Firstly some definitions:- Accuracy is the degree of veracity while precision is the degree of reproducibility.
We can explain the difference between accuracy and precision using the analogy of an arrow fired at a target. Accuracy describes the closeness of an arrow to the bullseye.
If many arrows were shot, precision equates to the size of the cluster. When all arrows are grouped together, the cluster is considered precise.
Optical Encoder Accuracy
Optical encoders work by shining a light source on to or through an optical element – usually a glass disk. The light is either blocked or passes through the disk’s gratings and a signal, analogous to position, is generated.
The glass disks are amazing – with tiny features which allow manufacturers to claim high precision. What is often not explicit is what happens if these tiny features are obscured by dust, dirt, grease etc. For purposes of this paper, we will ignore the issues of obscuration, fragility of the glass, optoelectronics life and temperature limits since these problems are well known. What is less well known is the issue of accuracy in optical encoders and optical encoder kits in particular.
Consider an optical device using a 1″ nominal disk with a resolution of 18 bits. Typically the claimed accuracy for such a device might be +/-10 arc seconds. However, what should be in big bold print (but never is on optical encoder data sheets) is that the stated accuracy assumes that the disk rotates perfectly relative to the read head and that temperature is constant.
If we consider a more realistic example, the disk is mounted slightly eccentrically by 0.001″ (0.025mm)
Fig. 3 – Imperfectly mounted rotor
Eccentricity comes from several sources and the following is a list of just some:-
- concentricity of the glass disk on its hub
- concentricity of the hub’s through bore relative to the optical disk
- perpendicularity of the hub relative to the plane of the optical disk
- parallelism of the optical disk face with the plane of the read head
- concentricity of the shaft on which the hub is mounted
- clearances in the bearings and bearing mounts which support the main shaft
- imperfect alignment of the bearings
- roundness of the shaft and roundness of the hub’s through bore
- locating method (typically a grub-screw will pull the hub to one side)
- displacements due to stresses or strain from forces on the shaft’s bearings
- thermal effects
- etc. etc.
A perfectly mounted optical disk requires such fine engineering that cost becomes prohibitive. In reality, there is a measurement error because the optical disk is not where the read head thinks it is. If we consider a mounting error of say 0.001″ then the measurement error is equivalent to the angle subtended by 0.001″ at the optical track radius. To make the maths easy let’s assume that the tracks are at a radius of 0,5″.
The equates to an error of 2 milliradians or 412 arc-seconds. In other words, the device with a specification accuracy of 10 arc-seconds is more than 40 times less accurate than its data sheet.
From experience we can advise that if you get an optical disk to be positioned accurately to within 0.001″ of an inch you are doing really well. Realistically, youre more likely to be in the range 2-10 thousandths of an inch so the actual accuracy will be 80-400 times worse than you might have originally calculated.
Zettlex Encoder Accuracy
The measurement principle of a Zettlex encoder is fundamentally different. Measurement is based on the mutual inductance between the faces of the rotor (the disk) and the stator (reader). Rather than calculating position from readings taken at a point, measurements are generated over the full face of both the stator and rotor. Consequently, discrepancies caused by non-concentricity in one part of the device are negated by opposing effects at the opposite part of the device.
Accuracies quoted by Zettlex are not quoted based on perfect alignment of rotor and stator. Realistically achievable tolerances – typically +/-0,2mm – are accounted for in any quoted resolutions, repeatabilities and accuracies.
Further, stated performance for Zettlex encoders are not subject to variation due to foreign matter, humidity, life-time, bearing wear or vibration.
Fig 4 – Zettlex IncOders offer accurate and reliable position measurement in tough conditions
If you’re thinking about using an optical encoder – and optical encoder kits, in particular:-
1. Read the spec. sheet very carefully
2. If your project has any of the following:
- mechanical assembly tolerances
- shock and vibration
- temperature variability
- potential ingress of liquids or foreign matter
then drop us an email for a competitive quote and a no-nonsense spec.
For more information on new generation inductive position sensors, please email firstname.lastname@example.org.