A dwell mechanism (either a linkage or cam-follower type) is an intermittent motion mechanism that alternates forward and return motion with holding position(s). [1]
A single dwell cam mechanism has a motion function which follows a dwell-rise-fall sequence that repeats.
It is sometimes desired to use a rise function such that the acceleration of the follower is non-zero at its end of the rise. To maintain continuity, the fall function often begins such that the follower has the same non-zero acceleration as it had at the end of the rise function. The follower then stays stationary during the dwell function, with its velocity, acceleration, and jerk zero. [2]
There are a number of functions that can provide these motion requirements. A double-harmonic function is a common example used for single dwell. [3]
A double dwell cam mechanism has a motion function which follows a rise-dwell-fall-dwell motion function sequence that repeats.
Unlike the single dwell, the rise function position ends with the follower's acceleration becoming zero, as it is stationary in the first dwell. Only after the first dwell does the acceleration become non-zero again during the fall function, returning the follower back to its original position and staying stationary again during the second dwell.
Dwell linkages cannot achieve a perfect dwell, unlike cam mechanisms, but rather have approximate dwells, where the output is remains roughly still.
A single dwell linkage takes advantage of the behavior of a link with a stationary revolute (hinge) or prismatic (sliding) joint and its interaction with special coupler curves.
There are two methods of producing a single-dwell: arc-based and linear-based.
An arc-based single dwell linkage uses the approximation of a circular arc. The concept for linkage dwell mechanisms is that a node located at the center of the circular arc segment of a coupler curve will remain relatively stationary.
This is achieved first by choosing a desired coupler curve created by a four-bar linkage. Once a coupler curve is chosen, a circle is fit as closely as possible to a section of the coupler curve. The center of the fitted circle then becomes the position of the dyad: two links which are connected to the joint producing the coupler curve on one end by a revolute joint, and a stationary revolute joint on the other, creating the single dwell. [4]
A linear-based dwell uses a similar approach, but using an approximated straight line and a prismatic joint, in replacement of the revolute joint, for the dyad's connection to the coupler curve.
There are several approaches to producing a double dwell linkage.
One such approach is to use a coupler curve with multiple approximated straight lines. Then, a dyad is positioned such that it is roughly tangential to both approximate straight lines. If the approximate straight lines are parallel, a pin and slider similar to that used in a scotch yoke mechanism can be used.
An extension to this approach is to use a quarter-circle arc produced by an oscillating link. A dyad with a prismatic joint can then be positioned such that the output link oscillates 90 degrees opposite of the oscillating link. This allows the design to be tile-able, with dyads to be appended onto each-other, and allow for double-dwells of any desirable occupied duration of the input stroke.
Although using the arc-based approach–the other method used for single-dwell linkages–is theoretically possible, it is significantly more difficult to execute and is impractical in practice, as both approximated arcs must share a circle of the same radius.
Mechanisms have also been developed based on buckling beams and arcs. [5]
The actual dwell time will depend on the length of the approximate circular arc or straight line in the coupler curve. Initial designs may need optimization to improve the dwell characteristics. [6]
Cam-follower dwell mechanisms are used in pairs in sewing machines to operate the four motion feed dogs, with one cam moving the dog up and down, and the other cam moving the dog forwards and backwards. The cams in this application are usually phased 90 degrees apart allowing a pause in the up/down movement of the dog while it is being moved forwards/backwards. A separate adjustable sliding block or link is used to control the amount of forwards/backwards movement of the dog.
Industrial applications include loading and unloading parts, or transporting a part to a machine and holding it in place for a manufacturing process. [7]
Other applications include assembly lines, packaging machinery, machine tools, etc.
In physics, jerk (also known as jolt) is the rate of change of an object's acceleration over time. It is a vector quantity (having both magnitude and direction). Jerk is most commonly denoted by the symbol j and expressed in m/s3 (SI units) or standard gravities per second (g0/s).
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path. The cam can be a simple tooth, as is used to deliver pulses of power to a steam hammer, for example, or an eccentric disc or other shape that produces a smooth reciprocating motion in the follower, which is a lever making contact with the cam. A cam timer is similar, and were widely used for electric machine control before the advent of inexpensive electronics, microcontrollers, integrated circuits, programmable logic controllers and digital control.
A machine is a physical system that uses power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromolecules, such as molecular machines. Machines can be driven by animals and people, by natural forces such as wind and water, and by chemical, thermal, or electrical power, and include a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement. They can also include computers and sensors that monitor performance and plan movement, often called mechanical systems.
