Null Current of Integrating Actuator


Thread Starter


Have a TM-55 Integrating Actuator with a SPC controller of woodword. The commissioning, the controller requires me to input a parameter called " Actuator Null Current", and it shall set equal to the actual null current of the actuator, otherwise, a steady errors will exist.

According to the SPC: At a certain input current of an integrating actuator, called the null current, its position does not change.

I went through the TM-55 manual, there is a parameter shows 160 mA null, and also said the null current can be changed.

First of all, what is null current?

Second, if the 160 mA null is the factory set null current of the actuator?

Third, why reaching null current, the position of the actuator will not change any more?

Last, the SPC controller need this parameter be set?

Thanks a lot

Robert Scott

The TM-55 Actuators come in two types. One is the Proportional type. (This is not the type you have.) In this type of actuator, the control signal from the SPC controller is interpreted as a position command - more current means a higher shaft angle, less current means a lower shaft angle. This type of controller can be operated open-loop (no position feedback).

The second type of TM-55, the one you have, is an Integrating actuator. That means the command from the controller is not a position command but is actually a velocity command. It tells the actuator how fast to move and in which direction. This type of controller cannot be operated open-loop, but must be operated closed-loop using position feedback to the SPC controller, which you have. The reason it cannot be operated open-loop is that when using a velocity command, the actuator is not paying any attention to its position. It is trusting that the controller knows what it is doing by commanding this or that velocity. So unless the controller has a direct way of determining position, it cannot intelligently command velocity to achieve any given position.

So once we realize that the command from the SPC controller to the TM-55 is a velocity, there must be some way of specifying a positive or a negative velocity. But the control command from the SPC controller is a positive current ranging from 20 ma. to 200 ma. By setting a "null current" of 160 ma., the SPC controller and the TM-55 can agree that 160 ma. means zero velocity. Any current above 160 ma. means a positive velocity, and any current below 160 ma. means a negative velocity. So in this mode, if you are going to have velocities that go just as fast in one direction as they do in the other, the command range would have to be from 120 ma. to 200 ma., with 160 ma. (meaning zero velocity) sitting right in the middle.

So what happens if the SPC controller and the TM-55 do not exactly agree on the null current? Well, if the SPC controller is sending out 160 ma. because it sees the desired position has been reached, it expects the TM-55 to stay right where it is. But if the TM-55 thinks that 150 ma. is the null current, it is going to interpret the 160 ma. command as a small positive velocity command and start moving. This will change the position feedback, and the SPC controller will eventually see the change in position and start commanding a negative velocity to the TM-55 to compensate for the undesirable position. This will eventually bring the TM-55 to a stop, and there everything will remain.

But exactly where will it be when it stops? Well, it will not be at the original desired position. It will be a little bit beyond the desired position. It will be at such a position that the SPC control decides to issue the small negative velocity command to correct the position, and the TM-55 will interpret that small negative velocity command as the command to "stand still". So the result is a static position error. (Unless the SPC controller has some integration of its own, in which case it will eventually correct the static position error to zero.)

Therefore you want to have both the SPC controller and the TM-55 to agree on what the "null current" should be in order to minimize static position error. If you are not sure where to set the null current parameter in the SPC controller, then just try different settings and see what the static position error is for each null current settings (assuming you have some way to measure static position error). When you have eliminated static position error, then the setting of the null current parameter is exactly right.

Robert Scott
Real-Time Specialties (embedded systems design)
Hopkins, MN
Wow, you are the best, Robert. I got more than I could from the woodward technical support. Thanks Robert.

In the SPC Service Tool, when configuring the Controller type, there are three parameters to fill, one is Actuator Minimum Current, the other is Actuator Maximum Current, the last one is Actuator Null Current. Now I understand the Actuator Null Current. So what is the Maximum and Minimum Current? Shall the Actuator Null Current needs to be seated right in the middle between Min and Max, as your example, 160 mA between 120mA and 200 mA. If not, any negative impact?

Our SPC controller is currently configured with 0 Minimum and 180 mA Maximum, and the null current is 157 mA.

It is recommendable to set null current at 100ma, so as for any change in demand have enough margin in negative and positive side. in normal operation of turbine at constant load null current remains constant say if we set 100 ma, remains at 100 ma. It can be set in the range of 90-110 ma (advisable).

In case if there is any issue and actuator/valve doesn't respond, then temporary null current may move towards max say 200 ma and to min say 20 ma.

Please ensure position error alarms to be set nicely.