# Chapter 12: Force Mode

## About This Chapter

Normal robot control controls:

```
Position
```

Force Mode, on the other hand, controls:

```
Force
```

For example, it can be used to:

- Push with a constant load.
- Press a component against another component.
- Slide along a surface.
- Produce spring-like motion.

---

# 12.1 Position Control and Force Control

Normal `moveJ` and `moveL` commands use:

```
Position control
```

For example, the robot attempts to move to:

```
100 mm
```

Even if an obstacle is encountered on the way, it attempts to reach the target position as far as possible.

---

Force Mode continues to push with:

```
10 N
```

The position is not guaranteed. Instead, the force is maintained at a constant value.

---

# 12.2 Applications of Force Mode

Typical examples:

- Connector insertion
- Component assembly
- Polishing
- Deburring
- Button pressing
- Spring measurement
- Load testing

---

# 12.3 Basic Sequence

Force Mode follows this sequence:

```
Start

↓

Force Mode

↓

Control at regular intervals

↓

End Force Mode
```

Unlike `moveJ`, it is not a command that is sent only once.

---

# 12.4 Basic API

Example:

```python
rtde_c.forceMode(
    task_frame,
    selection_vector,
    wrench,
    force_type,
    limits,
)
```

While Force Mode is active, the controller performs force control internally.

---

# 12.5 task_frame

This specifies the coordinate system in which force control is performed.

For example:

```
Base coordinates

Tool coordinates

Feature coordinates
```

In many cases, the following is used:

```
TCP coordinates
```

---

# 12.6 selection_vector

This specifies which degrees of freedom are subject to force control.

```
[
1,
1,
1,
0,
0,
0
]
```

In this example, only the XYZ directions are force-controlled. The rotational directions remain under position control.

---

For example:

```
[
0,
0,
1,
0,
0,
0
]
```

This applies force only along the Z direction.

---

# 12.7 wrench

The target force and torque are specified as:

```
[
Fx,
Fy,
Fz,
Mx,
My,
Mz
]
```

Units:

```
N

Nm
```

For example:

```
Fz=10 N
```

This applies a downward force of 10 N.

---

# 12.8 force_type

This specifies the controller's control mode.

Normally, the standard setting is sufficient. It is rarely changed except for special applications.

---

# 12.9 limits

This specifies the allowable displacement.

For example, if the robot has pushed as far as:

```
10 mm
```

without reaching:

```
10 N
```

it will not move any farther. This also serves as a safety feature.

---

# 12.10 Operation During Force Mode

The controller performs the following control cycle internally:

```
Current force

↓

Difference from target force

↓

PID

↓

Joint correction

↓

Motor
```

The user does not need to calculate the force on every cycle.

---

# 12.11 Reading the Force

API used together with Force Mode:

```python
rtde_r.getActualTCPForce()
```

Return values:

```
Fx

Fy

Fz

Mx

My

Mz
```

These values can be used to check the current force and torque.

---

# 12.12 Stopping Force Mode

Force Mode is terminated using:

```
forceModeStop()
```

Afterward, normal motion commands such as `moveJ` and `moveL` can be used again.

---

# 12.13 Common Applications

### Connector Insertion

```
moveJ

↓

moveL

↓

ForceMode

↓

10 N

↓

Insertion

↓

Stop
```

---

### Button Pressing

```
moveL

↓

ForceMode

↓

5 N

↓

Hold for 0.5 seconds

↓

Stop
```

---

### Surface Following

```
ForceMode

↓

Constant force

↓

Move in the XY directions
```

This is used in applications such as polishing.

---

# 12.14 Precautions

Force Mode may not produce the expected force unless the following are configured correctly:

- TCP settings
- Payload settings

Payload calibration is important when using an actual robot.

---

# 12.15 Difference from moveJ

`moveJ`:

```
Position

↓

Stop
```

Force Mode:

```
Force

↓

Maintain a constant value
```

Their purposes are completely different.

---

# 12.16 Design in ur_ctrl.py

For a high-level API, it is convenient to separate starting and stopping:

```python
start_force_mode()

stop_force_mode()
```

Combining these with:

```python
get_tcp_force()
```

also makes force monitoring easy to implement.

---

# Summary

Force Mode controls force rather than position.

Its purpose differs from that of ordinary motion APIs. It is used for:

- Assembly
- Pressing operations
- Polishing
- Contact tasks

When using Force Mode, it is important to configure and use the following appropriately:

- TCP settings
- Payload settings
- `ForceModeStop()`

The next chapter describes Freedrive Mode.