Kinematics is a subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies without considering the forces that cause them to move. Kinematics, as a field of study, is often referred to as the "geometry of motion" and is occasionally seen as a branch of mathematics. A kinematics problem begins by describing the geometry of the system and declaring the initial conditions of any known values of position, velocity and/or acceleration of points within the system. Then, using arguments from geometry, the position, velocity and acceleration of any unknown parts of the system can be determined. The study of how forces act on bodies falls within kinetics, not kinematics. For further details, see analytical dynamics.
Mechanical toys are toys powered by mechanical energy. Depending on the mechanism used they can perform a range of motions, from simple to very complex.
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In the study of mechanisms, a four-bar linkage, also called a four-bar, is the simplest closed-chain movable linkage. It consists of four bodies, called bars or links, connected in a loop by four joints. Generally, the joints are configured so the links move in parallel planes, and the assembly is called a planar four-bar linkage. Spherical and spatial four-bar linkages also exist and are used in practice.
A mechanical linkage is an assembly of systems connected to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to be rigid. The connections between links are modeled as providing ideal movement, pure rotation or sliding for example, and are called joints. A linkage modeled as a network of rigid links and ideal joints is called a kinematic chain.
In physics, the degrees of freedom (DOF) of a mechanical system is the number of independent parameters that define its configuration or state. It is important in the analysis of systems of bodies in mechanical engineering, structural engineering, aerospace engineering, robotics, and other fields.
In mechanical engineering, an overconstrained mechanism is a linkage that has more degrees of freedom than is predicted by the mobility formula. The mobility formula evaluates the degree of freedom of a system of rigid bodies that results when constraints are imposed in the form of joints between the links.
In classical mechanics, a kinematic pair is a connection between two physical objects that imposes constraints on their relative movement (kinematics). German engineer Franz Reuleaux introduced the kinematic pair as a new approach to the study of machines that provided an advance over the motion of elements consisting of simple machines.
A straight-line mechanism is a mechanism that converts any type of rotary or angular motion to perfect or near-perfect straight-line motion, or vice versa. Straight-line motion is linear motion of definite length or "stroke", every forward stroke being followed by a return stroke, giving reciprocating motion. The first such mechanism, patented in 1784 by James Watt, produced approximate straight-line motion, referred to by Watt as parallel motion.
In kinematics, cognate linkages are linkages that ensure the same coupler curve geometry or input-output relationship, while being dimensionally dissimilar. In case of four-bar linkage coupler cognates, the Roberts–Chebyshev Theorem, after Samuel Roberts and Pafnuty Chebyshev, states that each coupler curve can be generated by three different four-bar linkages. These four-bar linkages can be constructed using similar triangles and parallelograms, and the Cayley diagram.
The Klannlinkage is a planar mechanism designed to simulate the gait of legged animal and function as a wheel replacement, a leg mechanism. The linkage consists of the frame, a crank, two grounded rockers, and two couplers all connected by pivot joints. It was developed by Joe Klann in 1994 as an expansion of Burmester curves which are used to develop four-bar double-rocker linkages such as harbor crane booms. It is categorized as a modified Stephenson type III kinematic chain.
In engineering, a mechanism is a device that transforms input forces and movement into a desired set of output forces and movement. Mechanisms generally consist of moving components which may include:
A mechanical joint is a section of a machine which is used to connect one or more mechanical part to another. Mechanical joints may be temporary or permanent; most types are designed to be disassembled. Most mechanical joints are designed to allow relative movement of these mechanical parts of the machine in one degree of freedom, and restrict movement in one or more others.
A Hoberman mechanism, or Hoberman linkage, is a deployable mechanism that turns linear motion into radial motion.
A slider-crank linkage is a four-link mechanism with three revolute joints and one prismatic, or sliding, joint. The rotation of the crank drives the linear movement of the slider, or the expansion of gases against a sliding piston in a cylinder can drive the rotation of the crank.
In mechanical engineering, kinematic synthesis determines the size and configuration of mechanisms that shape the flow of power through a mechanical system, or machine, to achieve a desired performance. The word synthesis refers to combining parts to form a whole. Hartenberg and Denavit describe kinematic synthesis as
...it is design, the creation of something new. Kinematically, it is the conversion of a motion idea into hardware.
In robotics, Cartesian parallel manipulators are manipulators that move a platform using parallel-connected kinematic linkages ('limbs') lined up with a Cartesian coordinate system. Multiple limbs connect the moving platform to a base. Each limb is driven by a linear actuator and the linear actuators are mutually perpendicular. The term 'parallel' here refers to the way that the kinematic linkages are put together, it does not connote geometrically parallel; i.e., equidistant lines.
