NON-LINEAR ELEMENTS

• If our models include a device that is non linear we will need to linearize the model before we can proceed.

• A non-linear system can be approximated with a linear equation using the following method.

1. Pick an operating point or range for the component.

2. Find a constant value that relates a change in the input to a change in the output.

3. Develop a linear equation.

4. Use the linear equation in the analysis (Laplace or other)

• Consider the example below,

 

Figure 12.1 Linearizing non-linear elements

 

 

12.3.1 Time Variant

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- system parameters vary as a function of time.

 

12.3.2 Switching

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- system components turned on/off

- cables in tension/compression

- show an example where input conditions change

 

- give PWM (Pulse Width Modulation) example with ripple showing equivalent voltage. PWM is used to generate analog voltage equivalents. Show for a system with first order response with tau = 0.1s for a frequency of 1KHz, 10Hz and 1Hz. Point out the ripple and effective voltage.

- important to consider when doing system analysis

12.3.3 Deadband

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- Friction in all components

- costs money to reduce friction, so it is better to compensate in software

- small actuation signals not large enough to overcome friction

- This effect is normally known as ’stiction’, a combination of the words static and friction.

- Friction is common in less expensive motors, and when a motor is driving a mechanical system.

- In systems there are two type of friction that must be considered.

- The static friction, ’stiction’, will prevent initial motion. If the systems breaks free and starts turning, the kinetic friction will provide a roughly constant friction resistance.

- relationship in figure below.

- the region where the applied voltage has no effect is called the deadband.

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Figure 12.2 Motor deadband for a bidirectional motor

- deadband compensation as shown in figure below.

 

Figure 12.3 Deadband approximation for a bidirectional motor

- equations for these are shown in figure

 

Figure 12.4 Deadband approximation for a bidirectional motor

- c-code below

 

Figure 12.5 Deadband Compensation Subroutine

 

Figure 12.6 Deadband Breakaway Subroutine

12.3.4 Saturation and Clipping

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• Some devices have natural maximum values, such as voltage or pressure limitations caused by a regulated supply.

12.3.5 Hysteresis and Slip

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- windup resulting from springiness and friction

- backlash

-

 

- correct by tracking the previous motion direction and taking extra steps when reversing direction

 

- slip takeup can be done with the function below that adds a few steps when reversing direction.

 

Figure 12.7 Deadband Breakaway Subroutine

 

 

 

12.3.6 Delays and Lags

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• Time delays are common in systems

• In the simplest form this is a period of time between when an event occurs and when the effect occurs.

• If an output delay is larger than the control system step time it may be necessary to predict future states and initiate outputs ahead of those.

• If an input delay is larger than the control system it might be necessary to slow the control action, or build it into the control law.

 

 

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