Physical Limit Inputs

Each RMC axis allows for optional physical limit inputs to specify the boundaries in which the axis is allowed to operate. Each axis can have two physical limit inputs: Positive Limit Input and Negative Limit Input. Typically, these inputs are wired to limit switches.

The physical limit inputs differ from the Travel Limits, which limit the range of travel of an axis based on the feedback (position, pressure, etc.). The Travel Limits are required to be set on an axis; physical limit inputs are optional.

Setup

Use the Positive Limit Input and Negative Limit Input parameters to specify the inputs to be used for the physical limit inputs. The following options are possible:

Option	Description
none	This is the default setting.
Fault Input	RMC75: The Fault Input of the axis. RMC150: The Fault Input of the axis. Only available on the Quadrature Module. RMC200: n/a
Dedicated	Available on RMC75 QA and RMC150 Quadrature modules only. For the Positive Limit Input, this is the PosLim input. For the Negative Limit Input, this is the NegLim input.
general input	RMC75: any input from a D8 module, but only from the first 12 I/O points as listed in the Discrete I/O Monitor. RMC150: any general discrete input from any module. RMC200: any discrete input from any module.

Operation

The Positive Limit Input and Negative Limit Inputstatus bits indicate the state of the Limit Inputs. When one of these inputs becomes active (as indicated by the status bits), the corresponding Positive Limit Input or Negative Limit Inputerror bit will be latched. The error bit will cause a Halt to occur if the Positive Limit Input and Negative Limit Input Auto Stops are configured to do so and the Direct Output Status bit is off.

Issuing Commands while a Limit Input is Active

While a Limit Input is active, closed loop motion commands that move the axis away from the valid range will set the corresponding Limit Input error bit, causing a halt if the Auto Stops are configured to do so. Motion commands that move toward the valid range will not set the a Limit Input error bit. This allows you to move an axis back within the limits without causing further errors.

Therefore, if the Negative Limit Input is active, a motion command that moves the axis in the negative direction will cause an error. A motion command that moves the axis in the positive direction will not cause an error. Likewise, if the Positive Limit Input is active, a motion command that moves the axis in the positive direction will cause an error. A motion command that moves the axis in the negative direction will not cause an error.

Moving Toward the Valid Travel Range

If a Limit Input is active, and the axis is in open loop, and is drifting slightly, issuing a closed loop motion command may again trigger an overtravel error. This is because the target position starts at the actual position, velocity and acceleration at the time the command is issued. Therefore, the target may continue moving away from the valid range while accelerating to turn around. This will trigger an overtravel error. To avoid this, first issue a Hold Current Position (5) command, then the motion command. Or, simply use an open loop move to move into the valid range.

Limit Input Polarity

The Limit Input Polarity parameter determines whether Positive Limit Input and Negative Limit Input are active high or active low. The Positive Limit Input status bit and Negative Limit Input status bit indicate whether or not the corresponding limit input is active. For details on voltage levels, see the specifications for the module the input is located on.

Physical Placement of Sensors for Limit Inputs

Typically, the sensors for Limit Inputs should be installed at both ends of travel on the axis. For safe operation, they should be designed such that they become active close to the end of travel, and remain active to the physical end of travel. This increases safety, because motion commands (other than Direct Output) will only be allowed in the direction toward the valid range of travel. A diagram is shown below:

Good Practice

Do not install limit sensors such that they become active close to the end of travel, but then become inactive at the end of travel, as shown below. In this case, commands in the wrong direction will be allowed.

Poor Practice