MATLAB: What does the orientation imply in the context of a double acting hydraulic cylinder

cylinderdouble-actinghydraulicorientationSimscape

I am trying to use Double-Acting Hydraulic Cylinder block, but I am confused by the Cylinder orientation *parameter. *I can select either positive or negative. But since it is a double-acting Hydraulic cylinder, I was expecting this to be both positive and negative. Why should I select one?

http://www.mathworks.com/help/physmod/hydro/ref/doubleactinghydrauliccylinder.html

Best Answer

There is a chamber of fluid where hydraulic port A is connected. Let's call that chamber A. Similarly, there is a chamber of fluid where hydraulic port B is connected. Let's call that chamber B.

When the "Mechanical orientation" is set to "Acts in the positive direction", it means that when chamber A increases in volume (e.g., when flow enters port A) and chamber B decreases in volume (e.g., when flow leaves port B), there will be a positive displacement of mechanical port R relative to mechanical port C. Conversely, "Acts in the negative direction" means that when chamber A increases in volume and chamber B decreases in volume, there will be a negative displacement of mechanical port R relative to mechanical port C.

Consider the attached model (R2016b) which is a modified version of the shipped demo "sh_cyl_counterbalance_valve".

In this model, the relative flow rate (Q_A - Q_B) and the position and velocity for port R are plotted on the scope. Flip the orientation and observe the behavior in the displacement and velocity in the Scope.This should help clarify cylinder orientation.

So if you want port R to have positive velocity as chamber A expands, select a positive orientation and vice-versa.

Related Solutions

MATLAB: Cross-Sectional Area at Ports in Simscape

In general, "Cross-sectional area of port A/B" represents the area of the connection to that block. The cross-sectional area at ports A and B of the flow resistance block usually have small impact on the flow rate through the block. The parameter exists because we need to determine the direction of flow and provide a smoothing for the temperature at a connection node when the flow direction changes (i.e. flow reversal). The threshold for smoothing the nodal temperature based on a Mach number, so the cross-sectional area at the port is used to divide the mass flow rate to get the velocity and Mach number.

You can refer to Momentum Balance section under Description in this documentation.

MATLAB: How are the pressure and flow rate calculated for a fixed-area orifice in SimHydraulics 1.4 (R2008b)

The attached model shows a small example of how Simscape works in evaluating Across and Through variables, and specifically how the constant area orifice(same as fixed orifice) block works. In the attached model example.mdl, two orifices connect into a double-acting hydraulic cylinder whose mechanical piston is connected to a mass-damper-spring system. One end of the orifices have a pressure source that maintains the pressure constant (-4e6) at this end. Since pressure is an across variable, both the lower ports of orifices o1, and o2 have the same constant pressure kept by the pressure source.

At the other end of the orifices, the pressure is determined by the hydraulic cylinder, specifically the variable volume chambers within this cylinder. (To see this block, right click on the cylinder and select look under mask. Note that this has 2 variable volume chambers). The variable volume chamber's initial condition(such as initial piston position,etc) determine the initial pressure at this end of the orifices. Once the pressure difference is determined across the orifices, the flow rate is found out based on the pressure-flowrate equation of the orifices, as mentioned in the doc link:

 <http://www.mathworks.com/access/helpdesk/help/toolbox/physmod/hydro/ref/fixedorifice.html>

Once the current flow rate is obtained, the pressure of the orifices at the upper port at the next time is obtained from the current flow rate, and current pressure and the control signal input of the variable volume chamber, according to the flow rate-pressure equation shown in the below link:

 <http://www.mathworks.com/access/helpdesk/help/toolbox/physmod/simscape/ref/pistonchamber.html>

Thus, the pressure is internally determined, and the flow rate across the orifices obtained directly from the equation of the orifice.

To read more about how Simscape works, and how across and through variables are used to resolve the blocks internally, please visit the following links:

 <http://www.mathworks.com/access/helpdesk/help/toolbox/physmod/simscape/ug/bq89sba-1.html#bq89sba-2>
 <http://www.mathworks.com/access/helpdesk/help/toolbox/physmod/simscape/ug/bql2kkl-2.html>

To read more about the Double acting Hydraulic cylinder, visit here:

 <http://www.mathworks.com/access/helpdesk/help/toolbox/physmod/hydro/ref/doubleactinghydrauliccylinder.html>

A good debugging technique on a Simscape hydraulics model, is to use Ideal Pressure Sensors and Flow Rate Sensors at different points to measure the pressure and flow rates there. In the attached model, an ideal pressure sensor is used to measure the pressure at the lower ports of the orifices.(Please see the scope 'pressure at lower ports')

It might also be helpful to think of pressure-flowrate in the same terms as voltage-current, following kirchoffs laws.

Best Answer

Related Solutions

MATLAB: Cross-Sectional Area at Ports in Simscape

MATLAB: How are the pressure and flow rate calculated for a fixed-area orifice in SimHydraulics 1.4 (R2008b)

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