By Peter Nachtwey
Delta Computer Systems Inc.
Edited by Victoria Reitz
Advanced material handling goes far beyond simply moving objects on conveyor belts. In some cases, applications involve complex motion-control sequences to position, hold, transport, and steer materials through one or more production processes. Motion control becomes even more challenging when it involves large, heavy items such as logs or metal parts. Fluid power is often used for these material-handling challenges.
Fluid power excels in lifting and holding heavy objects, applying pressure, and squeezing objects with a controllable force. Hydraulic cylinders simply need to be "locked" to maintain even pressure, whereas electric motors doing the same job must continually exert force, which may lead to overheating and failure.
And the fluid-power advantage is that a single source of pressure and flow — such as a compressor or pump — can power many fluid-power devices, including a mix of rotary and linear-motion devices. Power sources can be located away from the motion device, saving weight and space in cramped locations. And fluid-power actuators are typically smaller and cost less than electric motors with the same power.
In addition, because accumulators are typically used to store energy in fluid-power applications, smoothing out pressure transients as the system operates, pumps or compressors can typically be sized to provide little more than the average pressure and flow required. By comparison, electric motors used in material-transport systems must typically be sized to handle maximum loads.
Engineers should consider a number of specific motion-control issues when designing fluid-power-based material-handling systems.
Correct control algorithms are critical. For example, to control transitions between holding position, to applying pressure, to moving while applying pressure, engineers must predict the motion of one or more of the axes. Given object size and position (from a PLC or other input), the motion controller can decrease hydraulic pressure in one axis before the second axis is engaged to grip the object. This minimizes pressure spikes and reduces hydraulic shock, avoiding damage to the object and lengthening life of the machine. Incorporating predictive factors in motion-control algorithms is called feedforward and is important for smooth material transfers.
Without incorporating predictive factors in the control algorithm, the traditional proportional, integral, and derivative (PID) feedback loop must do all the work. But PID control is responsive only, generating control inputs that eliminate errors between actual and target conditions. Therefore, there can be time lag as the error is reduced to zero. During this lag period, material-handling errors due to pressure overshoot or undershoot can damage or misalign the object. Ideally, control systems should be set up so that most work is done using the feedforward term, with PID inputs used only to tweak position or pressure.
Incorporate pressure control along with position control. It is the secret to gripping. Pressure limits should let actuators grab objects with enough force to hold them, not break them.
For example, a system could have the motion of one or more axes "geared" to the motion of another and use a combination of position and pressure controls to engage or grip an object. Once gripped, the object is moved by controlling the velocities of the multiple axes moving in tandem, and the system maintains position and pressure controls to keep the object from slipping.
The transitions between position, pressure, and velocity control may be coordinated by a programmable logic controller (PLC) or sequential programming of the motion controller. Data such as object size and position, production process details, and motioncontroller status help determine the correct time and position to transition the motion controller from position control (with pressure override), to "geared" control (with pressure override), and then velocity control (without pressure override).
Ensure the system selfadjusts for line conditions. Temperature changes affect viscosity of hydraulic fluid, which causes cylinders to overshoot or undershoot and leads to material or system damage. A motion controller can adjust to viscosity changes as they occur.
Keep it flexible. Material handling often requires rotary, linear, and pressure controls. Motion controllers that handle multiple loops can reduce the number of hydraulic valves, and thereby reduce system complexity and cost.
Design for expandability. Take advantage of standard networking and fieldbus protocols. They make expanding the system a matter of simply adding a network drop for additional motion controllers.
Keep them doggies moving
One system designed for optimal motion control is the "end-dogger" by Maxi Mill Inc., Albany, Oreg. It coordinates between multiple-motion axes to keep both ends of a log clamped as it is cut into boards. While the log is being cut, the system must also deftly manage position and pressure control to make sure the log is clamped with enough force to hold it and move it through the saws.
"Dogs" are the stops that come into direct contact with the log on each end to grip and move the log toward the saws and chippers. In this application, logs are first loaded onto turning rolls with four axes of position control. The logs are laser scanned, and an optimizing computer calculates offset and skew, positioning the log to be cut for the best board yield and assigning a gripping pressure threshold based on the diameter of the log. The log is then loaded onto the conveyor from the side, and the lower conveyor is moved so that dog A pushes the log into contact with dog B on the upper conveyor. Once the dogs are engaged, they move the log through the chip heads and saw, then disengage and release the board.
In this case, there are five distinct segments of speed control to cut each log: velocity to engage the back dog, velocity to push the log into the upper dog, velocity to enter the chip heads/saw, velocity in the saw, and exit velocity. A motion profile controls the full process.
The conveyors are driven by hydraulic motors, and position is controlled using quadrature inputs from sensors mounted on the conveyor drive axes. The top dog's pressure setpoint is set to a low value to receive the log. Pressure transducers are mounted in hydraulic lines going to and from both hydraulic motors. When dog B begins to move and pressure rises in the line, the system knows the log has come into contact with dog B.
Once the pressure measured at the hydraulic motor for the upper dog conveyor exceeds a set threshold, the pressure ramps up to the gripping setpoint value. At the same time, a control loop for the top conveyor motor tries to hold the position of dog B, while the lower conveyor continues to push the log to the right. This creates a position error that will create gripping pressure.
When target pressure on the log is met, based on information from the optimizing computer, the appropriate amount of grip on the log holds it steady without damaging it. The log has been engaged, or "dogged." At this point, the control system changes to position controlled (while still monitoring and controlling the maximum gripping pressure), and the top dog moves in a one-to-one ratio with the bottom dog. With this lock-step motion in process, the upper dog conveyor is "geared" to the operation of the lower conveyor. Gripping pressure is maintained even if the bottom dog stops and reverses drive direction. The control system continually adjusts the force applied by each dog independently, making sure the force is enough to hold the log firmly, yet not oppose movement.
Once the log is cut, the system changes the top dog out of gearing mode into velocity mode, sets the pressure high, and accelerates the top dog away from the log. The bottom dog pushes the log out, the position error is cleared from the motion controller, and the top dog goes around and back to the home position.
Gearing is the coordination of two or more motion axes, such that the motion of one axis depends on the motion of the other. Gearing can be as simple as moving two axes in lock step (1:1 ratio), such as in a simple conveyor, or as complex as moving one axis at a variable rate compared to another, as in a tensioning or pouring operation.
Gripping is common in material handling in which items must be gripped between two or more motion axes and sent through a process. Items must be gripped tightly enough that they don't move out of position during processing, but not so tightly that they are damaged. This is usually done by using pressure limiting and gearing the motion of one axis to the motion of another.
Splines define nonlinear, input-based motion. Time or position inputs, which may come from operators or designed into the system let the motion controller calculate the smoothly varying motion of an axis.
Camming uses the motion of an axis as a variable time function of the motion of another axis, such as when an axis approaches to remove parts from a moving conveyor and sends them to another process.
Synchronizing lets multiple axes be at their target point at the same time. When moving large objects, for example, there are often two control axes, one at each end of the object, which must be perfectly synchronized to hold the item in place and reduce wear and tear on the machine.
Superimposed moves let one axis follow another at a predetermined time or distance. For instance, it can initiate a production process that occurs at a given time or place on the conveyor.