Optical Encoders


Enhance “smart-drive” performance with CONTREX controls


Variable speed drive manufacturers have steadily crammed motion control features into their drives, earning them the label “smart-drives”.  The argument for this approach was to combine control and drive electronics into one unit, thus meeting the need of equipment manufacturers who required a reduction in the physical space consumed by these electronics, and to reduce the install time and cabling costs.


This combination of drive and control electronics came at the expense of usability, and ultimately performance.  Drive manufacturers found themselves in unfamiliar territory developing user interfaces and complex control software.  At the same time, hardware features like keypads and I/Os had to be kept to a minimum to contain cost.  Consequently, this made the control features of the “smart-drive” cumbersome to use, or simply inadequate.  Ironically, some have worked to alleviate the shortcomings by providing a remote keypad - this negates all original purpose of combining the electronics in the first place.


A common example of “smart-drives” are flux vector drives.  While they offer reasonable performance over a broad speed range, they have many limitations.  These limitations include:


  • Limited operator interface (keypad, readout) or no interface at all

  • Require an expensive, factory installed, motor mounted encoder

  • The encoders have fragile glass disks and are prone to failure

  • If used with potentiometer input, the analog error limits the accuracy

  • Cannot accept surface speed encoders, common in web control applications


An excellent way to use the best features of these drives while avoiding their pitfalls is to combine them with the CONTREX MLP-Trim.  Most closed-loop flux vector drive manufacturers offer an open-loop version of the same drive, at a significantly reduced price.  Usually it is the same hardware platform without the closed-loop option card.  The MLP-Trim can use the rugged, and inexpensive CONTREX Quadrature Ring Kit instead of the fragile encoder.  This ring kit can be installed or moved by any maintenance person.  Naturally, the flexible MLP-Trim can also use the encoder signal if required.


The accuracy requirement that leads a customer towards “smart-drives” prequalifies the application for needing an operator interface, making the MLP-Trim very appealing.  The MLP-Trim allows for operator entered speed and/or ratio, scaled set points and speed display, alarms and troubleshooting diagnostics, and of course the expert assistance of the CONTREX team.  Drive company support traditionally ends at the motor shaft.


The Bottom Line:  A given Brand “X” closed-loop flux vector and motor with encoder costs about $500 more than the same Brand “X” open-loop flux vector with a CONTREX MLP-Trim and Quadrature Ring Kit.  You get all of the great motion control features of the MLP-Trim while reducing total system costs.