EtherCAT Mobile Base

This tutorial drives a four-wheel skid-steer base with four EtherCAT servo drives from Stellar X. It is the Quick Start system on real motors: Strider still drives the base, and an EtherCAT gateway takes the place of the simulation gateway. You will:

1. wire the four drives to Stellar X,
2. add the gateway and Strider on Stellar Station and load their example documents,
3. bring up the EtherCAT network,
4. turn each wheel on its own from the gateway's test console, and
5. jog the wheels with Strider.

flowchart TB
    B["Your browser<br>Stellar Station · Strider"]
    subgraph SX ["Stellar X"]
        S["Strider<br>sodero.mlc.base"] -- servo_link --> G["EtherCAT gateway<br>sodero.actuator_gateway.wheels"]
    end
    subgraph EC ["EtherCAT line"]
        direction LR
        D1["Drive 1<br>front left"] --> D2["Drive 2<br>front right"] --> D3["Drive 3<br>rear left"] --> D4["Drive 4<br>rear right"]
    end
    B --> S
    G -- "EtherCAT Out" --> EC

Real motors move. Lift the base so that its wheels are off the ground until every wheel turns the right way. Wire the drives' safe torque off (STO) and an emergency stop, and keep the emergency stop within reach (Stop Controls).

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Before You Start

You need	Details
Stellar X	Powered on and reachable from your PC (Network Settings). In this guide its address is 192.168.60.18.
Four servo drives	EtherCAT drives with the CiA 402 profile and cyclic synchronous velocity (CSV). Their RxPDO must include Target velocity (0x60FF) (EtherCAT Gateway › What You Need).
Motors	One motor per wheel. The example uses motors with a rated torque of 2.2222 Nm, a peak torque of 6.6667 Nm, a maximum speed of 1,890 rpm and an encoder with 524,288 counts per turn, each behind a 7.75 : 1 gear.
A four-wheel base	The example base has wheels of 0.098 m radius at ±0.131 m front and back and ±0.188 m left and right of the base center.
Example files	wheels.gw (EtherCAT gateway with four drives) and base.sddm (Strider).

Your drives, motors and base are probably not the example's. Load the example files anyway and change the values in the Gateway Designer and Stellar Designer, as the steps below describe.

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Step 1: Wire the Drives

1. Plug Stellar X's EtherCAT Out port into the IN port of the front-left drive.
2. Connect its OUT port to the IN port of the front-right drive, then on to the rear-left and the rear-right drive.
3. Power the drives and release their STO and the emergency stop.

The cable order is the actuator order of the gateway, and the Strider document assigns the actuators to the wheels:

Drive on the cable	Gateway actuator	Wheel
1st	1 (index 0)	fl, front left
2nd	2 (index 1)	fr, front right
3rd	3 (index 2)	rl, rear left
4th	4 (index 3)	rr, rear right

Do not use a network switch. EtherCAT frames must pass through the drives in a line (EtherCAT Gateway › Wire the Drives).

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Step 2: Add the Gateway and Load wheels.gw

1. Open http://192.168.60.18 in your browser.
2. Click Add node, choose Gateway › Actuator gateway and click Next. Name it wheels and click Add node (Quick Start › Step 1).
3. Select sodero.actuator_gateway.wheels. In its inspector, click the upload icon in DEFINITION (Import .gw), choose wheels.gw on From your PC and click Import.

4. Click Edit Stellar Gateway Designer to check the document.

   

   ① four actuators, one per drive in cable order ② the Motor tab of actuator 1

   The gateway has four actuators ①, one per drive. Every actuator has the same Motor tab ②:

   Field	Example value
   Rated torque	2.2222 Nm
   Max torque	6.6667 Nm
   Max speed	1,890 rpm
   Resolution	524,288 PPR
   Encoder offset	0 pulse
   Gear ratio	7.75

   The EtherCAT tab of each actuator holds the drive's position on the cable (Idx 0 to 3), its label (drive_fl to drive_rr) and the PDO mapping: the six RxPDO and the six TxPDO objects listed in Process Data, including Target velocity (0x60FF).

5. If your hardware differs, change it here: select your drive in the EtherCAT tab and enter your motor in the Motor tab of each actuator (EtherCAT Gateway › Create the Gateway Document).

6. Open 02 Verification, click Verify and fix any errors. Then click the back arrow to return to the Station.

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Step 3: Add Strider and Connect It

1. Click Add node, choose Controller › Strider and click Next. Name it base and click Add node.
2. Select sodero.mlc.base, click the upload icon in DEFINITION (Import .sddm), choose base.sddm and click Import.
3. Connect the two nodes: drag from the Strider's servo_link to the gateway's servo_link, or click Auto-connect next to servo_link in the Strider's inspector.

