically, after a while miniscule cracks start to initiate. Once they are there, every cycle the crack will propagate a bit until it eventually reaches the surfaces and causes a fatigue crack. The higher the cyclic load, the sooner fatigue will occur. Even with stresses that are way below the yield stress, in the elastic regime. Fatigue usually occurs at some point. The picture below shows a tool that broke due to a fatigue crack as well as fatigue surface damage for component from traction devices.
A robot that combines mobility and manipulation capabilities, allowing them to move around and interact with objects or perform tasks in various locations.
An engineering term that is best described as a rotational force. A moment is defined by the force times the distance and is expressed in Nm. On a seesaw you will see it’s balanced if the moments on both sides are equal. For example, if one person is twice as heavy as the other, but sits halfway on the seesaw, it’s still in balance. Note that within mechanics a moment refers to a static force of a device that is not in motion. If a device is rotating, the rotational force is called torque.
The condition where the applied torque exceeds the designed limit of the transmission system. This can occur due to sudden load changes, start-up or shutdown operations, extreme wear of mechanical parts, or mechanical errors. Overtorquing can lead to catastrophic failures, such as tooth breakage, resulting in costly repairs and downtime.
Mechanisms or devices designed to prevent or limit the effects of overtorque, protecting the system from damage by automatically disengaging or absorbing excess torque. The Archimedes Drive possesses the unique ability to enter into a sliding regime when subjected to conditions involving excessive torque. This slip mechanism acts as a safety feature, absorbing the excess forces, and thereby preventing catastrophic damage.
Gear systems known for their high torque and reasonable precision, typically with a backlash range of 3-8 arc-minutes. Over time, wear and tear can increase this range by 1-3 arc-minutes.
The ratio of the output power to the input power, usually expressed as a percentage. Efficiency indicates how much of the input energy is converted into useful work. For example, in a mechanical system, it is the ratio between the energy required to do a task and the total energy spent. The formula for efficiency is:
Efficiency is crucial not only for reducing electricity costs but also for optimizing the use of the electric grid. In industrial settings, such as factories, the electric grid’s capacity (the amount of electricity available) must be efficiently managed. Higher efficiency in robots and machinery means lower energy consumption, allowing more robots to operate within the same electric grid capacity.
Mobile robots are battery powered, so here the benefit of high efficiency is that the action radius, the travel distance before the battery runs out, is higher. Furthermore, efficiency allows for easier control algorithms for precise applications, because it is easier to predict movements with at least energy losses as possible.
Reach in the context of robotics refers to the maximum distance that a robotic arm or manipulator can extend from its base to its end effector (such as a gripper or tool). This distance is measured in a straight line from the base of the robot to the farthest point the arm can extend. Reach is a critical specification in determining the robot’s working envelope or workspace, which is the area within which the robot can operate effectively.
Here the reach are the black numbers while the working area is represented in red
Working Range in robotics refers to the total area or volume within which a robot can effectively perform its tasks. This includes not only the maximum reach (how far the robot can extend its arm) but also considers the vertical and horizontal movement limits, angular movement of joints, and any other degrees of freedom that the robot possesses.
A type of robotic joint that allows rotational motion around one axis. It is commonly used in robotic arms and other systems requiring angular movement.
Sensors in robotics are devices that detect and measure various physical properties such as temperature, pressure, distance, motion, and more. They convert these measurements into electrical signals that can be used for monitoring, control, and decision-making processes within robotic systems. These sensors are critical for enabling robots to interact with their environment and perform tasks accurately and efficiently.
Electric motors equipped with sensors and controllers to provide precise control of position, speed, and acceleration. They are commonly used in robotics, CNC machinery, and automation systems.
A condition in mechanical systems where components slide over each other.
The sliding regime of the flexrollers in the Archimedes Drive occurs when the rollers spin on their own axes without moving the outer ring. Normally, these rollers roll smoothly between the inner and outer rings to transfer motion, but in the sliding regime, the contact force between the rollers and the outer ring isn’t enough to make the ring move. This typically happens when the required torque exceeds the system’s capacity or during a max impact torque event. It acts as a protective mode, allowing the system to handle overtorque by letting the rollers slip, rather than causing damage or wear. This helps to compensate for the excessive load and ensures the longevity and reliability of the drive.
