Recon Outdoors Talon IR Spécifications télécharger pdf (Page 179)

Software

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The counters help determine the zero velocity condition however, a problem arises

when selecting the appropriate threshold number. This number corresponds to the

number of control loop cycles executed before the actual velocity is assumed zero so

consideration is given to the loop period and the resulting control response. Low

values give a faster response to change whereas high values reduce the oscillatory

behaviour at low velocities. This is because a number of control loops may execute

between encoder counts thus causing the zero velocity condition and spurious bursts

of velocity data at the pulses. This is remedied by specifying different counter

thresholds based on the previous operating velocity. High velocities have a low

threshold to rapidly detect a halt while low velocities have a higher threshold to

ensure a stable readout for slow track movement.

The current motor velocities are entered into a matrix of the last 10 values and

averaged to reduce the effects of noise and drive system irregularities (friction,

terrain). An error value between the target and actual velocities is generated to enable

feedback control however, it is set to zero if the magnitude is less than 5 cm/s. This

error margin helps increase the control system stability without affecting the overall

robot response due to the nature of its operation in an outdoors environment.

The control effort is calculated using a pseudo PI (Proportional Integral) algorithm

given by Equation 5.6. This method is preferred over conventional PID (PI

Derivative) control due to delays and inaccuracies inherent to the system. Derivative

components would amplify these errors and produce undesirable control system

oscillation and instability. Full PID control should be reinvestigated once a complete

system model of the mechatron is generated. Saturation conditions are applied to the

control effort to ensure it does not exceed the analogue voltage limitations of the

DAQ card and microcontroller hardware. Special conditions such as starting from rest

and a zero target velocity are accounted for by rapidly increasing or decreasing the

control effort as required before writing to the analogue output channels.

Equation 5.6

where: u(k) = current control effort u(k-1) = previous control effort

K = controller gain e(k) = current error value

)()()( kKekuku +

−

= 1

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Recon Outdoors Talon IR Spécifications Page 179