# piezo actuator - higher displacement when under load?

When looking at the spec sheet for this Piezo Actuator https://www.thorlabs.com/thorproduct.cfm?partnumber=PK4FXH3P2, it states that maximum displacement is achieved when under 300N. How does this work? Everything I've read and just intuitively, it seems that displacement will decrease until you reach your blocking force for a given voltage. Not increase and then decrease.

Does anyone have any thoughts?

• I doubt that means what it looks like it says. I suspect it really means you can get the specified deflection at loads 0-300 N, maybe more, but not specified, and less above 300 N load. I too would expect a monotonic reduction in displacement with increasing load. Are data sheets in the piezo field written with a different style than those in other fields? Nov 30, 2021 at 20:33
• Question is about mechanical issues, not electrical. This is probably the wrong stackexchange site.
– Davide Andrea
Nov 30, 2021 at 21:49
• @Neil_UK, I like that thought. That could very well be what I'm missing. I'm fairly green when it comes to Piezo field (been doing a fair bit of research though) so I wasn't sure if there was some property of these materials I was unaware of that would explain my original assumption. Dec 1, 2021 at 14:12

To my understanding the blocking force is a measure of the maximum force generated by the actuator.

The way the blocking force is measured might explain better the discrepancy.

The piezo actuator length before operation $$L_0$$ is recorded. Subsequently, the piezo actuator is activated without a load to the maximum displacement $$\Delta L_{nom}$$. Once the nominal displacement (i.e. the final length is $$L_0 + \Delta L_{nom}$$ is reached then the piezo is pushed back to the initial position with an increasing external force. The force recorded when reaching the initial required to reach $$L_0$$ is reported as the blocking force.

A more formal definition of the blocking force is that it is the force that is achieved when the displacement of the actuator is completely blocked i.e. it works against a load with an infinitely high stiffness. However that cannot be achieved in real life, hence the above measurement.

• Yes, well put, that is my understanding as well. However, that's why this spec doesn't make sense to me. I would imagine it would be a fairly linear relationship between change in length to the force exerted. yet it states load for maximum displacement is 300N. I think the comment from Neil_UK could what i'm missing. Dec 1, 2021 at 14:07
• I'm not sure I follow. When its extended to 40 um, if you "reverse" the deflection, you'd need to exert more force. I intrerpret the 300N that at 40 um, it can apply a force of 300N without budging. (Does that make sense)?
– NMech
Dec 1, 2021 at 14:13
• When I look at that spec I read "load needed to get a maximum displacement". You're reading it as "maximum capable load while maintaining maximum displacement" correct? I guess I get confused when I look at a graph like this: <static.piceramic.com/fileadmin/_processed_/0/4/…> that describes a how a stack actuator functions. Based on this, the maximum displacement would be with zero force and decrease with any kind of force (and down 37% when at 300N). Another assumption i make is that these specs are all at 150V. Dec 1, 2021 at 14:44
• Just to be clear, I read the "Load for maximum displacement" as "maximum capable load while maintaining maximum displacement". I don't read the same way the blocking force spec. Regarding the plot, yes I think I interpret it the same way. Although it seems to be from another manufacturer.
– NMech
Dec 1, 2021 at 14:54
• Yeah, I believe you're correct and this conversation has been very helpful in my understanding. And yes, it is from another manufacture but I think it would apply as it was referenced as just a basic graph for stack actuators in general. I actually just found a similar one on the manufactures site of the spec sheet I posted. Dec 1, 2021 at 15:16