and the motor that creates delayed response times , leading to reduced system bandwidth .
When a large inertia mismatch is introduced to the system , such as a small , high torque motor connected to an exceptionally large load via a coupling device , the compliance problem is magnified . When the motor quickly applies torque , the large load hesitates to respond due to its high inertia ; the delay is a result of coupling compliance between the motor and load that introduces windup before the load begins to move . As the load finally synchronises with the motor , the large inertia causes overshoot of the target speed , resulting in the motor adjusting to slow down . When the system adjusts the overspeed of the inertia , the target speed is again passed , triggering the motor to adjust once more . This causes a continued cycle of repeated adjustment that creates resonance and an unstable system .
The compliance challenge
Most mechanical systems can be mathematically modelled and
simulated using various excitation frequencies to identify the response point where resonance occurs . However , the bandwidth of a system can never exceed the initial antiresonance point . In fact , the higher the compliance , the lower the frequency of the initial resonance point , that reduces bandwidth accordingly . When the driven load is directly coupled to the motor to minimise compliance , the mismatch is mitigated , increasing the initial resonance frequency and creating a higher bandwidth system .
Mathematical models show that the ultimate solution for a higher bandwidth and cost-effective system is to increase the mechanical stiffness and reduce total system inertia . Consider a direct drive solution where the load is directly coupled to the motor with near zero compliance ; in cases like this , precisely controlling the system with high bandwidth can be achieved with inertia mismatches as high as 30:1 . As direct drive solutions are not suited to all applications , compliant links will inevitably be introduced . However , advanced analytical tools , such as the bode plot , can readily identify the compliant elements that reduce system performance .
Motion engineering
Applications often characterised by a high inertia mismatch can include printing and labelling , as well as various robot designs . Although inertia mismatch is no longer the main challenge , resolving the imbalance requires careful specification across a range of aspects , from motor sizing , through to tuning and analytics of the control algorithms , and mechanical architecture .
Assistance in specifying a system is particularly useful for machines with an inherent inertia mismatch . With comprehensive application sizing and best practices in designing a stiff mechanism , this can achieve a high-performance motion system capable of higher bandwidths , improved move and settle times , and robust dynamic control .
For further information , please visit www . inmoco . co . uk
Issue 71 PECM 27