Introduction

---

This guide gives a brief overview of common gauge movements, and suggests their pros and cons.

Physical

---

In physical gauges, the needle is attached directly to the value being measured. For instance, a boost gauge measures pressure, and a physical boost gauge uses that pressure to move its needle.



In this gauge, boost inflates or deflates a metallic bladder, applying a torque to a lever-arm, which rotates a gear, and finally rotates the gauge needle. The needle must be lightweight, as the holding torque is small at the desired position, and to minimize needle movement induced by road bumps. Gauge calibration depends on the rest position and flexibility of the metallic bladder, which may change over time.

Problems:

• Long-term calibration
• Accuracy
• Susceptibility to road bumps
• Lightweight needle required

Analog Volt Meter (d'Arsonval)

---

This gauge type consists of a coil, permanent magnet, and damped return spring; it's the electrical equivalent of the physical gauge. The needle shaft has a coil surrounded by a fixed permanent magnet, or vice versa, and a spring to return the needle to zero. When a current is applied to the coil, it creates a magnetic field that tries to align itself with the permanent magnet's field, but is fought by the return spring. More current through the coil strengthens the field, positioning the needle further away from zero, and less current weakens the field, positioning the needle closer to zero. With the proper series resistor, this gauge can be hooked to any voltage source, such as an oil pressure sensor, to measure its voltage. The system is damped to reduce needle oscillations and road bumps inducing needle movement, resulting in a slow reacting gauge. Finally, the needle position with a given applied voltage depends on the return spring's rest position and spring constant, which may change over time, throwing off gauge calibration.



Problems:

• Slow needle movement
• Long-term calibration
• Accuracy
• Susceptibility to road bumps
• Lightweight needle required

Air-Core Motor

---

Air-core gauges are the first that require circuitry to operate. The device consists of two independent, perpendicular coils surrounding a hollow chamber. The needle shaft protrudes into the chamber, where the shaft has a permanent magnet affixed. When current flows through the perpendicular coils, their magnetic fields super-impose, creating an overall magnetic field whose direction is approximately:



Where x and y are the coils' respective currents. The permanent magnet is free to align itself with that field, eventually settling near theta. In this way, by proportioning the current through each coil, the needle can reach all 360-degrees of gauge. However, because applied torque is maximum perpendicular to the field, and zero when aligned (near theta), the magnet rarely settles at theta; any friction in the shaft bearings or gravity acting on the needle will keep the needle from reaching theta. Further, because holding torque falls to zero near theta, most air-core motor gauges have simple, lightweight, stamped metal needles; the lower the needle's weight, the lower its overshoot, and less likely it is for bumps in the road to cause the needle to jump. You won't see a beefy back lit needle on an air-core gauge.



Problems:

• Accuracy
• Susceptibility to road bumps
• Lightweight needle required

Stepper Motor

---

Stepper motors are the most complicated to operate, but have certain advantages. Unlike the other gauge types, the stepper motor provides holding torque at the desired angle, allowing for beefier needles, and reducing both overshoot and susceptibility to bouncing over bumps. The stepper motor can be thought of as a gear and ratchet. In its simplest operation, the stepper motor moves in one-notch increments, or steps, and a 360-degree rotation may have 60 or more steps. The control board "tells" the needle to move one step forward or backward, and doing this repeatedly creates a continuous movement. It's up to the controller to remember where the needle is pointing, since it can tell the needle to move forward or backward, but cannot determine where the needle is. In more complicated modes of operation, each step can act like an entire 360-degrees of air-core rotation; with clever current control, the needle can be positioned to any angle between steps (called fine- or micro- stepping).



Problems:

• Needle movement can appear jerky

See Also...

---

• [FC Modification Manual]