Etymologically, Hexapod comes from the Greek words “Hexa” (six) and “podos” (feet). It refers to a platform that has six feet… well, six linear actuators. This is a complex system that allows for movements along the 3 Cartesian axes: movements along the vertical, lateral and frontal axis, as well as random yaw, pitch and roll movements around the axes mentioned above. This system is quite widespread in simulators for automotive companies, as well as flight simulators, theme parks and gaming equipment. The most powerful ones are hydraulic, though electrical technology is starting to tolerate increasingly heavy loads.
Vertical vibration testers allow for simulating the effects of vertical vibrations. The Safe Load TT-patented pitch & roll (P&R) module can add the P&R effects that take place during transportation. These transport simulation systems are used in leading-edge laboratories, where they are used in packaging system research, design and validation.
There are many different models with significantly different characteristics, so we will focus only on those that apply to transport simulation. The purpose of any laboratory machine used for quality control or for studying an item is for the available means to be used to validate or evaluate the characteristics or capabilities of that item. One such example would be considering how to evaluate the ability to withstand the handling process during the distribution cycle by taking into account the effect time has on a package that supports loads stacked on top of it during storage. Basically, there are two ways to do this: either we wait until the samples suffer the deformation caused by time while they are subjected to the maximum loads that they could withstand based on their design, subsequently testing the distribution cycle, or they are deformed using a compressometer and they get tested without the need to wait for months or even years. Both methods condition and test the sample until it is damaged as if it had undergone storage for as long as required.
This said, laboratories do not aim to replicate everything that may happen to the sample being tested. The goal is only to simulate that which has been proven to affect it. For instance, the package that we were testing above –simulating its storage- is not affected by the warehouse lights being turned off by night or when no one is working with it. It would be pointless to subject it to a light-on/light-off cycle as long as no study demonstrates that this causes damages that might affect its integrity during the distribution cycle. Even if it did suffer something as common –and important from a sales viewpoint– as discoloration, it is not something that would affect its properties within the distribution cycle, and it is unnecessary to simulate it to understand how it behaves within that cycle.
Vibrations during the distribution cycle
Now, the question is, if we were a package or a product being transported… what do we feel? The effect of the motions on the goods being transported during transportation have been studied all throughout the 20th century. Initially, a set of tests would be performed that would include vibrations at a fixed frequency and intensity as well as shocks, and as technology improved, vibrations at varying intensities and frequencies started being applied. The same happened to motion-related technology for systems that control motions, which have gone from something relatively simple as is controlling a motion at a fixed intensity and frequency, to highly complex systems that allow for controlling motions at constantly-changing frequencies and intensities, as experienced during real transportation.
Today, there are mostly 3 techniques being applied: replication, frequency sweep and random vibration.
- Replication means recording a trip and repeating it. It is like listening to the same song over and over, for whatever transport, destination or road it may be. Strictly speaking, a specific trip is tested independently to study whether it has been the best of routes or pure hell. Any change in the type of transport, destination, vehicle, or luck o’ the Irish of the driver, and the test will no longer be reliable.
- Frequency sweep consists of studying the system’s frequency-related response for the range of frequencies generated during transportation (typically between 0.5 and 300Hz for the vertical axis). Once the resonant frequency of the system has been identified, it is stress-tested at that fixed frequency. In other words, it is like hitting it where it hurts.
- Random vibration test. All it takes is riding a vehicle to note that vibrations do not remain the same throughout the trip, even if you travel the same route. Not even automated systems do everything exactly the same, not to mention what happens when we take the human or weather factor into the equation. Random vibration transport simulation tests allow for reproducing what transpires during transportation, adding a factor by which those vibrations happen randomly, but controlling that, statistically speaking, the characteristics of the distribution cycle are present at all times. This form of transport simulation has two main variants: using recorded and analyzed data to yield the PSD, which is then used in the simulation, or applying PSDs as defined in a testing standard or procedure.
Transport simulation standards include frequency sweep tests and random vibration tests for frequencies that range from 0.5 to 300Hz. With some exceptions, they focus on simulating the phenomena that take place for the vertical component. At the time, some technical committees of standard organizations are studying the matter, though they have made little headway. In addition to this, vibration testing standards include a horizontal component for the testing table that is difficult to guarantee.
Solutions for vibration simulation
If we take a look at the devices that are supposed to perform the tests, we will notice that going beyond vertical motions requires technology to take another leap. Nowadays, the best expressions of such a leap are hexapods and vertical vibration testers with P&R.
Hexapods allow for replicating movements very realistically, so much so that they are used to train the pilots of every airline in the world. No airline transport pilot can be rated in a specific model without successfully racking up a minimum number of hours in a flight simulator. Formula 1 pilots also use them because FIA restricts the number of driving hours for single-seaters. Some automotive makes have taken the concept one step further and mounted a hexapod on a platform that allows the hexapod to move all over a huge facility. On top of that platform, they mount cars and study the driving feel in a safe environment. In both cases, the platforms have projector systems for the occupants to “see” what goes on around them.
The vibration system with P&R is being mostly applied to transport simulation. It consists of an actuator that performs the vertical vibration motion, with two other actuators added on the moving platform which allows for P&R motions. Depending on the P&R model, it may consist of a module that is integrated into the entire machine, or a module that can be removed, thereby increasing the operating frequency range and the vertical actuator load limit.
