Navigating the complexities of linear motion
Author : Josh Roberts, Festo GB
04 November 2024
Linear motion technology is no longer a simple choice of pneumatic or electric drives
Linear motion on machines used to involve a straight choice between pneumatic or electric drives – but now there are other alternatives. Josh Roberts, Applications Engineering Manager at Festo GB, explains the range of linear motion technologies available today and explores the design considerations that will result in the best selection for your application.
Traditionally, machines have used pneumatic or electric drives for linear motion. However, recent technological advancements have expanded the range of available options, often beyond what designers might expect. For instance, point-to-point electric drives are now more cost-effective and can substitute for some movements that were typically performed by pneumatic cylinders. Equally, the latest pneumatic technologies now include features that were once exclusive to electric drives. New servo and digital pneumatic technologies have further diversified the choices. This is making the selection of an optimal solution that balances performance, cost, and energy efficiency a more complex task for designers and machine-builders.
Design criteria
With so many linear motion technologies to choose from, beginning with the application parameters and then deciding which product option will deliver the best solution is a sensible approach. Key design considerations include the payload (what mass is to be moved) and the force that will be required. For example, does the application require a defined pressing force, and does it vary based on the product? What repeatability is needed and are multiple positions involved? Designers should also ascertain the motion control requirements for speed, synchronisation, or interpolated motion.
Physical and practical aspects also require due consideration. What is the physical footprint: is this a desktop machine or end of arm tooling on a 6-axis robot, for example? What energy source will be used? If this is pneumatic, the supply pressure and if its electric phasing (single or three-phase) needs to be determined. Do you need to meet any energy efficiency and carbon footprint targets? What about cost: does the customer value TCO over capital expenditure? Finally, is the expertise in place to service and maintain the preferred technological solution?
Linear motion options
Once the design parameters are established, we need to consider the linear motion technologies available and examine their characteristics.
Pneumatics is a robust, simple to use technology. Pneumatic devices offer exceptional force output for the defined footprint and weight, together with highly repeatable end-to-end positioning. Flow controls and sensors are used to define speed and end position, with force control linked directly to the applied pressure. Using best practice design and operation, pneumatics can achieve good energy efficiency.
Servo pneumatics offer all the benefits of pneumatics, plus positioning capabilities. Servo pneumatic devices have a linear displacement encoder attached to the cylinder in question. The positioning commands and the encoder feedback create a closed loop control. The output is fed via a proportional directional control valve back to the actuator. The cylinder is positioned by the controller using motion control to achieve the defined speeds and accelerations. This technology is specifically useful for applications not suitable for electric drives (due to size, weight, energy source or environment).
Digital pneumatics combine energy and diagnostics benefits with precise motion control. For example, the Festo Motion Terminal (VTEM) combines fast switching and virtually silent piezo valves with integrated sensors for closed loop control. This smart terminal is accessed using the wide range of downloadable Apps (i.e. proportional control, soft-stop, pre-set travel time, leakage, etc.), which makes it very flexible. Each valve position houses four piezo valves with boosters in a bridge format. Each port has both pressure and flow sensors for closed loop feedback of the pneumatic output. This configuration enables a multitude of different functions, which are engaged by the motion apps. For example, the function of a single valve can be changed from a directional control valve to a proportional valve – in real time.
End-to-end electric actuators are available in an array of mechanical axes designs, such as gantry or cantilever design with either a ball screw or tooth belt. The integrated stepper motor with an encoder and controller offers connections for both power supply and communications (digital IO or IO-Link). Simple to commission, these devices can be taught the desired end position and perform point to point positioning under a defined motion profile (set over the push buttons or IO link). Mid-positions are also possible with IO-Link control.
Electric drives combine advanced motion control with high dynamic motion. Common servo controllers range from 24V DC power supply to 3 phase 480V power supply. Based on the motor type selected for the application (stepper, BLDC, servo), designers can deploy an array of different feedback encoder types. Position, speed, and force can be controlled from the PLC via the servo controller. Multiple drives can be combined via motion controllers for complex handling solutions. Enhanced motion control can also be achieved, including synchronisation, flying saw and interpolated motion. Functions such as DC link can be enabled for improved energy efficiency.
Factoring in sustainability and TCO
Total cost of ownership is becoming increasingly important, making the selection of sustainable linear motion technology both time-consuming and complex. To streamline this decision-making process, Festo offers a software tool called the ‘CO2 & TCO Guide’. This tool assists designers in comparing electric and pneumatic drives from Festo’s product portfolio, providing a clear comparison of energy consumption, CO2 emissions, initial purchasing costs (tailored to your company’s conditions), and the total cost of ownership. By entering key application criteria, users receive a graphical, illustrated cost calculation for periods of up to 10 years.
More resources
Keeping pace with advances in linear motion technology will help designers to make the right choices for their application, based on the relevant criteria. Festo supports all five of the linear motion technologies discussed in this article, which means we can give technology neutral advice. We also offer online selection tools that support our customers in identifying the best linear motion solution for their application.
Further support, including a more in-depth explanation of the linear motion technologies available and how to relate them to a real machine design, is available in this 'on-demand' webinar: www.festo.co.uk/optimaldriveswebinar .
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