Both axes arrive, but the tool takes the wrong path
An XY dispenser travels from (0,0) to (300,400) mm. Independently starting an X move and a Y move does not generally guarantee a straight line. Each axis has its own profile, so one can reach its target early and the tool path bends.
Coordinated motion plans a path for an axis group. The planner converts that path into member-axis demands while respecting applicable path and axis limits. PLCopen Part 4 covers coordinated motion; use the edition and features supported by the installed vendor library.
Split path speed into axis components
The straight-line length is sqrt(300² + 400²) = 500 mm. At an ideal constant path speed of 100 mm/s, X velocity is 60 mm/s and Y velocity is 80 mm/s. Their vector magnitude is 100 mm/s. Giving both axes 100 mm/s produces a different direction and a path-speed magnitude of about 141.4 mm/s while both are moving.
path_length = sqrt(dx² + dy²)
vx = path_speed * dx / path_length
vy = path_speed * dy / path_lengthThe ideal travel time of five seconds excludes acceleration and braking. The path bench makes this exclusion explicit. A real planner adds the appropriate ramps and may reduce path speed to respect an individual axis's limits.
Group ownership and coordinate systems
Powering each drive and enabling the axis group are distinct operations. Group enable transfers control according to the runtime. Do not let an independent manual-axis routine command a member while the group owns it unless the library supports a deliberate transition.
A machine coordinate system describes the machine frame. A workpiece coordinate system locates the part. A tool coordinate or transformation relates the tool geometry to the programmed point. A correct numerical path in the wrong frame can put the tool into the fixture.
For a Cartesian gantry, the mapping is relatively direct. Robots and other mechanisms require kinematic transformations, with possible singularities, branch choices and workspace restrictions. A Cartesian destination that looks reachable on a drawing may exceed an individual joint limit.
Lines, circles and corners
MC_MoveLinearAbsolute and related operations describe coordinated lines. Circular operations need the vendor's definitions for endpoint, centre or auxiliary point and plane. A path made of line segments has a corner where direction changes. Exact corner passage at nonzero speed would require an instantaneous velocity-direction change.
Choose either a stop/slowdown at the exact corner or a permitted rounded transition. BufferMode concerns operation relationship; TransitionMode and tolerance concern path geometry in libraries that expose them. A speed setting alone does not define the allowed corner deviation.
Moving products
Conveyor tracking introduces a moving coordinate system. The product's frame changes while the tool moves. Include the captured product location, master encoder relation and tracking-in/tracking-out lifecycle. Applying a fixed machine-frame endpoint to a moving product gives the correct coordinate for the wrong instant.
TryPLC's path bench is an analytic Cartesian line. It does not execute group blocks, inverse kinematics, transitions or conveyor tracking. The ST engine currently rejects the coordinated block families so their absence is visible.
Bench investigation
Use X 300 mm, Y 400 mm and path speed 100 mm/s. At 50% completion, the tool should be at (150,200) mm. Verify the length and velocity components. Then set Y to zero; all path speed belongs to X. Change the endpoint and observe how the components change while their magnitude stays at the entered speed.
Exercises
Specify a dispenser path with a 90-degree corner. Decide whether bead geometry requires exact corner position or permits a rounded transition. Name the tolerance and axis limits that must be checked. For a pick from a moving conveyor, draw both frames and state how a capture becomes the product's tracked reference.
References
CODESYS group setup, linear motion and dynamic tracking document these separate operations.