Mitsubishi M800/M80 servo monitor: why Droop and the three maximum currents are not directly comparable
IB-1501273-L distinguishes position-following error, current feedback and command peaks. Learn how power-on and two-second windows affect diagnosis before condemning a drive from one percentage.
Contents
If Load current is low but Max current 1 is high on the same axis, is the current feedback faulty? Not necessarily. The fields differ in both time window and signal type. Interpret their definitions before treating every percentage as the load at one instant.
Scope: servo units, not spindle units
This article uses the Mitsubishi M800/M80/E80 Maintenance Manual, IB-1501273-L, revision L, section 4.7.1, printed pages 100–101. Its screen path is Diagn → Drv mon → Servo unit, covering NC and PLC axes. Other series, M800V/M80V controls, spindle units and machine-builder screens require separate verification. This is an interpretation of existing records, not a request to move the machine or change parameters.
Three kinds of information on one page
| Screen item | Manual definition and comparison limits |
|---|---|
| Droop (i) | Error of actual machine position relative to commanded position; the display unit follows #1003 iunit. It is not a current percentage or a direct measurement of workpiece dimensional error. |
| Speed / Feedrate | Actual motor speed and machine-side feedrate respectively. For a linear motor, Speed uses mm/s, not r/min. |
| Load current (%) | Motor-current feedback expressed relative to continuous stall current. |
| Max current 1 (%) | Absolute peak current command sampled since power-on, relative to the same continuous stall current. |
| Max current 2 (%) | Absolute peak current command sampled in the most recent 2 seconds, on the same basis. |
| Max current 3 (%) | Absolute peak current feedback sampled in the most recent 2 seconds, on the same basis. |
The key difference between Max current 1 and 2 is the time window. Between 2 and 3 it is command versus feedback. Absolute peaks do not retain a sign from which to infer motion direction. These percentages are not percentages of the drive's rated power.
Larger Droop does not identify a failed component
The manual states that Droop is proportional to commanded speed and gives the position-loop Gain relationship as feedrate (mm/s) divided by tracking-delay error (mm). A larger Droop in a record taken at a different speed is therefore insufficient evidence of a failed motor or encoder. Match speed, axis, units and operating phase before comparison.
This relationship is not a field-tuning recipe. Acceleration, deceleration and steady-feed records cannot be mixed without qualification, nor should a ratio be forced from near-zero stopped readings. This article neither derives recommended gains from the display nor claims that every error changing with speed is normal.
An example that interprets records only
These figures are hypothetical, not measured: a stopped-machine record shows Load current 10%, Max current 1 160%, Max current 2 25% and Max current 3 24%. They are not inherently contradictory. The 160% is a command peak since power-on, not evidence that present feedback remains at 160%. The 25% and 24% are peaks of different signals within the last 2 seconds. Subtracting them does not yield an instantaneous current-tracking error because the peaks need not occur at the same time.
Likewise, 160% alone does not establish an overload. The same section separately defines Overload as data for monitoring motor and drive-unit overload, and Regen load as resistance-overload monitoring when a resistance-regenerative supply is connected. Neither is interchangeable with Max current. No universal alarm threshold or permissible duration is proposed here.
Making the comparison useful
- Label existing records with the axis, control, drive and motor models, units and operating phase. Identical percentages on different motors do not imply identical amperes.
- Identify whether records belong to the same power-on period and note image times. Different periods give Max current 1 different histories. Do not power-cycle merely to obtain a fresh peak.
- Keep Load current, all three Max current fields and Droop separate, alongside existing speed and alarm evidence. Screenshots taken at different times are not a simultaneously sampled waveform.
- If changes remain unexplained in comparable historical records, pass the complete set to responsible personnel. One screen cannot isolate mechanical resistance, command and feedback problems or identify a component to replace.
Stop conditions and reference
The comparison method above is editorial, read-only guidance. For abnormal movement, noise or related alarms, stop safely under the machine builder's procedure. Do not test this article by raising gain, widening error limits, disabling protection or making live electrical measurements. Waveform capture or test motion requires a separate safe plan from qualified personnel.
Reference: Mitsubishi M800/M80/E80 Series Maintenance Manual, IB-1501273-L, revision L, §4.7.1, printed pages 100–101. Definitions come from the manufacturer's manual; hypothetical figures and the comparison method are editorial explanations. No machine testing was performed.