The existence of P&R motions in transportation has been demonstrated in international congresses specialized in containers, packaging and transport simulation. What
is being claimed –and proven– is that, when these motions are added to the transport random vibration simulation, the results are even closer to the real conditions experienced during transportation. The P&R motions can be simulated using both systems –hexapods and P&R modules.
Hexapods allow for certain degrees of rotation around the vertical axis. It is clear that all transport systems yaw in order to change directions. During this yawing, not only does the direction change, but lateral forces are also exerted, which can be replicated using the pitch and roll motions performed by the machines. Despite being more realistic, the motion that causes the change in direction does not structurally affect the load. The speed of rotation around the vertical axis causing any effect on the load would require motions that would be close to acrobatic –transporting the load while doing spins and barrel rolls. Testing for that is pointless.
Regarding the manner in which the motions are performed, we need to analyze how fast the system we are planning to use can perform those motions. The response time of any mechanical system is usually defined by the transfer function. That is, so to speak, how fast a system responds when given a command. Individual limits are added to the real-time control limits of many actuators, while another problem is added on top of that: the effect that the phase has in relation to the control. Small differences like the length of the cables can alter the response of a system as a whole.
Usually, a hydraulic actuator can have dynamic responses that can exceed 500Hz. For the electrical actuators of some hexapods, they hardly ever exceed 25-30Hz. This results from the complexity of controlling 6 axes at the same time with the required precision using the power generated by an electrical actuator, as opposed to a hydraulic one.
We already mentioned the operating frequency range when discussing the transport simulation vibration standards. Depending on the standard and transport being tested, most start at 0.5Hz and 4Hz and must reach 150Hz to 300Hz. This, is in addition to the flatness requirement, makes hexapods non-compliant with the standards. The solution applied by many is to perform their own data recording and apply a filter to rule out higher frequencies, thereby adapting the test to the machine rather than the machine to the conditions experienced during transportation. One example are electric hexapods, which can hardly reach 30Hz –they can only simulate 20% of the bandwidth of the conditions up to 150Hz, and 10% up to 300Hz. For hydraulic hexapods, the upper cutoff frequency is higher, but it does not meet the requirements of the standard either. When looking at your everyday airport baggage cart, for example, and cargo containers, they have a very characteristic profile that reaches 300Hz almost as white noise. Not simulating the 30-300Hz range leaves out the frequencies that typically affect small items, such as toy motor and circuit boards, among many other similarly-sized items. The case is the same for road, rail or sea freight.
These hexapods are restricted by their own strokes and by the technical complexity that commanding 6 actuators that move one same platform at the same time implies. This means that the motion of the 6 actuators defines the motion that the platform transfers to the sample. The system is so complex that so-called real-time controllers are used to control it. These are embedded PCs that reserve a percentage of their microprocessor clock cycle and memory to execute the control without other areas of the operating system interrupting them.
The vertical vibration plus P&R system is relatively simple. Each actuator performs independent motions that only affect the vibration axis that corresponds to it, without interfering on the movement of all other axes. Since each actuator controls a single axis, one could even use completely independent systems for each actuator, such as independent hydraulic power sources, independent controllers, and completely independent control programs. By having 3 actuators to perform the motions instead of six, it needs a much lower flow rate. As a whole, a lower flow rate means a smaller hydraulic power source, fewer actuators and a less complex controller, which makes it a more affordable system.
In summary, it is necessary to tell the different technologies apart; on one hand, we have hydraulic vs. electric, and on the other, hexapod vs. vertical + P&R:
Advantages of hydraulic actuator-based systems vs. electric systems
- The operating frequency range is higher. Machines can be made for heavier loads. They require less electric power. Despite the lower efficiency of converting electrical power into hydraulic power to perform the same motion as that of the electric actuator, the hydraulic power source converts the electrical power and uses hydraulic accumulators, which makes their response smoother and homogeneous, while the electric actuator needs to have all the power required by the motion at the same instant when it is required. To put it simply, the hydraulic system requires a power that is slightly higher than the average needed by the electric actuator, but the electric actuator must have all the power that is required at a specific moment available or it will not be able to perform the motion required by the simulation. This implies that the electrical actuator requires a more expensive electrical installation and a more expensive electric utility service. In addition, the electric actuator has a worse total harmonic distortion (THD). The fact of being simulating mechanical noise that is converted into electrical noise if applied directly by means of electric actuators. In some places, the THD also affects the price of electricity, since it forces the electric grid to add filters and other devices.
- The advantage of systems based on electric actuators as opposed to hydraulic ones is their cleanliness (they obviously do not suffer from oil leaks). The circuit breaker that they require is not as large as the one required for a hydraulic power source, and the installation is less complex.
Advantages of the hexapod vs. vertical vibration + P&R system.
- It allows for rotation motions around the vertical axis. It is apparently more realistic. It enables the building of simulators for training or entertainment.
- Advantages of vibration + P&R vs. the hexapod. The main advantage is that it can perform tests that are standard-compliant. It requires less vertical space for its installation. It requires a smaller seismic mass. The hydraulic power source that it requires for the same kind of profiles is smaller, and consequently, so is the power use. The control system is cheaper. It is overall cheaper for comparable machines.