The actuation of cams, detents, and levers has long been the forte of pneumatic or hydraulic systems in machines. These systems are generally open loop. The only verification that the system has executed a command is the absence of an alarm signal as generated by some sensor, usually a mechanical or Hall-effect switch located near the mechanical stop. The speed at which events happen under open-loop control is dictated by physical factors such as mechanical friction and the amount of force the actuator generates. An adjustable mechanical stop usually sets the final position of an air cylinder.
Open-loop pneumatics are increasingly giving way to closed-loop servosystems. Closed-loop servos continuously monitor feedback from speed or position sensors as a means of controlling not just the distance actuators move, but also their speed and acceleration while they travel to their final destination.
There are many actuator types to consider when replacing a pneumatic system with a servocontrolled system. Actuator types available include electric cylinders with ball screws, rodless actuators with belts, or direct-drive linear systems. Many times the amount of force required for movement may lead the engineer to think a pneumatic actuator is the only possibility. Yet, electric actuators are quite capable of exerting as much force as a pneumatic cylinder. For example, electric cylinders from Danaher Motion range from approximately 30-lb continuous force to well over 4,000 lb. Linear-motor forces only range as high as 2,700 lb, but at significantly higher velocities. Rotary possibilities use carriage assemblies that contain small leadscrews with a rotary mount. When mated with NEMA-17 motors and indexers, these assemblies provide precise reciprocating linear action.
The typical way of implementing servocontrol in systems otherwise handled by air is with an iron-core linear motor. The linear motor takes the place of the air cylinder. Use of servocontrol can improve factors in pneumatic systems that range from mechanical wear to precision. To see why, it is useful to review the principal physical factors that bear on how pneumatic systems operate.
ACCURACY AND REPEATABILITY
Air cylinders are adjustable for repeatability, but the velocity and position of the cylinder during an open-loop move are affected by environmental conditions such as altitude, temperature, and humidity.
In contrast, velocity and acceleration of servo-driven linear motors and leadscrews are both programmable and repeatable. Servocontrol doggedly follows the same time-position profile every cycle. Varying friction and heat produce little impact on programmed parameters when given adequate design margins, typically on the order of 15%.
Air cylinders incur large frictional forces on their first excursion after prolonged periods of inactivity. This breakaway friction — often referred to as "stiction" — causes an interesting effect. The system builds pressure behind the air cylinder while the backside exhausts. There is a moment of nonmotion as the buildup occurs causing a jerk when the cylinder breaks free.
To counter this, pneumatic systems employ floating O-rings for pressures below 100 psi. These O-rings reside in an oversized captive area of the piston that lets them breakaway first. Their inertia helps reduce the breakaway force required for the piston. Of course, breakaway forces drop once the system cycles and O-ring lubricant spreads across the cylinder. Thus consistency and repeatability are not a given constant for air-cylinder operation.
Servosystems have their own static-friction issues. Quite often the first move of an actuator requires as much as 10 to 20% more current than ensuing moves. With the system operating closed loop, however, there is no difference in actuation time from one cycle to the next.
One must not overlook the solenoid valves used to control airflow into the cylinder. Both speed and reaction time of air cylinders vary with each opening of the valve from factors such as available air pressure, cylinder back pressure, and atmospheric changes.
Direct-current valves are sometimes hot fired to counter these variances. Hot firing means that a 12-V valve is hit with as much as 48 V at turn on. Then the voltage drops to 12 V for holding the valve open. The higher voltage forces a faster buildup of current through the inductive windings of the valve. On the other hand, ac valves are subject to the limitations of the ac cycle. Worst case they may not engage for anywhere from 8 to 10 msec, the time required for the ac voltage to reach its peak in the ac cycle.
Valves in an air-cylinder system require oil. Typically, atomizers maintain an oiled-air environment within the valve. Too much oil washes out the O-ring lubricant of the air cylinder and makes it fail. Too little oil and the valves fail by sticking closed. Simple oil systems drip oil into the air tubing ahead of the valve via gravity. Daily monitoring assures the proper amount of oil is set measured in the number of drops/minute.
Many air systems also need a muffler or reclassifier. This is a device that muffles exhaust noise and removes the oil from the air.
VIBRATIONS AND NOISE
Certainly, all mechanical systems generate noise. Any jerk forces, or changes in acceleration, in a system compound noise. The typical hard stops required of a pneumatic system can generate a lot of sound.
Cushioned air cylinders are manufactured with the ability to use back pressure to cushion the hard stop. Adjustment screws on the side of the cylinder control the cushioning effect. Pressure changes in the system necessitate readjustment of the screws. Other systems fire a solenoid with a small venturi space at the end of the cylinder stroke to reduce shock. Once again these are subject to all of the tolerances of air.
Designers have studied the vibrational spectrum of metal-to-metal impacts on pneumatic systems and found high proportions of upper frequencies. These high frequencies may damage collateral equipment such as bearings. The high frequencies conduct to frame members, components, and eventually to ball bearings producing a phenomenon known as false brinelling. Vibration of the balls within the bearing wipes away bearing grease, wearing a groove at the ball location.
Dampers help reduce noise and vibration when air cylinders reach their stop. Dampers take many forms but are often comprised of polymers with a certain durometer or hardness that cushions the cylinders as it reaches its final position. However, certain conditions such as high temperatures may require the use of metal dampers. Accommodating metal dampers requires that the cylinder follow a deceleration profile to lessen impact at the stop.
In contrast, servosystems perform this cushioning automatically through their time-position profile. Although certainly not noise-free, servosystems are significantly quieter than openloop actuators by any standard of measure. A typical iron-core linear motor used as an actuator is so quiet, observers can hear the rolling of the bearings. Even ball screws can fall below the 60dB level, about that of normal office noise or close conversation.
Mechanical stops are not necessary in servosystems. The servo takes the actuator to the final destination and stops. Brake mechanisms applied after reaching the desired position provide additional rigidity if necessary. With no metal-tometal contact, subsequent damage from impact shock or vibration is eliminated.
Even under the best operating conditions, air cylinders have wear issues. Those with varying velocity and acceleration tend to wear out quicker. Controlled mechanisms tend to wear out as well as the cylinder itself.
The O-ring is the compliant portion designed to wear before the cylinder scores or is damaged. Considering operational friction and heat, a life expectancy of 10 million cycles may seem like a large number. But in automated electronic placement machines, which can typically hit 30 to 60 thousand placements/hr, it can equate to as little as 170 hr of operation.
The greatest wear factor in a servocontrolled system is with the bearings within the mechanism. Under normal circumstances, bearings in servocontrolled systems should provide several thousand hours of continuous operation before wear requires replacement.
It's generally easier to troubleshoot a servosystem than a pneumatic system. Many servo/indexers have multi-channel oscilloscope options that display specific parameters during operation. Maintenance personnel may this way monitor current, velocity commands, actual velocity, and position. Drive status is also usually available.
Certainly there are many noncritical operations suited for open-loop air cylinders. However, closed-loop servoactuation is a viable alternative; particularly when the operation requires reporting information or is time qualified.
Danaher Motion Assistance Center, (540) 633 3400, danahermotion.com