System with compressed air seems more expensive and harder to build and integrate neither using simple electromagnetic drives to open public transport doors.
$\begingroup$ Where I live, trains and metros use motors, not air. $\endgroup$– martMay 23, 2015 at 20:59
Do you mean, for example, a bus entry door? It may be as simple as the "service factor" for motors ("electromagnetic devices") as compared to pneumatics. Inexpensive motors with sintered bronze bushings wear out, and better ones with actual bearings must be routinely lubricated. Air-operated cylinders are cheap and last a very long time, with the air typically containing traces of added oil.
The two are really not all that different, and this has been a hot-button topic for a long time. Electrical systems require a generator/alternator, regulator, wiring, control device, and actuator. Pneumatic systems require a compressor, tank/oiler, tubing, valve, and piston. Electrical systems (wiring) can short or open; pneumatic systems can plug or leak.
This article gives some pros and cons between the two, with interesting cost-analysis.
- Provide precise control and positioning.
- Help adapt machines to flexible processes.
- Low operating cost.
- Most economical when deployed in a moderate scale in processes where their performance advantages can be a benefit and when the electronics are separate from the actuator to segment and minimize replacement costs.
- Stepper motors are an economical choice for accurate positioning at lower speeds. However, steppers may lose synchronization with the controller when employed open loop without an encoder or if they are undersized for an application.
- Servos, by definition, are closed loop and provide superior performance at high speeds, albeit at a higher cost.
- Components of an electric actuator include the mechanical actuator that translates motor rotation to linear speed and thrust, the motor, an electronic driver or amplifier to power the motor, and a controller to control motion. The total cost for these components ranges from 800 USD to 3,000 USD and up.
- While component costs of electric actuators are high, operating costs are low. High component costs often deter the use of electric actuators because savings in operating costs compared to pneumatics are often not adequately considered or are outright ignored.
- Provide more force and speed per unit size than any other actuator except hydraulic.
- Force and speed are easily adjustable and are independent of each other.
- Are most economical when the scale of deployment matches the capacity of the compressor.
- Small compressors are efficient and economical when used for a small number of devices.
- Low component costs.
- High operating costs (install, replacement cylinders, electricity for compressor (76%))
Added: In addition to the points @ratchet freak has made, mechanical break-away (limit) systems are possible for motors; however they are seldom used. Typically the motor is sized to generate torque below a threshold value as to not cause harm. However, two issues arise: 1. This is a mechanical linkage, and would not be as repeatable, reliable, or self-correcting as a (pressure regulated) pneumatic system. And 2. If the motor failed, the actuator is usually locked due to mechanical gearing (thus relying on break-away linkage.)
It is possible to implement a cylinder as a linear motor; essentially a solenoid with two or more coils. But it would be heavy, use lots of copper wire, and use an elaborate drive system (especially if to limit maximum thrust.)
2$\begingroup$ These are all good and very true points. I suspect however that the biggest influencing factor in transit applications is resistance to dirt/dust/water and vibration. Electromechanical actuators have a lot more moving parts than a piston, and therefore more places for damage to occur. Electrical parts can short together, bearings and gears can become fouled with particulates. By comparison only very large particles will ruin a pneumatic valve or cylinder, and the only fragile component is the sealing ring. $\endgroup$– Ethan48May 23, 2015 at 16:21
They are smoother and if something gets caught in between them then the maximum force it can exert is capped depending on the pressure and the piston area. This means that a trapped limb is less likely to get crushed.
In an emergency (a typical concern for public transport) you only need to open a valve (pulling the emergency open will do that) and push to open them. This is simpler and more reliable than disconnecting a electronic actuator.
As a last point most public transport vehicles already have an existing pneumatic system for the brakes and suspension (as demonstrated by all the hissing a typical bus does) to tap into.