I currently have an application where both feedback types may be required in order to control a DC motor axis correctly. The motor has a tachometer mounted on the back that is currently used for velocity feedback, but the axis drifts due to the nature of the velocity control. A quadrature encoder has been fitted separately (not on the motor) because the motor rotations and axis motion far exceeds the encoder capability. It has been geared such that position information is sufficiently resolved. Is it possible to use BOTH feedback types to control motor speed as well as absolute position of the axis? I would consider this a Position-Velocity axis configured as a dual loop, but that is not one of the axis options. I cannot seem to tell from documentation whether this is possible to implement with a virtual axis or user program. If so, should I tailor the card selection to the tachometer or quadrature feedback? (Q, H, U, or A cards?)
Sounds like you should be able to set this up as Cascade Control where you use the Tachometer for feedback for the inner loop and the encoder for feedback for the outer loop.
You can look up Cascade Control in the RMC help.
If you use an RMC75 you will need an QA1 axis plus an A2 expansion module.
If you have lots of backlash between the motor and the encoder you may need to add some programming to reduce the hunting that the inner loop does as the motor moves back and forth through the dead zone.
Note that with a quadrature encoder you will not have absolute position. You will need to home the axis. You could use an SSI encoder rather than quadrature to obtain absolute positions without homing. If you do this you would need to use an MA1 rather than a QA1 axis on the controller.
It may be easier to put the encoder on the motor since you will have to home it anyway. Then you would just have a single loop to set up and control.
If you have other axes you are controlling or want to use an RMC150 for other reasons then you would need a Q1 card for the quadrature encoder and an H1 card for the tachometer. If you use SSI you would need an S1 rather than the Q1.
Thanks David. That is helpful.
The hardware and mechanical configuration is already existing, so I have no choice in the matter for device selection, etc. The mechanics of the axis cause the motor to run hundreds of rotations, so even if the encoder was on the back of the motor, I can guarantee a pulse counter would rollover from one end of travel to the other, hence the reason for the tachometer. The encoder is placed down on a pivot point for the axis with the largest gear ratio possible in the space given to achieve the highest pulse resolution per degree of travel. We have an existing controller from the mid-90’s that is operating this axis and 3 other axes, but it is incapable of performing these complicated types of control and it is unsupported now. We are upgrading the control system and I’m trying to find out if we can achieve a better method of position control to eliminate drift.
I looked into the cascade control option, but it was unclear to me if this was a good solution. I will look into it some more and tailor the hardware selection if it fits. I believe we need an RMC150 for the 4 axes of motion and other digital interlocks and controls.
I am a bit concerned about the deadband for tachometer control, as well as backlash with the encoder, but I’d like to think the gearing is tight “enough” to have minimal backlash and provide good position indication. We currently monitor the encoder and calculate position and it is repeatable and linear.
Many multi-turn SSI encoders have 4096 turns, and 4096 or more counts per turn. Would that cover all the rotation?
For example, the Sick AHS/AHM36: http://www.sick.com/group/EN/home/products/product_portfolio/encoders/Pages/rotary_absolute_multiturn.aspx