The watchdog¶
The watchdog stops the motor when the host stops talking: a host program that crashes, a cable that comes out, a USB adapter that hangs. It is a timer in the driver.
Every valid motion frame and every request the driver answers starts it again; when it runs out, the driver raises the watchdog error and does what its action says. From firmware 109 the action can hold the position, brake, coast, or let a joint down slowly; before, the driver always stopped into its idle behaviour.
Setting it¶
Send_Watchdog (ID 15): the time in ms (4 bytes) and the action (1 byte). Or the parameters 6 (time) and 7 (action) with Send_Parameter.
Watchdog ms in the settings.
0 ms switches it off, the default. Set it to a few of your host's cycle times: with a 100 Hz loop, 30 to 100 ms. Send_Save_config keeps it, and a kept watchdog runs from power-up: a host that does not start sending within the time finds the driver in watchdog error and clears it with Send_Clear_Error.
What starts the timer again: Send_data_pack_1, Send_data_pack_PD, Send_data_pack_HALL, Send_gripper_data_pack, the status requests (Send_Respond_…), Send_Get_Parameter and Send_Buffer_Read. From firmware 109 Send_Watchdog (or parameter 6) starts a fresh window, and Send_Save_config starts the timer again too.
What does not: the other setting frames (gains, limits, Idle, Clear, Settings, Send_Parameter) and every UART command (known issue KI-11). While the encoder table or the anti-cogging is measured, the driver feeds it itself.
What a trip does¶
| Action | Name | What the driver does |
|---|---|---|
| 0 | legacy | what firmware 108 and older did: idle, which brakes (the coils shorted) unless #Coast or Send_Settings chose coasting |
| 1 | hold | holds the position the motor had at the trip, for the fallback time; then the after-action |
| 2 | brake | shorts the coils at once |
| 3 | coast | leaves the coils open at once: the motor turns freely |
| 4 | descend | applies the fallback current, but never lets the motor turn faster than the fallback speed, for the fallback time; then the after-action |
Any other action byte acts as 0. 0 is also what an unchanged host sends: the Python library's default, and what the PAR6 runtime sends.
Three rules keep a gentler action from softening the response to a real fault:
- Only a trip of the watchdog alone gets the action. If another error is active as well (E-stop, encoder, supply voltage, temperature, not calibrated, a gripper E-stop), the driver gives the legacy response.
- Hold and descend take over only from a running motor: position, velocity, current, PD or gripper mode. A driver that was idle when the watchdog ran out stays as idle was. From firmware 110 that holds also when a motion frame comes before the error is cleared; on 109 such a frame started the hold or the descend late, for the whole fallback time.
- Hold and descend always end after the fallback time, at most 60 s, so a lost host never leaves a motor driven for good. The after-action follows: 0 legacy, 2 brake or 3 coast.
The driver reports the watchdog error the whole time, and Send_Clear_Error (or #Clear) ends the action at
once and goes to idle; the next motion frame takes over.
Hold¶
Hold keeps the position loop on the position the shaft had at the trip. Coming from position or velocity mode it keeps the velocity integral too, so a joint under load does not sag at the moment of the trip.
A host that recovers within the fallback time (a USB hiccup) finds the joint where it left it. On the bench a bare NEMA 17 held within ±40 ticks (0.9°) for 5 s while it was pushed by hand, then braked.
Descend¶
Descend applies the fallback current and limits the speed in both directions to the fallback speed. Inside the cap the speed limiter's integral relaxes back to zero (within about 0.25 s), so it holds back only what keeps the speed at the cap and never more:
- a joint under gravity with 0 mA lowers itself at no more than the cap speed and rests on whatever stops it;
- a gripper with its gripping current keeps its grip, and its jaws cannot slam shut;
- a free motor with a current turns at about the cap speed (on the bench, 200 mA and a 1000 ticks/s cap: 587 ticks/s on average; the stepper's detents slow it).
It starts with the velocity integral at zero, so gravity can take the joint down.
The fallback¶
Four values, kept by Send_Save_config in settings block 2:
| Parameter | #WdFb position |
What | Default |
|---|---|---|---|
| 2 | 1st | how long hold or descend runs, ms (0 to 60000) | 2000 |
| 3 | 2nd | the current descend applies, mA (signed) | 0 |
| 4 | 3rd | the speed cap of descend, ticks/s (0 to 65535) | 1000 |
| 5 | 4th | the after-action: 0 legacy, 2 brake, 3 coast | 0 |
Choosing an action¶
- A bare motor on the bench, a conveyor, a wheel: brake (2) stops it fastest; coast (3) lets it run out.
- A joint that carries a load: hold (1) for a second or two, so a short loss of the host does not move it, then descend (4) with 0 mA for a joint that gravity pulls down, or brake for one it does not. Choose the cap speed for a joint you can still catch by hand.
- A gripper: descend with the gripping current, its sign the closing direction, keeps an object held.
These suggestions are untested on an assembled robot arm; try the action on your machine before you rely on it. The braking of brake and of the legacy response is not current-limited (known issue KI-15): a load that drives a braking motor fast drives a large current through the coils.
Seeing a trip¶
A capture armed with the error trigger (parameter 28 = 2) records the moments before and after a trip, the watchdog's included: Status and diagnostics.
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