The main purpose of the seat is to support the machine operator. The seat should provide a level of comfort that allows the operator to operate a machine in a safe and comfortable environment. A safe and comfortable environment has been shown to impact the productivity and the job satisfaction of the operator. A safe and comfortable environment plays a significant role in the first impression a customer gets when operating a new machine.
Earthmoving machines have a seat suspension for two reasons:
- Keep the operator isolated from the harsh machine inputs
- Keep the operator connected to the machine controls and the machine as a whole
The three main components of a suspension system are:
- the Mass
- the Spring
- the Damper
The mass represents the object to be isolated from an input excitation (in this case the operator).
The spring is a device that is used to hold up the weight of the mass (in this case is an air spring).
The characteristic property of a spring is stiffness. Stiffness is a measure of energy storage. The act of compressing a spring stores energy in the spring which can later be released when the compressing force is removed. A spring-mass system would keep bouncing if not for the damper.
The damper is used to limit the amount of motion of the system (in this case is a hydraulic damper).
The characteristic property of a damper is damping. Damping is a measure of energy dissipation. The act of compressing a damper dissipates energy which cannot be recovered when the compressing force is removed. The damper keeps the spring-mass system from bouncing excessively. A hydraulic damper works on the principle that fluid is forced from one chamber to another through a series of restrictive passages. As the fluid passes through the passages, the kinetic energy of the system is dissipated and transformed into viscous heating of the damper fluid.
Illustration 1 | g03496617 |
Illustration of the influence of damping on ride performance |
A common misconception is that changing a damper affects stiffness. Changing a damper may increase the damping. Changing a damper may result in a harsher ride (or "stiffer" ride) but the stiffness of the system has not changed. The only way to affect stiffness is by changing the spring.
There are three types of suspension system: passive, active, and semiactive suspension. The passive suspension is the type of suspension that most people are familiar with. Most automobiles, motorcycles, trucks, trains, busses, and similar machines. have a passive suspension at each wheel. What makes a suspension passive is the fact that the stiffness and damping cannot be changed in real time in response to the input excitation. A passive system has no means of adding energy to the system and merely dissipates the energy that the system is exposed to. A passive system must be tuned for the harshest conditions. This tuning often leads to less than ideal ride performance in everyday operation.
The polar opposite of a passive suspension system is the active suspension. In an active suspension, the damper (and sometimes the spring) is replaced by some type of actuator that dissipates energy. The actuator also puts energy into the system. The actuator can adapt and provide the best possible performance in any conditions within the operating range of the active system. An active system can be most easily seen in the implement linkages of tractors, backhoes, excavators, dump trucks, and similar machines. This type of suspension system relies on one or more sensors, a controller, and an actuator and therefore is more complex. This type of suspension system requires significant power input, and is more expensive than the passive system. Therefore, a seat suspension system is not practical.
The semiactive suspension system is a mix between a passive and a fully active system. The semiactive suspension system relies on a spring and a damper to dissipate energy. One of the components may contain active components that allow the system to adapt to the operating conditions.
Illustration 2 | g03496737 |
Functional schematic of passive, active, and semiactive suspension types |
The CARM system is a semiactive system with an active orifice damper. Unlike the more commonly used Magneto-Rheological (MR) dampers, the active orifice type of damper can default to a highly damped state in case of malfunction. The MR system always defaults to the lowest damped state.
The active element in a CARM damper is an adjustable valve controlled by a controller. The valve is driven by a voice coil. The voice coil opens and closes the orifice that allows fluid to pass from one side of the damper piston to another. The controller reads sensor inputs and uses the information to determine the appropriate opening and closing of the valve.
The perception of ride in a suspension is influenced by two main components: the amount and feel of relative motion, and the harshness of endstop collisions. The latter has the biggest impact on ride comfort. More relative motion is preferred as long as the motion is smooth and allows the operator to reach the pedals and controls. Unfortunately, more motion leads to more endstop collisions. A passive suspension can be designed that avoids one or the other. The key is in designing a suspension that can allow enough relative motion to feel comfortable without allowing the suspension to affect the endstops. This goal is easily accomplished with a passive system in on-road applications. This goal is difficult to accomplish in the rough earthmoving environment where inputs are highly unpredictable. The solution is a semiactive system that can adapt and change based on the operating conditions.