Actuator is not reaching commanded displacement or force end levels.

Problem

 

 

Actuator is not reaching commanded displacement or force end levels. 



 
Case Study:

 
Test procedure to ramp in displacement to an end level of 0 inches then ramp to 2 inches never reaches 0 inches before ramping to 2 inches and consistently falls short by the same amount of 0.25inches in each test run (Figure 1).  

 


 
Figure 1 – Displacement Feedback

 

 

Answer

 

Collect force data to compare with the displacement data to see if force is stalling at the same time (Figure 1).

 

 

 
Figure 1 – Force and displacement feedback both plateauing at same time



 
If this is happening, it suggests that the actuator is unable to produce any more force on the specimen.  If that value is below the rated value of the actuator then there is likely not enough pressure being delivered to the actuator. 

 
First check the high pressure output at the hydraulic service manifold (HSM) pressure gauge if you have one.  It should be close to full system pressure.  Most systems operate at 3,000 psi.  You can calculate the maximum force the actuator can generate by multiplying the measured pressure by the area of the actuator piston.  The area of the actuator piston can be found on the actuator’s nomenclature tag. 

 
In the example above the measured pressure was approximately 2,500 psi.  The actuator’s maximum force capacity was 250 kN.  The maximum force produced was around 208 kN (47 kip) which was expected with a reduction to only 83% of full system pressure. 

 
Next measure the pressure at the hydraulic power unit (HPU). This will tell you if the pressure loss is originating at the HPU.  

 
Note:  If you don’t have a pressure gauge at the HSM you can measure the pressure at the HPU too to see if that if you lost pressure there but if you read 3,000 psi at the HPU you still will need to verify if you have lost pressure after the HSM to the actuator. 

 
If you find a loss in pressure you will need to verify the cause and fix it.  It is recommended that an MTS field service engineer come on site to do this. 

K
Keary is the author of this solution article.

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