E-Stop Service

The “estop” service uses a heartbeat-based, software-implemented service to help guarantee a communications link between one or more off-robot host machines. The robot will not allow the motors to be powered without properly setting up this service.

In short, one or more pre-determined clients must confirm that they can talk to the robot by sending a special message. In the parlance of the service, one or more Endpoints that have been Registered against the Configuration must send CheckIn messages.

Clients can set one of the following “stop levels”:

  • CUT – actuator power is cut immediately.

  • SETTLE_THEN_CUT – the robot attempts to come to a stop, sit down, and then cut actuator power.

  • NONE – no stop level. Actuators are allowed to power on. This does NOT turn the actuators on.

The set up happens over the course of several steps: Configuration, Registration, and CheckIn. Once all registered endpoints have checked in, actuators can be powered on.

The Python library takes care of these steps for you. See the E-Stop example in estop_nogui example and the client implementation in bosdyn/client/estop.py for an example usage and implementation.

Configuration

The client sending the SetEstopConfigRequest is saying “This is the exact number of endpoints I expect to exist, and how often they must check in.” Endpoints in the configuration have two important values they must fill out:

  1. role

  2. timeout

See the protobuf’s documentation for more details.

There are some limitations on the configuration:

  1. There must be exactly one Endpoint with a role set to PDB_rooted.

  2. The PDB_rooted Endpoint must have a timeout of 60 seconds or less.

The PDB_rooted endpoint has its communications verified in the firmwares, which is why it has special requirements on its configuration.

Once the configuration is set, the robot waits for individual endpoints to register themselves. In the simplest case, a client will set a configuration with one endpoint.

By setting a configuration, any currently registered endpoints are forgotten about. This means the robot will behave as if the CUT stop level was sent by a client

Registration

One or more clients must now send one or more RegisterEstopEndpointRequest messages. The request must specify target_endpoint.role, as well as the relevant fields in new_endpoint. See the protobuf’s documentation for more details.

In the simplest case, the same client that sent the configuration with a single endpoint will immediately register itself as that endpoint.

CheckIn

A registered endpoint can now send an EstopCheckInRequest. Each such request contains a challenge and a response, to help guarantee a live communications link between the robot and a responsive client. The response must be the 1’s compliment of the provided challenge.

The EstopCheckInResponse received by the client will have a new challenge that should be responded to.

The first check-in sent by an endpoint will not have a valid challenge/response pair, so the first response will provide a INCORRECT_CHALLENGE_RESPONSE status, but it can be safely ignored.

If an incorrect challenge/response pair is received by the server, that check-in does not reset the timeout.

Operation

The E-Stop service will allow actuators to be powered on as long as the following conditions are met:

  1. There is a valid configuration

  2. All endpoints in the configuration have been registered

  3. All registered endpoints have checked in with the NONE stop level

  4. The number of seconds between each endpoint’s previous check-in and now is less than each endpoint’s timeout