Carl Vangsness
Emerson EMC
Emerson Electric Co.
Chanhassen, Minn.
Many machines are too expensive to replace entirely when they can’t meet increasingly stringent production goals. Rather than trash outdated equipment, it’s often possible to trade in the old embedded motion controls for new servosystems. However, an upgrade is not always easy. Getting deeper into a machine analysis may uncover problems disguised as hapless bandages.
For instance, belts and pulleys of various diameters may have been added to increase speed and change torque for manufacturing new products. Unfortunately, this type of change also increases load inertia, which may be acceptable or not. Also, a clutch and brake may have been added when another technique might have served better. These changes were originally perceived as improvements and added periodically during the machine’s life. But, in the face of a retrofit, all such fixes need reevaluating to determine why they were installed so they can become an integral part of the new design — if still needed.
Load, Distance, and TimeThe first step in a redesign considers the machine’s main function, which is usually to move an object or a load a specified distance in a certain a
mount of time. The load, distance, and time are the three primary design factors. All others stem from them.
After defining the load in terms of weight and size, consider the distance the load moves next. Analyze the longest distance first, since it establishes the machine’s throughput, making it the most difficult path to work with. For example, loads composed of 10-ft tubes require longer actuators to move and position for machining than do 10-in. tubes. In the case of ballscrew actuators, speeds and tolerances are more difficult to control. Also check the shorter paths for throughput, especially if machine timing changes when product size changes.
The distance a load moves is usually linked to machine output cycle time and the load’s position, so the time for that move can be quickly determined. However, look for redesign requirements that the basic machine might not be able to deliver, such as moves that can’t be made in an allotted period without unusually expensive modifications. Likewise, when the move can be made in the specified time, check acceleration forces, because those that exceed 1 g could destroy the product the machine makes.
After calculating distance and timing, select an actuator. For linear motion less than 25 ips, a ball-screw system fits particularly well. Screws produce high mechanical efficiency and accuracy because their motors turn more times per inch than in other systems. But screws have a practical rotational speed limit which comes from vibration generated by a combination of unsupported length and the diameter of the screw. All ball-screw manufacturers offer sizing specifications for long operating life.
Screw vibration usually becomes a problem when linear speeds exceed 25 ips. Belt and pulley drives provide an alternative. They come in 40-ft lengths and can move objects up to 200 ips. Their downside comes from backlash in tooth-totooth variations that affect accuracy. In addition, belt and pulley drives do not have the built-in mechanical advantage of screw systems. Screw actuators typically run at higher shaft-to-load speed ratios than belt and pulley systems.
However, because a driven pulley speed is likely to be slower than the servomotor driving it, a speed reducer can lower the needed motor torque. The speed reducer also diminishes the load inertia reflected back to the motor by the square of the reduction, while raising the driving speed by a multiple of the reduction.
Speed reducers provide another advantage: Servomotor speed can now be in a range where it is most efficient, typically between 1,500 and 2,500 rpm. Servomotors can run at speeds above 3,000 rpm, but only when the system operates from full-rated input voltage and current. For example, when only 208 Vac is available for a system rated at 230 Vac, the motor may not reach full-rated speed. A conservative design, however, lets the motor normally run below maximum rated speeds. This means production won’t suffer when the line voltage is low.
Position accuracy is the final consideration. Many factors influence system accuracy, but a design engineer must be clear about the machine’s expectations. Users tend to overspecify systems requirements for the upgrade, hoping to have power in reserve for further increases in throughput.
One user, for instance, requested a servosystem vendor design an applicator machine that would deposit a drop of glue in the middle of a 0.25-in. paper tab moving at 600 ft/min. The user wanted position accuracy of ±0.00010 in., a wish the designer’s best controller couldn’t meet. When the servosystem designer informed the user that ±0.004 in. was possible, the user replied that would be good enough. The user offhandedly thought that ±0.00010 in. would be a good place to start.
Servosystems are controlled by microprocessors and other hardware components with a finite capability. Limits should be considered in the original specifications. If someone had provided this demanding customer with a system that could reach the accuracy at the speeds specified, it would have been more expensive, extremely difficult to install, and almost impossible to maintain and operate.
Getting Real
Another manufacturer provides a more realistic example of expectations. It ordered a new servosystem to replace an older one as a first step in modernizing a production machine.
The working mechanism consisted of two arms linked together which oscillated over a 90° sweep angle. The arms were driven by a set of elliptical gears coupled to an ac motor. The elliptical gears provided a sine-wave motion to the arms which was necessary to produce linear motion at the tips of the arms. The tips moved a sheet of paper over a straight-line path.
One goal was to eliminate the elliptical gears, so the new servosystem had to be programmed to duplicate the sinewave motion. This would also allow the manufacturer to make quick adjustments for different product lengths.
Analysis of the process led the servosystem supplier to an alternative approach based on the fundamental objective — to move the paper in a straight line. The replacement actuator relies on a packaged belt and pulley drive available from a number of suppliers. This approach removes a large inertial mass from the motor load, and a fairly complex servoprogramming task from the operating system. The belt and pulley actuator and servosystem can be programmed to move in inches or millimeters, with the servosystem responsible for all math conversion.
Software eases motor sizing Input move parameters include index distance and time, dwell, acceleration, and deceleration times. A user-supplied speed reducer mounted between the load and motor introduces a transfer function that the program considers. The algorithm then calculates servosystem hardware requirements such as total inertia, maximum speeds, and acceleration and deceleration speeds. It also computes the minimum values for acceleration, deceleration, run, and rms torques. Finally, the program searches its database for a match between these parameters and a motor and amplifier, and prints out a speed versus torque graph. The process lets designers quickly fine-tune a system or project requirements as systems needs grow. |