TechArchive

Original article date: April 1999

Quick to learn and fast to execute motion control programming is becoming an attractive alternative to traditional mechanical linkages. MILES CLEE of Baldor Optimised Control explains the features of the company’s MINT language.

Programs developed in Baldor’s motion control programming language MINT can be used across the company’s line of motion control products ranging from a single-axis standalone controller through to eight axis PC controllers.

MINT can be used to position a single axis system fast and accurately with positions able to be taught and replayed. Typical applications here would be press feeders packaging machine actuators and cut-to-length systems.

MINT supports linear interpolation across eight axes and circular interpolation across two. Circular and linear moves can be blended together using the keyword CONTOUR to achieve a smooth continuous motion along a complex path. This is aimed at applications such as sign cutting wood routing and glue laying .

MINT’s memory ARRAY feature is for teaching and replaying the positions of a pick-and-place robot. XYZ axis positions can be numerically programmed from Baldor’s CAN operator panel or manually taught by jogging each axis to the desired position. Arrays provide an efficient way of storing a large number of position points and they offer the flexibility of editing deleting or replaying a small segment of the data.

Software cams are useful to replace traditional mechanical cams with a software data table and a servo or vector drive. They provide additional axes of motion on packaging machines and allow cam changes to be performed at the touch of a button. Typical applications would be packaging machines rotating machines and reciprocating mechanisms.

Advanced software gearbox and software clutch functions enable two or more axes to be linked together in precise synchronisation. Motion control systems can be configured to follow a master encoder or pulse and direction inputs. The position of the follower axis is locked to the position of the master axis in much the same way as a mechanical gear with the gearing ratio set in software using the MINT keyword FOLLOW. The input shaft (Master) can be any open loop system with position feedback. The output shaft (follower) may be any closed loop drive such as a vector or servo. Multiple output shafts may be geared to a single Master input

A software programmable gearing ratio makes it possible to change the ratio on demand. This makes it unnecessary to shut down a machine to change gear sizes. The gear ratio could then be changed via the serial port operator panel or a simple thumbwheel switch connected to digital inputs. This provides a replacement for variable ratio mechanical gearboxes and would be useful for continuous flow process lines and webs and flexible format packaging machines.

MINT’s software clutch can be used to accelerate a following axis from standstill to a speed set by the software gearbox and Master speed. The acceleration can be controlled over a defined distance on the Master machine to maintain precise position registration. Applications for the software clutch include clutch-brake replacements rotary knives and flying shears where cutting printing or labelling have to be performed on a moving web. MINT’s flying shear functions perform operations at accurate intervals on a moving product on the basis of feedback form a Master encoder. In a typical application the cutter is mounted on a linear reciprocating slide and is synchronised with the moving product for each cut. The acceleration synchronisation and return stroke are linked by software to the movement of the product so accurate position reference is always maintained.

In most systems it is necessary to correct for irregularities in processed materials or machine deficiencies. Examples are plastic film webs used for wrap packaging stretch and distort; cartons exiting a filling machine in random orientations and spacings; worn mechanical elements introducing backlash into a line process; product slips and slides on a continuous conveyor line.

Such irregularities can be compensated for by advancing or retarding the position of a following axis in relation to its expected position. MINT’s OFFSET keyword can be used a positive or negative positional offset on top of the base speed of an axis. The base speed may be a function of software gear ration and Master speed or simply a set speed.

The direction and duration of the position offset is calculated in MINT based upon the sensed position of a registration mark or product on the web. A sensor connected to the Fast Position Capture input of any MINT motion controller can capture the position of all axes within 30us or better.

If a product arrives at the sensor ahead of where it is expected to be a negative position offset is applied. If it arrives behind where it is expected a positive position offset is applied.

About MINT

The language is similar to BASIC in many ways with enhancements for real-time machine control. It supports a wide range of control keywords that are designed to provide fast execution of complex motion control functions. Examples are CAM and FOLLOW.

A single program written in MINT can control motion I/O communications operator interface and error handling. For applications requiring faster program execution a C motion library is available for the Nextmove PC and BX controllers.

Products are supplied with a programmers toolkit a comprehensive program development package for Windows 3.X/95/NT. The toolkit provides a program editor a software oscilloscope for aid system commissioning file upload and download facilities a communications debugger and terminal emulator. Also included is a library of common application examples and some C routines including file upload/download to assist in the implementation of host computer communications.

The MINT interface library produces a common application program interface (API) for the complete range of MINT products supplied as a DLL or device driver for the various Windows implementations. This allows applications running on a PC to access the functionality of MINT motion controllers. Communication is via the ISA bus for Nextmove PC and via an RS232/485 serial interface for other controllers. Full C++ source code is also provided allowing the libraries to be compiled for use with other operating systems.

• Baldor Optimised Control
• Miles Clee
• 0117 987 3100

April 1999