① the gateway and Strider connected through servo_link ② the gateway's document ③ the gateway's log after a start

base.sddm describes the example base: a skid-steer chassis with four drive wheels, 0.098 m wheel radius and 17 kg. For your own base, select sodero.mlc.base, click Edit Stellar Designer and enter its dimensions and limits (Mobile Robot (.sddm)).

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Step 4: Start the Gateway

1. Select the gateway and click Start node. Wait until its state is Operating.

2. In its LOGS (stream Gateway events) ③, check that all four drives answered and reached operation:

   Gateway sodero.actuator_gateway.wheels starting: EtherCAT, 4 actuator(s), 1.0 ms cycle.
   EtherCAT: starting on network port 'eth0' (4 drive(s) expected).
   EtherCAT: found drive 1 (…).
   EtherCAT: found drive 2 (…).
   EtherCAT: found drive 3 (…).
   EtherCAT: found drive 4 (…).
   EtherCAT ready: 4 drive(s) operational, 1.0 ms cycle.
   

   Each found drive line names the drive as the drive reports itself.

If the gateway stops before EtherCAT ready, its log says why: no cable link, a drive missing from the chain, a PDO mapping that does not match the drive, or a drive that refuses the configuration. See EtherCAT Gateway › Troubleshooting.

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Step 5: Test the Wheels with the Test Console

Check every wheel with the gateway's test console before Strider drives them. That page describes the console and how to open and end a test; this step gives the values for the four wheels.

1. Select the gateway and click Test Stellar Gateway in its CONTROL section. The test console opens in a new browser tab. While the test runs, the graph shows it as sodero.actuator_test.wheels ①, and the gateway's CONTROL section shows Open test console and Stop test ②.

   

   ① the test console, connected to the gateway's servo_link ② the gateway's test console buttons

2. In the console, check that the top bar shows Gateway Connected and that there are four cards, A1 to A4. Card A1 is actuator 1, the first drive on the cable.

   

   ① EMERGENCY STOP ② Servo ON and Servo OFF ③ the drive mode of all actuators ④ the target velocity of actuator 1

3. Under DRIVE MODE — APPLY TO ALL AXES ③, choose VELOCITY and click Apply.

4. Click Servo ON ②. Check the gateway's log on the Station page for Servo ON: all 4 drive(s) enabled.
5. On card A1, press and hold the dqa_d · Target Velocity slider ④ and drag it to the right, to about 60 deg/s. The wheel turns while you hold the mouse button and stops when you release it. Keep the console's tab open while a wheel turns.
6. Do the same on cards A2, A3 and A4.

Each card must turn its own wheel (Step 1) forward, the way the base drives forward. At 60 deg/s a wheel needs 6 s for one turn.

What you see	Cause and fix
A different wheel turns	The cable order does not match the wheel order. Rewire the drives in the order of Step 1.
The wheel turns backward	The motor is mounted the other way round. Invert its direction in the drive, for example with the drive's polarity parameter as a start-up SDO in the EtherCAT tab. Leave the influence factor in the Strider document at 1 (Mobile Robot › Actuator Mapping).
One turn takes clearly more or less than 6 s	Gear ratio or Resolution in the gateway is wrong.

Other problems, such as a drive that does not enable, are in Test Console › Troubleshooting. To change the gateway document, for example a drive's polarity, end the test first (Test Console › Change the Gateway Document).

When every wheel passes, click Servo OFF in the console, then Stop test in the gateway's CONTROL section (Test Console › End the Test).

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Step 6: Jog the Wheels with Strider

1. Select the Strider and click Start node. The gateway's log shows Robot controller connected.
2. In the Strider's inspector, click Strider in CONTROL and open the Drive page (Strider › Jog the Base).
3. Under Drive Mode, click Twist. Under Servo / Safety, switch Servo On. The gateway's log shows Servo ON: all 4 drive(s) enabled.
4. Set Linear Velocity Limit to a low value, such as 0.1 m/s.
5. Press and hold Forward (W) for a moment. All four wheels turn forward.
6. Press and hold Turn Left (A). The left wheels turn backward and the right wheels forward.

The wheels passed the test console, so a wrong wheel or direction here comes from the Strider document: its actuator mapping must assign actuators 1 to 4 to fl, fr, rl and rr, each with the influence factor 1 (Mobile Robot › Actuator Mapping). If the wheels turn too fast or too slow, check the wheel radius in the Strider document.

When every wheel turns the right way, switch the servo off and click Idle. Put the base on the ground and drive it slowly, with the limits still low.

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What's Next

Next step	Page
Every Strider control, odometry calibration and programs	Strider
Localize the base on a map and drive it to goals	Localization and Navigation
EtherCAT parameters, process data and all gateway messages	EtherCAT Gateway
Your own base's dimensions, wheels and limits	Mobile Robot (.sddm)

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