How the Hydraulics Juggle with 1.2 Tons

06.11.2020 | written by Michael Pyper

In the last blog post we introduced the inverse pendulum. It demonstrates the performance and efficiency of the AX when used in conjunction with a synchronous motor in a closed control loop. The direct AX drive technology without valves combines the high force and power density of hydraulics with precision and energy efficiency of the electric drive technology.

The inverted pendulum – an overview

In the explainer video above we show how easily our hydraulics juggle with 1.2 tons. At the beginning, we position the carriage very slowly and precisely. Then there is a highly dynamic swinging operation to bring the pendulum to its top position, followed by delicate balancing. A manual "Stop" allows the pendulum to fall down. Immediately afterwards we effortlessly "re-synch" with it and balance it again.

In the second part we make the pendulum "dance" – in this phase, a lot of braking energy can be recovered. For clarity, we show this section in slow motion.

Making throttling losses a thing of the past

Even today, hydraulics offer many advantages that can't be achieved with any electromechanical solution. Particularly when it's a matter of getting maximum forces from a small space, there is no way of avoiding hydraulics. Not disputed, however, is its most serious drawback: the poor energy efficiency. This is mainly due to throttling losses at the control valves and the present efficiency of conventional components.

Closed-loop linear axes are always the right choice when fast and precise control is required, and energy needs to be recovered. These subsystems can be flexibly positioned in machines and plants and can therefore be decentralized. The other well-known advantages of conventional hydraulics such as overload protection, heat dissipation, damping or emergency stop functions are retained and can be easily and safely incorporated using proven components.

Sounds simple, but not exactly trivial in practice. The reliability and performance of many of today's pumps are not optimal at every operating point. Especially with high forces and low speeds, or when reversing, they quickly reach their limits. Our new AX pumps can do this much better!

Ideal for variable speeds

With its 24 pistons, the AX pump has very low pulsation levels, and it can be operated without problem and with the highest reliability in all four quadrants. Thanks to the hydrostatic lubrication and the compensating forces, a minimum permissible speed no longer needs to be observed, and the dreaded stick-slip effect of conventional pumps no longer exists. With an efficiency of up to 94 percent, it competes in the same league as the best electric motors and gearboxes. The overall efficiency of the drive system, consisting of inverter, servomotor, AX pump and cylinder, is up to 82 percent. When recovering potential energy from lowering and braking processes, an efficiency of as much as 84 percent is possible. And the AX's are also quiet.

The development of such a system requires a powerful test stand that, in conjunction with the appropriate electronics, can be used to develop and test new control strategies and systems architectures for hydraulic systems. The inverse pendulum provides the most graphic evidence of the performance of linear axes equipped with AX pumps. It is also clear to see how the disadvantages of conventional hydraulics can be eliminated by using the new AX pump generation. The results are outstanding, and there are many possibilities for transferring this performance and the acquired know-how to other applications.

For development and demonstration

The "inverse pendulum" is considered to be the benchmark in control technology. Like a juggler, the system has to balance a pendulum at its unstable highest point. We packed 1.2 tons of weight onto the carriage, which is moved by the linear axis. The carriage swings the pendulum to top dead center and then keeps it there in continuous operation for many hours without any cooling.

The AX24 weighs only twelve kilograms (26.5 lbs) and has a displacement of 24 cubic centimeters (1.46 cu. in.). It can be driven at any speed between zero and 3500 rpm, and delivers a maximum torque of 150 Newton meters (110 ft lbf), which is equivalent to the engine of a small car.

The entire test stand weighs more than five tons, and the overall length is almost six meters. The pendulum itself consists of a one meter long rod with a disc weighing two kilograms attached to one end. The test stand is portable and is not fixed to the hall floor. The way the inverse pendulum is controlled gives an impressive demonstration of the performance of the no-cooler-needed hydraulic drive system.

The direct drive system for the pendulum would not be possible without the AX pump, which was specially developed for variable-speed applications. With its optimal reliability at all operating points – especially at high force and low speed, and when reversing – the full performance envelope of the electric linear drive can be utilized 1:1.

As already described, the minimum speed limit of conventional pumps no longer applies with the AX pumps. This is also demonstrated by the live view through a window in the bell housing of the AX pump coupling. The video shows the exact positioning of the 1.2 ton pendulum carriage, shortly before the highly dynamic swinging begins.

Electronics and software

The electronics that we employ, together with appropriate software, implement the commands from the test stand controller and take over control of the inverter for the motor–pump unit. This means that this kind of linear axis can be incorporated into a machine as a subsystem in a closed control loop in a particularly simple and flexible way, without requiring any hydraulic knowledge. There is no need to observe minimum permissible speeds and other limitations of today's hydraulics. Consequently, the linear axis from Bucher Hydraulics can be commissioned easily, safely and smoothly with just a knowledge of electrical drive technology.