Top 4 Reasons to Use Direct Load Monitoring in Electric Servo Systems

How would you like to improve your electric servo system using an alternative type of position feedback?  Direct Load Monitoring has these 4 key advantages.

1.  Delivers accuracy that is independent of actuator drive precision or backlash.

2.  Maintains constant position accuracy over time regardless of drive wear and tear.

3.  Enables the implementation of less precise, less expensive drive mechanisms without sacrificing accuracy.

4.  Does not add significant overall cost.

 This alternative approach is called Direct Load Monitoring.  To understand what Direct Load Monitoring is and how it can deliver these benefits, we need to review how position feedback is typically implemented on electric servo systems. 

Load position feedback is commonly derived from a rotary encoder that is coupled to the motor shaft.   Since the rotary encoder is monitoring motor shaft rotations and only indirectly indicating the actual load position, this type of position feedback is called Indirect Load Monitoring

 A typical electric servosystem with a rotary encoder is pictured in Fig. 1 below.

Fig. 1 – Typical Electric Servosystem with Indirect Load Monitoring.
Fig. 1 – Typical Electric Servosystem with Indirect Load Monitoring. The top connector is for the position signals from the integrated rotary encoder connected to the shaft of the electric motor. The side connector is for motor power. The load that is being positioned is attached to the moving sled, which is represented by the two threaded rails on top of the aluminum housing.

 While this setup generates an accurate indication of the motor shaft position, it does not necessarily deliver an accurate indication of the true load position

Why not?  How about another Top 4 List, only this time it’s the…

 Top 4 Reasons Not to Use Indirect Load Monitoring

 1.  Motor shaft rotations are only an estimate of the actual load position.

2.  Load position accuracy is impacted by the backlash & non-linearity of the mechanical drive components connected between the motor and the load.

3.  Overall system accuracy decreases over time as drive wear and tear increases, because the full inaccuracy of the mechanical components is accumulated by the shaft-mounted encoder. 

4.  In order to serve the accuracy needs of the shaft-mounted encoder, higher-precision drive mechanisms are needed, increasing overall system costs. 

 Also, consider this: if a system is expected to deliver a specified level of accuracy over a warranty period, for example, it is necessary to design-in excess accuracy up front in order to guarantee the specified accuracy at the end of the coverage period.    

So, Direct Load Monitoring is clearly the way to go if you want the most consistent accuracy at the lowest overall cost.  Fig. 2 below shows one way to implement Direct Load Monitoring.  In this example, a magnetic linear encoder is set up to provide direct position feedback at the moving load. 

Fig. 2 –Electric Servosystem with Direct Load Monitoring.
Fig. 2 –Electric Servosystem with Direct Load Monitoring. The dark band along the side of the housing is a precision-encoded incremental magnetic tape. The silver sensor mounted on the bracket is the incremental encoder read head, which provides digital position and velocity signals to the motion control system.

There is one key disadvantage to this kind of Direct Load Monitoring: the cable moves with the read head, meaning that it needs to be flex-rated as well as properly managed as the load moves back and forth.  However, the benefits of Direct Load Monitoring often outweigh the disadvantages, so the extra effort required to address cable management is usually worth it.


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