The means and methods of stopping automated equipment in an integrated material handling system are governed by many factors including, but not limited to:
- Location and usage of the equipment per the application
- Operator or non-maintenance personnel access to moving equipment
- Federal, State, and/or local ordinance or code
- Accepted electrical design practice
- Customer or end user preference
With all of these variables in play; ConveyStop cannot claim or be advertised as a de facto “approved” stopping method for all conveyor system stopping situations including an emergency stop situation. The purpose of this section is to define a recognized standard for control system stop classifications or categories and describe how ConveyStop can be applied for each.
It is the responsibility of the integrator of a ConveyLinx equipped system utilizing ConveyStop to assess all of the aforementioned factors before deeming a ConveyStop generated Stop Command as being applicable and suited for an emergency stop situation.
The contains accepted definitions for stopping functionality and emergency operations that are, in general, applicable to the conveyor and material handling industry.
NFPA® 79 section 220.127.116.11 Emergency Operations is as follows:
(1) This section specifies the requirements for the emergency stop and the emergency switching-off functions of the emergency operations, both of which are initiated by a single human action
(2) Once active operation of an emergency stop or emergency switching off actuator has ceased following a command, the effect of this command shall be sustained until it is reset. This reset shall be
possible only at the location where the command has been initiated. The reset of the command shall not restart the machinery but only permit restarting
(3) It shall not be possible to restart the machinery until all emergency stop commands have been reset. It shall not be possible to re-energize the machinery until all emergency switching off commands have
How ConveyStop Applies to Emergency Operations
The section 18.104.22.168 Item (1) criterion is met when physical buttons or switches are assigned and enabled with ConveyStop. Please note that a networked PLC or PC can generate a Stop Command and as such can occur programmatically and not necessarily by “single human action”. In this case, the integrator would be responsible for assuring that the PLC or PC based Stop Command is always initiated from a “single human action” integrated with said PLC or PC controls.
The section 22.214.171.124 Item (2) criterion is met when physical buttons or switches are assigned and enabled with ConveyStop and these devices are proper maintained contact type. As long as the button or switch is in its “emergency” position; ConveyStop will not issue or respond to any Start Command regardless of source (hard-wired button or networked PLC or PC). Also, resetting the physical device to its “non-emergency” state will not restart the modules to operation nor will this resetting of the device initiate any Start Command.
The section 126.96.36.199 Item (3) criterion is met in ConveyStop by design. If multiple physical buttons or switches are assigned and enabled with ConveyStop; all have to be placed into their “non-emergency” state before ConveyStop will issue or respond to any Start Command regardless of source. This is true regardless of which device first initiated the stop.
NFPA® 79 section 9.2.2 Stop Functions defines stop functionality as:
Stop functions shall operate by de-energizing that relevant circuit and shall override related start functions. The reset of the stop functions shall not initiate any hazardous conditions.
This section further defines three (3) Categories for stop functionality:
Category 0 – An uncontrolled stop by immediately removing power to the machines actuators
Category 1 – A controlled stop with power to the machine actuators available to achieve the stop then remove power when the stop is achieved
Category 2 – A controlled stop with power left available to the machine actuators
ConveyStop and Stop Function Definition
In general, ConveyStop follows the intent of the Stop Function definition for section 9.2.2. The manner in which “de-energizing that relevant circuit” it is accomplished differs between the ERSC and CNIP modules.
CNIP and Stop Function
Four of the six digital output circuits on the CNIP modules have their control power source internally wired to a contact relay. When a Stop Command is active, this relay is de-energized and control power is disconnected. In this state, the digital output circuit is de-energized regardless of the state of the logical output. This is a common stop circuit design for PLC based I/O systems.
ERSC and Stop Function
In situations where a Stop Command is active and control power is maintained to an ERSC, the de-energizing of the relevant circuit is accomplished by the on board processor. The ERSC utilizes a single processor and this processor directly controls (among other things) the power MOSFET transistor gates that commutate the motor. When a Stop Command is active, the processor places all MOSFET gates in their open or non-conductive state and then by-passes the task in the processor that produces motor commutation.
ConveyStop and Stop Function Categories
Utilizing ConveyStop as designed and intended adheres to the criterion of Category 2. Maintaining control power to all ConveyLinx modules is desirable because they maintain their “pre stopped” state making for faster and more reliable recovery. Another added benefit of maintaining control power to all ConveyLinx modules is that the diagnostic features of both ConveyLinx and ConveyStop are available to PLC and/or PC (including the ConveyStop PC software monitoring capability) as an aid in troubleshooting and event logging.
Category 1 And Category 0
Both of these categories involve disconnecting power to ConveyLinx modules and either of these can be implemented on a ConveyLinx system with or without ConveyStop being enabled.
If ConveyStop is implemented, the behavior would essentially be as described in section 0 Power Loss. ERSC modules will retain their state to flash memory as power is being dropped. When power is restored; each Stop Group affected by the power disconnect will have to receive a Start Command from either an assigned and enabled button or switch or from a network source (PLC or PC).
For CNIP modules, each is equipped with separate power terminals. One set of terminals is for module logic and input bus power and the other set powers only the output bus. In a Category 1 or Category 0 system, the motion producing control power can be disconnected from the CNIP’s output bus while leaving logic and input control power on. This is a typical strategy applied to PLC I/O systems and can be implemented with CNIP modules with or without ConveyStop installed and enabled. It must be noted that this strategy requires 2 separate power systems to be field wired.
What Happens When You Do Not Use ConveyStop?
Operationally, ConveyStop is not required for any ConveyLinx system to function. If ConveyStop is not implemented, disconnecting power does cause all motor motion to stop for ERSC’s and all outputs to be de-energized for CNIP’s. However it is important to note that without ConveyStop enabled and when power is restored; ERSC modules will be enabled to run and if conveyor conditions dictate, motors will run and packages will begin to move with no other or separate start signal or command.
Another important note is that ConveyLinx devices are Ethernet based, and upon cycle of power, modules will individually reinitialize at different rates making exact power up behavior unpredictable.
By no means is NFPA® 79 the only specification or criteria for defining the stop function of an automated system. The NFPA® 79 is a general standard for the USA and the descriptions above are based upon general experience for US installations.
The bottom line is that it is always up to the integrator to understand and adhere to the applicable specifications, codes, and standards on a per system basis. ConveyStop can be a valuable tool to achieve desired system stop functionality, enhanced diagnostics, and lower installed cost.