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7.12 Improving the dynamic performance of an open-loop motor servodrive using a PSC and pressure derivative feedback
195
7.12 Improving the dynamic performance of an open-loop motor servodrive
196
where +SG is the pressure transducer gain and IJ is a suitably selected time constant from a linearised analysis and, inevitably, practical testing. The open-loop transfer function now becomes:
SG D OLQ TL
Q OLQ
Q
P D
TL
G N * +
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IJ Ȧ @
IJ V IJ IJ Ȧ VIJ V
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It then follows that:
• this 3rd order open-loop transfer function is inherently stable and independent of the choice of the time constant IJ selected
• the value of the time constant τ selected does affect the speed transient behaviour, the very reason for introducing it
(7.43)
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There are other issues to be considered before digital implementation is intoduced as follows:
• in reality the accumulation of servovalve dynamics and proportional relief valve dynamics, together with system noise, has a small but detectable effect on the system performance
• the effect of higher-order dynamics is to create a small phase shift with respect to the algebraic linearisation process and this is compounded if digital filtering is used to minimise signal noise
• signal noise arises from both fluid flow effects and the pressure ripple generated by the supply pump and the motor, and may be filtered to some extent by using the filter translator available on the PSC card
• the supply pump operates at a nominal speed of 1440 rpm and the load line pressure ripple is rich in harmonics at both the pumping frequency and the motor frequency. The time domain signal shows that the motor fundamental frequency dominates the pressure ripple and will of course vary as the motor speed is varied. The pressure ripple fundamental frequency does not present particular computation problems using the PSC card, providing higher-frequency component effects are minimised.
In practice minimising unwanted frequency components is achieved quite simply by using digital low- pass filters. The PSC card filter translator is provided to allow the user to input filter data in a high level floating point format which is then converted to low level integer form for use with the filter function block. The PSC card was set to operate with a sample time of T = 4ms, that is a sampling frequency of 250Hz.
Some results are shown as figure 7.8 at a constant minimum load where the motor speed was demanded to change from 500rpm to 800rpm. Comparisons are made with conventional control at a fixed supply pressure of 3V= 90bar and “intelligent” control with variable supply pressure, optimum efficiency control and dynamic pressure feedback.
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Figure 7.8 A comparison of conventional control and “intelligent” control of an openloop motor servodrive [Davies RM and Watton J. Intelligent Control of an Electrohydraulic Motor Drive System. Journal of Mechatronics, Vol 5, No 5, 1995, 527–540]
The major conclusions from this project are:
• the conventional dynamic behaviour is highly non-linear, the response being sluggish at high motor speeds and oscillatory at lower motor speeds as expected under conventional constant supply pressure conditions
• active control of supply pressure showed only marginal changes in steady-state values as the steady-state speed is changed
• under ‘intelligent’ control the non-linear transient speed effects are reduced, combined with a reduction in the required supply pressure when appropriate
• intelligent control has reduced the tendency of the active line pressure to cavitate, that is, fall to the fluid vapor pressure
• symmetry of dynamic behaviour at the different speeds is not exactly achieved due to the desire to also achieve optimum efficiency at a servovalve pressure drop of 15bar
• at the chosen servovalve pressure drop of 15bar the flow characteristic is not ideally linearised at higher speeds. This could be improved by moving to a higher pressure differential at the expense of a reduction in efficiency
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8 Pressure and force control
8.1 Aim
To understand the behaviour of other types of electrohydraulic closed-loop systems via the following objectives:
• to consider the response, control and stability of a pressurised chamber
• to consider servodrive structural force control and stability
• to develop the concepts using worked examples