Motor Current Monitoring

Introduction

Motor current monitoring is a joint sensing method for force and torque. For a joint driven by a DC motor, you measure the armature current. This current is proportional to the motor torque.

How it works

$$T=K_T{I}_a$$

where:

• $I_a$ is the armature current
• $T$ is the total torque from the motor
• $K_T$ is the motor torque constant

A rotary to linear converter, such as a rack and pinion, changes this torque into a force. With a pinion gear of radius $R$ and efficiency $\eta$ (for example $\eta =0.9$):

$$F=\frac{T}{R}\times \eta$$

So the relation between motor current and force is:

$$\boxed{F=\frac{K_T{I}_a\eta }{R}}$$

Problems

• $K_T$ and the armature resistance $R_a$ change with temperature. $K_T$ decreases and $R_a$ increases as the temperature rises, so the force decreases
• brush noise and changes in brush resistance make it hard to monitor the current accurately