Volume 6 Issue 8 September 2005
Make It Precise
by Al Simone
When it comes to automated machinery, it is likely either stepper motors or servo motors are being used to control movement. The main purpose of using these types of motors is for precise positioning of some part of the machine.
Have you ever thought about how the computer tells that part to move to the correct location quickly and accurately?
As a simple example, consider an automated lineal optimizing saw for cutting vinyl profiles. Information, which has been downloaded into the machine, is used to move the guide to a specific location. After the operator places the profile against the guide and activates the cutting operation, the profile is cut to the desired size. Letís assume that the information downloaded or entered into the machineís computer in this case is the value 24, and that this value means a profile length of 24 inches. Now, how is this information translated to movement?
First, on the mechanical side, we have a guide which is connected to a long linear bearing. This bearing holds the guide in the correct position above the cutting table but allows movement in the left and right directions, positioning the guide nearer or farther away from the saw blade. One side of this guide is connected to a timing belt which wraps around a 3-inch diameter pulley on the left end of the table. The timing belt continues under the table, and then wraps around another pulley at the right end of the table before connecting back to the other side of the guide. Assume that the servo motor is connected to one of the three pulleys via a 5-to-1 gear head (5 turns of the motor equals 1 turn of the pulley). This servo motor has a 4,000 count-per-revolution encoder, which is a device connected to the motor that is used to ensure that the motor is moved to the correct position. Using feedback, the computer can read the encoder and adjust the motor movement to verify the accurate final positioning of the motor. In this example, the encoder can resolve up to 4,000 unique positions every rotation of the motor for high resolution positioning. This means the computer can use the encoder to position the guide in exactly the desired position.
With this information in hand, we can now calculate how much to turn the motor, which turns the pulley and moves the timing belt to accurately set the guide at 24 inches. Since the pulley diameter is 3 inches, every turn of this pulley will move the guide 9.425 inches (3xp). Calculating backwards, we divide 24 inches by 9.425 inches-per-rotation to arrive at 2.546 rotations of the pulley. Now, we continue working backwards from the pulley to the motor. Since the pulley is connected to the 5- to-1 gear head, the motor must turn five times for every one pulley rotation, or 5 x 2.546 for a total of 12.73 rotations of the motor. Remember that the computer doesnít know about motor rotations, only encoder counts. The computer must issue a distance command to the motor of 4,000 counts per rotation x 12.73 rotations or 50,920 total encoder counts. In other words, the guide will then move until the encoder records 50,920 counts, and then the guide will stop.
Making Sense of the Numbers
So what do all these numbers mean to you in terms of accuracy and resolution? This system can resolve up to .0005 of an inch (Ĺ of 1 thousandths of an inch). This is calculated from the numbers above (24 inches/50,920 counts per inch). Higher accuracy can be pursued by increasing the resolution of the encoder to 8,000 counts per revolution or by increasing the gear head ration from 5-to-1 to 10-to-1. Unfortunately, accuracy in the system is also extremely dependent on the stiffness of the mechanical system. Small problems such as loose belts or bearings introduce large errors in the system positioning.
Al Simone serves as president/owner for Spadix Technologies in Middlesex, N.J.
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