Stiffness in a robotic joint refers to the resistance of the joint or component to deformation or deflection under load. This can either mean the axial stiffness which is measured in Newton per meter (N/m) or the torsional stiffness, which is measured in Newton-meters per arc-minute (Nm/arcmin). This property is crucial as it determines how much a system resists deformation when subjected to external forces. The stiffer the system, the larger the load required to cause deformation.
Joint stiffness has a significant impact on the motion accuracy of a robotic end effector, especially at high speeds. Higher stiffness ensures that the robot maintains precise positioning and control, which is essential for tasks requiring high accuracy. Conversely, lower stiffness can lead to increased flexibility, potentially causing errors in the robot’s movements and reducing the precision of the end effector’s operations.
A type of gear that’s also known as a Harmonic Drive, which is also the name of the company that invented it. It consists of an oval bearing attached to the input that constantly stretches a flexible steel cup with gears on the outside (the flex spline) on two sides, pushing the gears on the outside into the next gear on a circular spline. This drive is known for its extremely low backlash, very high gear ratios and its compactness. It is, however, expensive and has a low efficiency.
Stress is an engineering term that describes the forces distributed throughout a system during deformation. It is typically defined as the force applied divided by the cross-sectional area of the material being deformed. Common types of deformation include tension, compression, shear, bending, and twisting. In traction applications, Hertzian Contact Stress is also significant—this type of stress occurs up to a few millimetres beneath the surface (for steel surfaces).
Stress is often represented in a stress-strain diagram. In the initial stages of strain, the stress increases linearly. Up to the maximum point in this region, known as the yield stress, the deformation is elastic: when the force is removed, the material returns to its original shape. Beyond this point, the material reaches ultimate tensile stress and eventually fracture stress. In this region, the material undergoes “plastic deformation’’, meaning the changes are permanent, and the material will not revert to its original shape after the force is released.
A tension-based transmission with flexrollers is a type of mechanical transmission system where tension is transmitted directly between cylindrical components, known as flexrollers, rather than using gears, belts or cables. These systems are designed to leverage the contact and friction between the cylinders to transfer motion and force.
Where the moment describes the rotational force of a static object by multiplying the force times the arm, torque refers to the rotational force of a moving object.
Tstart : Starting torque/break-away torque – Torque required to overcome the static friction and get a drive in motion.
Tback : Backdrive torque – Same as the starting torque, but now for a backdriving motion.
TN : Nominal torque/rated torque – The torque for which the desired lifetime is achieved. Within industrial robotics, this is typically an L10 of 1 billion cycles.
TR : Max repeated torque/Peak torque/Max acceleration torque – During a cycle the torque typically peaks during acceleration and deceleration. The desired L10 will still be achieved when the acceleration stays below this limit. Constantly running at this torque will typically lead to an L10 of 100 million cycles. For strain wave gears, TR is typically double the nominal torque.
TM : Momentary peak torque/Emergency stop torque – The maximum torque before direct failure will occur. Constantly running at this torque will typically lead to an L10 of 10 million cycles, so one tries to prevent this as much as one can. For strain wave gears, TM is typically four times the nominal torque.
TSlip : Torque limit specific to the Archimedes Drive where the drive starts slipping. There is no TM where the drive will break while exceeding this. There is no fixed relation between TSlip and TM.
Also known as the torque to weight ratio or T/W-ratio. For a lot of applications, it holds that the lighter a drive is, the better. However, since the maximum torque typically scales with the weight, weight itself is not a property that discloses a lot of information. Therefore, using the torque density is a convenient way to assess how the weights of different drive types and torque amounts compare.
Traction is formed by the friction between a driving wheel and the surface it moves upon.
It is the amount of force a wheel can apply to the surface before it slips. In traction applications, like the wheel of a car, one typically doesn’t want the wheel to slip, because then you will lose the tractive force that moves the wheel forward. Therefore, one typically tries to find ways to maximize the friction coefficient. Without slipping, the tractive force is always equal to the static friction. If a wheel slips, there is often still some traction, equal to dynamic friction.
The definition of a transmission differs per industry. Within robotics, it’s defined as all the components after the motor that transmit and adjust the torque all the way to the end effector. This includes components like the drive and belts that transmit the torque over a larger distance.
In engineering and robotics, a transparent mechanism refers to a system whose behavior can be predicted and effectively controlled, as there is a clear and consistent relationship between input actions and output responses. This transparency ensures that the system’s operation is intuitive and understandable, allowing for precise control and predictable performance.
