6 MM Features
Mk8 MM Manual Page | 125
6.2 Using 4-20mA Servomotors
6.2.1 Overview
A third party 4-20mA servomotor can be used with the Mk8 MM in conjunction with an Autoflame universal
Mk8 IO Module (part #DTI80022). The IO module acts as an interface between the MM and the 4-20mA
servomotors. This feature is used in sites where the fuel and/or air going into the burner cannot be controlled
by mains voltage or where pneumatic control already exists. The MM will send a 4-20mA signal out from the
IO module for the position of the servomotor for it to move according to the commissioned fuel-air curve and
receives a 4-20mA signal back to the IO to indicate where the external 4-20mA servomotor position is
currently.
The Mk8 MM channels which can be controlled using an external 4-20mA servomotor include channels 1, 2,
3, 4 and 7. Any or all of the 5 servomotor channels available on the Mk8 MM can be independently
configured to use an externa 4-20mA servomotor or an Autoflame servomotor.
IO Module Analogue Input/ Output
6.2.2 External 4-20mA Servomotor Requirements
For the MM to control the servomotor channels via external 4-20mA servomotors, the following requirements
must be met:
1. The external 4-20mA servomotor must be capable of being controlled to an accuracy no higher than
0.5° angular accuracy.
2. There must be both a 4-20mA input and 4-20mA positional feedback from the external 4-20mA
servomotor.
3. The feedback signal from the external 4-20mA servomotor must correlate directly to the input signal.
For example, if 8mA is outputted from the MM, then the servomotor should move to the position that
gives 8mA feedback to the MM.
4. The range of the external 4-20mA servomotor is 4mA for the closed position at 0.0° up to 20mA for
the open position at 90.0° position, with a linear scaling. For example, 12mA will correspond to a
position of 45.0°.
5. Neither the input or output signal from the external 4-20mA servomotor follows loop control; this
means that if the MM sends a signal of 10mA to the servomotor via the IO module, and the
servomotor does not have angular position to return a 10mA signal back to the MM, then a fault
condition will occur.
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