What is Stiff Load?

Question

In Control systems, when it comes to governer control, I came across with a term Stiff Load.

Can someone please explain what it means?

The jokes are endless, but shall remain unsaid. – Nov 01 '11 at 16:29 — , Nov 01 '11 at 16:29

score 4 · Answer 1 · answered Nov 01 '11 at 07:30

4

A "stiff load" in a mechanical system is one which does not undergo substantial deformation or interaction with forces applied to it.
ie the reaction to applied forces is from other flexible or variable parts of the system rather than to the load proper.

Wikipedia here says:

Stiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom (DOF) when a set of loading points and boundary conditions are prescribed on the elastic body.

Note that "stiffness' relates not only to the axis along the axis where a "force" is applied but also the degree of coupling to other axes.

In an electrical system a stiff system would be one where eg the load on a power supply did not vary as voltage varied - not a usual situation.

answered Nov 01 '11 at 07:30

Russell McMahon

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Minus switching regulators I cannot think of any other electrical system I have had that was "stiff." – Kortuk Nov 01 '11 at 08:35
@Kortuk - even linear regulators? – Connor Wolf Nov 01 '11 at 09:32
@FakeName, they will change power draw with different voltage. If a constant current is what is required for stiffness, then they would be one of the few things that would. I am not sure that I have ever really used stiffness for an electrical system. – Kortuk Nov 01 '11 at 10:05
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The only times I have heard "stiff" used in the context of electronics is regarding the voltage regulation characteristics of a powersupply. – Connor Wolf Nov 01 '11 at 10:28
He is asking in a control system context and it needs to be answered in that context. He mentioned only mechanical sysems BUT he may have an actuator, motor, psu etc. The question, as ever, needs to be developed. | As an EE "stiff" to me means unlikely to alter much when purturbed (voltage eetc). || To control engineer a stiff electrical load does not vary when voltage varies. Consider a buck converter feeding and output at constant V into a resistor. Pout = constant. Input load to converter is ~~ constant across the input voltage range. Efficiency variation with Vin affects stiffness. – Russell McMahon Nov 01 '11 at 12:45
An inverting opamp is a pretty good stiff load. – Optimal Cynic Nov 02 '11 at 06:28

score 3 · Answer 2 · answered Nov 01 '11 at 13:19

Stiff means the value of interest doesn't change much as varying pressure is applied that should change it. For example, concrete is stiff because it deflects little with applied force. Rubber is much less stiff.

In electronics stiff is usually used to describe a voltage source and means low impedance in that case. The voltage changes little with different currents. It is less common, but you could also say a current source is "stiff". In that case it means very high impedance. The current changes little with different voltages. In either case the value of interest doesn't change much as a function of applied conditions.

What is Stiff Load?

2 Answers2