By Frank Langro, Manager, Product Management, Festo North America
Selecting the correct motion control component or system for the application involves mechanics, the physics of speed and acceleration, and the electronics of precision control.
In terms of motion control, machine builders can use pneumatics or electrical energy to drive the motion of mechanical actuators. A machine may be based on pneumatic motion, electric motion, or a combination of both. The electrically powered motion control market is growing faster than that of pneumatics because the price/performance ratio of electrically driven motion control has improved dramatically. The human perception that electronics can meet every need also plays a role.
Figure 1: A typical rod-style pneumatic actuator.
It is important to understand that both pneumatic and electric powered mechanical motion offer a sweet spot in terms of applications. Pneumatic and electric powered motion control forms an application continuum from lower to higher cost and from lower to higher precision. Knowing where, when, and why to apply one form over the other gives the machine builder, and ultimately the end user, the greatest potential for optimum productivity and lowest total cost of ownership.
Where, when, and why to apply
Basic pneumatic control is typically 30% to 50% lower in cost than an electric motor solution for the same application. Pneumatics offers a reliable motion for simple in/out, up/down, and rotary applications requiring high-force and high-speed continuous motion.
With the typical standard pneumatically controlled actuators there is no mid-point positioning. These were previously referred to as “bang-bang” actuators because they slammed into shock absorbers to stop. Today, various forms of cushioning can be used to slow the actuator as it comes to the end position, creating a softer stop.
Pneumatic actuators typically have a small footprint on the machine, an advantage when space is at a premium. Pneumatic systems are easy to set up and maintain. Typical applications include flying knives for cutting, inserting press-fit components, or rejecting non-conforming parts.
In some manufacturing plants with hazardous environments, where sparking may be a safety issue, pneumatics are preferred. For example, in Class 1 Division 1 settings, pneumatics may be the only motion control solution allowed under the National Electric Code.
The myth that standard pneumatic motion control wastes energy comes from the reality that leaks in pneumatic lines can occur in improperly maintained systems.
While standard pneumatically powered actuators are not designed for mid-stroke positioning or controlled velocity, servo pneumatic actuators offer infinite positioning. These closed-loop actuators have proportional valves that can control the positioning of the cylinder’s piston. Servo-pneumatic systems began with analog signals. Today’s most advanced systems feature multi-bit digitally controlled valves. This precise control allows these systems to perform up to 30% more cycles per minute than standard pneumatic actuators.
Servo pneumatics offer:
• High power
• A small form factor
• Closed-loop control
Like all pneumatic systems, servo pneumatics is suitable for high speed, 24/7 duty cycles. For example, servo pneumatic motion control can be used for a manufacturing line where various sizes of product are being shuttled or for lines where heavy packages up to 150 lb have to be shuttled at high speed.
Figure 2: Servo-pneumatics are suitable when closed-loop control and large forces are combined, like in this volumetric filling application.
Moving out of pneumatics with stepmotors
A stepmotor is a permanent-magnet motor that moves in increments. Basic stepmotors do not require position feedback. They are typically the lowest cost electric power technology available for motion control and offer ease of use and low maintenance. More advanced stepmotor systems offer full closed-loop servo control. Stepmotors are ideal for high torque, low speed (less than 2,000 rpm) intermittent duty applications. Small increments of movement call for a micro-step motor.
Stepmotors are often used when short, repetitive movements are needed—indexing, for example. In one application, a stepmotor moves a welding gun to transverse door hinges—welding the hinges to a frame. The 5-lb gun was moved at 2 ft/sec and was controlled by a touch screen operator interface panel.
Toothed-belt actuators are ideally matched to the stepper system. The combination of the relatively low speed of the stepper, with the high amount of linear motion per rotation of the toothed-belt pulley, give a good price/performance ratio, as long as the weight is not excessive and the accuracy is not critical.
Table 1: A look at the benefits of standard and servo pneumatic systems.
Servomotors and mechanical drives
The fastest growing segment of the motion control industry is intelligent digital servo amplifier/motors, principally because servomotors coupled to the optimum actuator for the application deliver precise and highly repeatable motion control.
Table 2: Comparison of actuator technologies combined with an electric servo system.
Electric servomotors are recognized for the ability to boost productivity and lower direct labor costs. In the past, machine changeover between different sized products required skilled workers to make mechanical adjustments to the machine. The process took time and directly impacted throughput. Today precise changeover adjustments can be made automatically through servo control. Servomotor control spans a wide range of costs and applications.
Figure 3: Pick-and-Place systems with toothed-belt actuators are ideal applications for stepmotors and drives.
Finding the right supplier
While motion products from different suppliers all roughly perform the same functions, a wide variation exists in terms of set up, ease of use, reliability, troubleshooting, and the ability of the supplier to provide software and firmware upgrades. The selection of the motion control vendor definitely has an impact on overall performance of the machine and the satisfaction of the end-user customer.
Table 3: Application strengths of various motor/actuator Combinations
In determining the correct motion control solution, not only is it important to consider the motive force—pneumatic power or electrical power, but also how that force integrates with the actuator technology rods, belts, screws, ball screws, or linear motors. As the charts on pneumatics and electronics show, each actuator technology offers different sweet spots in terms of load, stroke, velocity, acceleration, noise, stiffness, cost and flexibility. The soft issues such as ease of use, set up, and support are also factors. The supplier of the motion components or systems must be proficient in the mechanics and electronics of motion.
Search for a supplier that will examine the machine and come back with an optimum solution — a solution that may be pneumatic, electric, or a combination of both.
Motion control applied correctly has the power to differentiate the machine builder’s product offering and contain costs. The machine with the optimum motion control will help the end user improve the bottom line through labor reduction, higher throughput, and faster return on investment. Motion control that is easy to use and set up, straightforward to troubleshoot, and backed by a quality support team adds to the advantages of the physical solution.