Selecting the optimum motion control solution for the application

A machine with optimum motion control will help the end user improve the bottom line through labour 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.

The successful machine builder develops products that offer superior price, performance, reliability and the ability for their customers to lower direct labour costs. The machine builder’s reputation for quality and support also influences a buyer’s decision, as does the total cost of ownership.

The majority of machine builders today incorporate off-the-shelf control components such as PLCs, HMI and I/O into their machines. These components and systems offer the best price/performance ratio in the industry’s history, and the same price/performance advantages apply to motion control technology for both pneumatic- and electric-powered motion. Motion control is far more complex, however, compared to discrete control. Selecting the correct motion control component or system for the application is not simply a matter of counting I/O and sizing the controller to match - the choice involves mechanics, the physics of speed and acceleration, and the electronics of precision control.

Each motion control application is a mix of considerations. For example, the back and forth repetitive motion of a cutting knife requires a far simpler and less expensive solution than the complex multiaxis movement of delicate silicon wafers.

Machine builders have a choice of using air pressure - pneumatics - or electrical energy to drive the motion of mechanical actuators. One 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, and the common perception that electronics can meet every need also plays a role.

Even with the bias towards electric-powered motion control, however, 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 pneumatic motion control 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.

Figure 1: A typical rod-style pneumatic actuator.

Pneumatic actuators typically have a small footprint on the machine - a real advantage when space is at a premium - and 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.

Servo pneumatics

While standard, pneumatically powered actuators are not designed for mid-stroke positioning or controlled velocity, servo pneumatic actuators, which have been on the market for a decade or more, 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 are suitable for high-speed, 24/7 duty cycles. For example, servo pneumatic motion control is ideal for a manufacturing line where various sizes of product are being shuttled or for lines where heavy packages up to 70 kg have to be shuttled at high speed.

Table 1 presents a comparison of standard and servo pneumatic systems.

Table 1: Comparison of standard and servo-pneumatic systems.

Drive technology	Standard pneumatics	Servo pneumatics
Load	Up to 100 kg	Up to 300 kg
Stroke	Up to 8.5 m	Up to 2 m
Velocity	3 m/s	6 m/s
Acceleration	30 m/s2	30 m/s2
Precision	100 μm	200 μm
Noise	Very noisy	Noisy
Stiffness	Medium	Low
Flexibility	Hard stops only	Infinite postions
Typical costs	1	1.5-2x standard pneumatics

Stepper motors - moving out of pneumatics and into electrically driven motion control

A stepper motor is a permanent magnet motor that moves in increments. Basic stepper motors 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 stepper motor systems offer full closed-loop servo control. Stepper motors are ideal for high-torque, low-speed - less than 2000 rpm - intermittent duty applications. Small increments of movement call for a micro-step motor.

Figure 2: Electric servo systems are ideal where high precision is required.

Stepper motors are often used when short, repetitive movements are needed - in indexing, for example.

Toothed-belt actuators are an ideal match for stepper systems. 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.

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.

Electric servomotors are recognised for the ability to boost productivity and lower direct labour 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.

The simplest control, the single-axis servomotor, overlaps stepper motors in terms of cost and functionality. Multiaxis precise applications calling for high acceleration increase the cost of the system. Electric servo solutions are at the top end of precision when used with high-resolution feedback. For example, wafer handling at a fabrication facility calls for this type of precise motion control.

Table 2 describes the key parameters of servomotors connected to mechanical drives, and Table 3 summarises some of the key strengths of the various pneumatic and electric options described in this article.

Table 2: Comparison of actuator technologies combined with an electric servo system.

Mechanical drive technology	Belt driven axis	Lead screw driven axis	Ball screw driven axis	Linear motor axis
Load	Up to 200 kg	Up to 100 kg	Up to 200 kg	Up to 30 kg
Stroke	Up to 10 m	Up to 2 m	Up to 2 m	Up to 10 m
Velocity	5-10 m/s	0.5 m/s	3-5 m/s	5-10 m/s
Acceleration	100 m/s2	30 m/s2	50 m/s2	150 m/s2
Precision	100 μm	50 μm	20 μm	3 μm
Noise	Noisy	Low	Medium	Low
Stiffness	Medium	Very High	High	High
Typical costs	1.5-2x standard pneumatics	2-3x standard pneumatics	2-3x standard pneumatics	3-5x standard pneumatics

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.

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.

Table 3: Application strengths of various motor/actuator combinations.

Drive       ⇒ technology	Standard pneumatics	Servo pneumatics	Belt drive axis w/ stepper	Belt drive axis w/ servo	Ball screw axis w/ servo	Linear motor axis
Application with:     ⇓
High load		•		•	•
Long stroke	•		•	•		•
High velocity			•	•		•
High acceleration				•		•
High precision					•	•
Low noise					•	•
High stiffness				•	•	•
Low cost	•	•	•
Multiple positions		•	•	•	•	•

Some suppliers specialise in pneumatic solutions, while others focus solely on electric motors. Some suppliers offer motion as part of a large product offering. The problem with suppliers offering only one range of solutions - pneumatic, electric or as an adjunct to an actuator line - is that they often can’t cover the gamut of possibilities.

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. The ideal motion supplier will be expert in the mechanics, physics and electronics of motion as well as a company that provides excellent customer service. This supplier’s products should be compatible with the major PLC and device-level networks for a de facto open solution.

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 labour 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.

By Joseph Biondo, Electronic Business Development Manager, Festo North America