EPOS4 / IDX: Object "Torque actual value"

Topic:

• What is the meaning and scaling of EPOS4's resp. IDX's object "Torque actual value"?
• How is the value of the object "Torque actual value" evaluated?

Solution:

1.) Scaling of "Torque actual value"

The value of the object "Torque actual value" (0x6077) and "Torque actual averaged" (0x30D2-01) is given in [per mill] (= 1/1000) resp. displayed in [%] by EPOS Studio's "Data recorder" and refer to the "Motor rated torque" (0x6076) in [μNm], i.e. the motor's "Nominal torque" (specified in [mNm]).

Please find more details in the "EPOS4 Firmware Specification" too:
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2.) "Motor rated torque" evaluation

The "Motor rated torque" (0x6076) is calculated by the EPOS4 resp. IDX by a multiplication of the motor's "Nominal current" (0x3001-01) and its "Torque constant" (0x3001-05). These both base motor data values have to be configured by EPOS Studio's "Startup" wizard during initial commissioning based on the motor's data sheet.

Example: "EC-i 40", PN: 496651
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The value of the object "Torque constant" (0x3001-05) can be configured manually by the "Startup" wizard (based on the motor's data sheet) or it can be identified automatically during "Regulation Tuning".

Important:
The value of the "Nominal current" and the "Torque constant" have to be configured properly. Otherwise the internally calculated "Motor rated torque" (0x6076) will be wrong and the reported "Torque values" might be misleading.

3.) Torque value calculation

Please note that there is no torque measuring device present to measure the motor torque!
Any object with a torque value just provides a calculated value. The calculation is based on the measured "Current actual value" (0x30D1-02) multiplied with the "Torque constant" (0x3001-05) which was configured or evaluated during tuning.

The [per mill] value of the "Torque actual value" (0x6077) is based on the following formula:

• "Torque actual value" [per mill] = ("Current actual value" [mA] / "Nominal current" [mA]) * 1000

The torque [mNm] can be calculated based on the reported [per mill] value of the "Torque actual value" (0x6077) by one of the following formulas:

• Torque [mNm] = ("Torque actual value" [per mill] / 1000) * ("Motor rated torque" [μNm] / 1000)
• Torque [mNm] = ("Torque actual value" [per mill] / 1000) * "Motor rated torque" [mNm]
• Torque [mNm] = ("Torque actual value" [per mill] / 1000) * ("Nominal current" [mA] / 1000) * ("Torque constant" [μNm/A] / 1000)
• Torque [mNm] = ("Torque actual value" [per mill] / 1000) * "Nominal current" [mA] / 1000) * "Torque constant" [mNm/A]
• Torque [mNm] = ("Torque actual value" [per mill] / 1000) * "Nominal current" [A] * "Torque constant" [mNm/A]

4.) Torque accuracy?

Due to the fact that the reported torque values by the controller are just calculated based on motor current, these will not(!) correspond precisely to a torque measured by an output shaft torque sensor. There are different possible root causes for such a discrepancy:

• There are a lot of "constants" part of the internal torque calculation which have to be configured properly but still have a tolerance of +/-10% typically.
• Motor winding temperature will also have some slight influence on the "constants".
• If a gear is present, its efficiency (at the actual working point) can have a remarkable impact on the reported torque compared to the output shaft torque measured dynamically by a torque sensor.

Final conclusion:

• The reported "Torque values" reported by the EPOS4 or IDX are [per mill] (= 1/1000) of the "Motor rated torque" (0x6076) resp. motor's nominal torque.
• The "Torque values" are just internally calculated values based on the motor current measurement and motor constants. These derived torque values are not(!) highly accurate. 
• If a precise output shaft torque data (esp. of a gear's output shaft torque) is required, a torque meter or torque sensor will be required which can measure the torque dynamically during motion.