APPLIED INDUSTRIAL CONTROL SOLUTIONS
DC Drive Speed Controller Design
Brian T. Boulter
© ApICS ® LLC 2000
A summary of the derivation along with a detailed description of dynamic models for single wrap and double wrap, 1:1 gearless, 2:1 gearless, and 1:1 geared elevator and hoist configurations is presented. Typical and proposed Elevator system controls configurations are described. Simulation comparisons between the typical and proposed elevator system configurations along with supporting field results that demonstrate a significant improvement in system performance and ride quality using the proposed configuration are then presented. Ease of set-up in the field is also demonstrated.
The combination of typical elevator drive/system software and typical elevator pattern generation software has proved difficult to tune in the field. On occasion, a tuning that provided adequate ride quality could not be found.
To address this problem the author was commissioned to develop a control strategy for a proposed elevator control algorithm that would be both easy to tune in the field and provide a "class ride" in the presence of low frequency system resonances and significant tachometer ripple. This in turn required the development of models for 1:1 gearless, 2:1 gearless and 1:1 geared elevator configurations. These models were used to investigate existing and potential control strategies as well as to arrive at an understanding of the fundamental problems responsible for the observed difficulties tuning the typical control algorithm.
The investigation of the typical control strategy, using the developed models of elevators, indicated that high speed loop bandwidths (10 to 15 [rad/sec]) resulted in forward path gains that significantly amplified tachometer ripple. The amplified tachometer ripple in turn excited parasitic system resonant frequencies at speeds where the rotational frequency of the tachometer ripple matched a system resonant frequency. The requirement for high speed loop bandwidths in the typical design came from a need to provide adequate tracking of the speed reference (or pattern).
Based on these observations and the knowledge that good tracking was paramount in any elevator speed regulator, a control strategy needed to be developed where excellent tracking is realized without amplifying tachometer ripple and exciting parasitic natural frequencies. This indicated a system where some sort of reference forcing is employed.
There will always be systems where a low frequency underdamped resonant mode exists. These resonant modes must be accommodated with some sort of attenuation mechanism. To accomplish this a feedback filter was designed that provides attenuation of the problematic frequency with minimal loss of phase margin at the speed loop crossover.
Finally a controller structure was designed that employed reference forcing along with the inclusion of the resonance compensation filter. The tuning of the proposed control algorithm was then approached as a loop-shaping problem. That is, a control scheme where the desired open loop Bode plot magnitude and phase characteristics are known a-priori. This enables the design of a controller where the correct adjustment of a single gain term optimizes the control design and yields the desired Bode plot open loop gain and phase characteristics. Optimal tuning can be accomplished in the field with the adjustment of one variable making the control system fairly simple to tune on-site.
Once the controller structure was identified it was successfully field tested at the Hyatt Regency in Chicago Illinois. Chapters  and  of this Systems Report. contain discussions of observations and conclusions based on the simulations and field testing and can be referred to for a quick summary statement of the technical content of this engineering report.
The following is a summary of observations and conclusions made from simulations of the three elevator models and field testing of the proposed control algorithm at the OEM Site.
- Higher speed loop bandwidths in elevator applications result in forward path gains that significantly amplify tachometer ripple. At particular speeds, the amplified tachometer ripple excites parasitic natural frequencies. This is especially noticeable when these frequencies are in the 5 - 10 [Hz] range.
- Higher speed loop bandwidths reduce damping of the dominant resonant frequency. This is especially noticeable when this frequency is below 10 [Hz].
- To realize similar tracking of the pattern, elevator systems using the typical control algorithm required speed loop bandwidths approximately 2 to 3 times higher than the proposed control algorithm.
- The use of the reference forcing filter in the proposed control algorithm significantly improves tracking of the pattern without the need for high speed loop bandwidths. Lower speed loop bandwidths translate into significant attenuation of parasitic resonant frequencies and reduced excitation of these frequencies by tachometer ripple. This in turn translates into a smoother ride.
- The use of the reference forcing filter in the proposed control algorithm results in a speed regulation that is first order. The step response does not have any overshoot. By coming into the target floor with no overshoot the time spent in leveling speed is reduced resulting in significantly improved floor-to-floor times.
- In systems where the dominant natural frequency is very low and under-damped the RES_COMP filter can be deployed to attenuate the parasitic frequency. This will require some adjustment to the pattern if a floor-to-floor time similar to that obtained without the RES_COMP filter is desired.